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**The Generic Mapping Tools**

**Technical Reference and Cookbook**

**Pål (Paul) Wessel**

**SOEST, University of Hawai'i at Manoa**

**Walter H. F. Smith**

**Laboratory for Satellite Altimetry, NOAA/NESDIS**

**Remko Scharroo**

**EUMETSAT, Darmstadt, Germany**

**Joaquim F. Luis**

**Universidade do Algarve, Faro, Portugal**

**Florian Wobbe**

**Alfred Wegener Institute, Germany**

.. figure:: /_images/GMT5_Summit_2011.jpg
   :height: 599 px
   :width: 1013 px
   :align: center
   :scale: 50 %

   The four horsemen of the GMT apocalypse: Remko Scharroo, Paul Wessel, Walter H.F. Smith,
   and Joaquim Luis at the GMT Developer Summit in Honolulu, Hawaii during February 14--18, 2011.

Acknowledgments
===============

The Generic Mapping Tools (GMT) could not have been designed without
the generous support of several people. We gratefully acknowledge A. B.
Watts and the late W. F. Haxby for supporting our efforts on the
original version 1.0 while we were their graduate students at
Lamont-Doherty Earth Observatory. Doug Shearer and Roger Davis patiently
answered many questions over e-mail. The subroutine ``gauss`` was
written and supplied by Bill Menke. Further development of versions
2.0--2.1 at SOEST would not have been possible without the support from
the HIGP/SOEST Post-Doctoral Fellowship program to Paul Wessel. Walter
H. F. Smith gratefully acknowledges the generous support of the C. H.
and I. M. Green Foundation for Earth Sciences at the Institute of
Geophysics and Planetary Physics, Scripps Institution of Oceanography,
University of California at San Diego. GMT series 3.x, 4.x, and 5.x
owe their existence to grants EAR-93-02272, OCE-95-29431, OCE-00-82552,
OCE-04-52126, and OCE-1029874 from the National Science Foundation,
which we gratefully acknowledge.

We would also like to acknowledge feedback, suggestions and bug reports
from Michael Barck, Manfred Brands, Allen Cogbill, Stephan Eickschen,
Ben Horner-Johnson, John Kuhn, Angel Li, Andrew Macrae, Alex Madon, Ken
McLean, Greg Neumann, Ameet Raval, Georg Schwarz, Richard Signell, Peter
Schmidt, Dirk Stoecker, Eduardo Suárez, Mikhail Tchernychev, Malte
Thoma, David Townsend, Garry Vaughan, William Weibel, and many others,
including their advice on how to make GMT portable to a wide range of
platforms. John Lillibridge and Stephan Eickschen provided the original
examples 11 and 32, respectively; Hanno von Lom helped resolve early
problems with DLL libraries for Win32; Lloyd Parkes enabled indexed
color images in PostScript; Kurt Schwehr maintains the packages; Wayne
Wilson implemented the full general perspective projection; and William
Yip helped translate GMT to POSIX ANSI C and incorporate netCDF 3. The
SOEST RCF staff (Ross Ishida, Pat Townsend, and Sharon Stahl) provided
valuable help on Linux and web server support.

Honolulu, HI, Silver Spring, MD, Faro, Portugal, Darmstadt and
Bremerhaven, Germany, March 2014


A Reminder
==========

If you feel it is appropriate, you may consider paying us back by citing
our EOS articles on GMT and technical papers on algorithms when you
publish papers containing results or illustrations obtained using GMT.
The EOS articles on GMT are

-  Wessel, P., W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe,
   Generic Mapping Tools: Improved Version Released, *EOS Trans. AGU*, 94(45),
   p. 409-410, 2013. `doi:10.1002/2013EO450001 <http://dx.doi.org/10.1002/2013EO450001>`_.

-  Wessel, P., and W. H. F. Smith, New, improved version of Generic
   Mapping Tools released, *EOS Trans. AGU*, 79(47),
   p. 579, 1998. `doi:10.1029/98EO00426 <http://dx.doi.org/10.1029/98EO00426>`_.

-  Wessel, P., and W. H. F. Smith, New version of the Generic Mapping
   Tools released, *EOS Trans. AGU*, 76(33), 329, 1995. `doi:10.1029/95EO00198 <http://dx.doi.org/10.1029/95EO00198>`_.

-  Wessel, P., and W. H. F. Smith, Free software helps map and display
   data, *EOS Trans. AGU*, 72(41), 445--446, 1991. `doi:10.1029/90EO00319 <http://dx.doi.org/10.1029/90EO00319>`_.


Some GMT programs are based on algorithms we have developed and
published separately, such as

-  Kim, S.-S., and P. Wessel, Directional median filtering for
   regional-residual separation of bathymetry, *Geochem. Geophys.
   Geosyst.*, 9, Q03005, 2008. `doi:10.1029/2007GC001850 <http://dx.doi.org/10.1029/2007GC001850>`_.
   [:doc:`dimfilter <supplements/misc/dimfilter>`, **misc** supplement]

-  Luis, J. F. and J. M. Miranda, Reevaluation of magnetic chrons in the
   North Atlantic between 35N and 47N: Implications for the formation of the
   Azores Triple Junction and associated plateau,
   *J. Geophys. Res.*, 113, B10105, 2008. `doi:10.1029/2007JB005573 <http://dx.doi.org/10.1029/2007JB005573>`_.
   [:doc:`grdredpol <supplements/potential/grdredpol>`, **potential** supplement]

-  Smith, W. H. F., and P. Wessel, Gridding with continuous curvature
   splines in tension, *Geophysics*, 55(3), 293--305, 1990. `doi:10.1190/1.1442837 <http://dx.doi.org/10.1190/1.1442837>`_.
   [:doc:`surface`]

-  Wessel, P., Tools for analyzing intersecting tracks: The x2sys
   package, *Computers & Geosciences*, 36, 348--354, 2010. `doi:10.1016/j.cageo.2009.05.009 <http://dx.doi.org/10.1016/j.cageo.2009.05.009>`_.
   [:doc:`x2sys <supplements/x2sys/x2sys_init>` supplement]

-  Wessel, P., A General-purpose Green's function-based interpolator,
   *Computers & Geosciences*, 35, 1247--1254, 2009. `doi:10.1016/j.cageo.2008.08.012 <http://dx.doi.org/10.1016/j.cageo.2008.08.012>`_.
   [:doc:`greenspline`]

-  Wessel, P. and J. M. Becker, Interpolation using a generalized
   Green's function for a spherical surface spline in tension, *Geophys.
   J. Int.*, 174, 21--28, 2008. `doi:10.1111/j.1365-246X.2008.03829.x <http://dx.doi.org/10.1111/j.1365-246X.2008.03829.x>`_.
   [:doc:`greenspline`]

Finally, GMT includes some code supplied by others, in particular the
Triangle code used for Delaunay triangulation. Its author, `Jonathan
Shewchuk <http://www.cs.berkeley.edu/~jrs/>`_, says

    "If you use Triangle, and especially if you use it to accomplish
    real work, I would like very much to hear from you. A short letter
    or email describing how you use Triangle will
    mean a lot to me. The more people I know are using this program, the
    more easily I can justify spending time on improvements and on the
    three-dimensional successor to Triangle, which in turn will benefit
    you."

A few GMT users take the time to write us letters, telling us of the
difference GMT is making in their work. We appreciate receiving these
letters. On days when we wonder why we ever released GMT we pull these
letters out and read them. Seriously, as financial support for
GMT depends on how well we can "sell" the idea to funding agencies and
our superiors, letter-writing is one area where GMT users can affect
such decisions by supporting the GMT project.

Copyright and Caveat Emptor!
============================

Copyright ©1991--2014 by P. Wessel, W. H. F. Smith, R. Scharroo, J.
Luis and F. Wobbe

The Generic Mapping Tools (GMT) is free software; you can
redistribute it and/or modify it under the terms of the GNU Lesser
General Public License as published by the Free Software Foundation.

The GMT package is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the file
``LICENSE.TXT`` in the GMT directory or the for more details.

Permission is granted to make and distribute verbatim copies of this
manual provided that the copyright notice and these paragraphs are
preserved on all copies. The GMT package may be included in a bundled
distribution of software for which a reasonable fee may be charged.

GMT does not come with any warranties, nor
is it guaranteed to work on your computer. The user assumes full
responsibility for the use of this system. In particular, the University
of Hawaii School of Ocean and Earth Science and Technology, the National
Oceanic and Atmospheric Administration, EUMETSAT, the
Universidade do Algarve, Alfred Wegener Institute, the National Science
Foundation, Paul Wessel, Walter H. F. Smith, Remko Scharroo, Joaquim F.
Luis, Florian Wobbe or any other individuals involved in the design and
maintenance of GMT are NOT responsible for any damage that may follow
from correct or incorrect use of these programs.

Preface
=======

While GMT has served the map-making and data processing needs of
scientists since 1988 [1]_, the current global use was heralded by the
first official release in *EOS Trans. AGU* in the fall of 1991. Since
then, GMT has grown to become a standard tool for many users,
particularly in the Earth and Ocean Sciences but the global collective
of GMT users is incredibly diverse. Development has at times been
rapid, and numerous releases have seen the light of day since the early
versions. For a detailed history of the changes from release to release,
see file ``ChangeLog`` in the main GMT directory. For a nightly snapshot of ongoing
activity, see the online page. For a historical perspective of the
origins and development of GMT see the video podcast "20 Years with
GMT -- The Generic Mapping Tools" produced following a seminar given by
Paul Wessel on the 20th anniversary of GMT; a link is available on the
GMT website.

The success of GMT is to a large degree due to the input of the user
community. In fact, most of the capabilities and options in
GMT programs originated as user requests. We would like to hear from
you should you have any suggestions for future enhancements and
modification. Please send your comments to the GMT help list or create
an issue in the bug tracker
(see `<http://gmt.soest.hawaii.edu/projects/gmt/issues/>`_).

New Features in GMT 5
=====================

GMT 5 represents a new branch of GMT development that mostly preserves the
capabilities of the previous versions while adding over 200 new features
to an already extensive bag of tricks.  Our PostScript library
:doc:`PSL <pslib>` has seen a complete rewrite as well
and produce shorter and more compact PostScript. However, the big news
is aimed for developers who wish to leverage GMT in their own applications.
We have completely revamped the code base so that high-level
GMT functionality is now accessible via GMT "modules". These are
high-level functions named after their corresponding programs (e.g.,
``GMT_grdimage``) that contains all of the functionality of that program
within the function. While currently callable from C/C++ only (with some
support for F77), we are making progress on the Matlab interface as well
and there are plans to start on the Python version. Developers should
consult the :doc:`GMT API <GMT_API>` documentation for more details.

We recommend that users of GMT 4 consider learning the new rules and defaults
since eventually (in some years) GMT 4 will be obsolete.
To ease the transition to GMT 5 you may run it in compatibility mode,
thus allowing the use of many obsolete default names and command
switches (you will receive a warning instead).  This is discussed below.

Below are six key areas of improvements in GMT 5.

New programs
------------

First, a few new programs have been added and some have been
promoted (and possibly renamed) from earlier supplements:

:doc:`gmt`
    This is the **only** program executable that is distributed with GMT 5. To avoid
    problems with namespace conflicts (e.g., there are other, non-GMT programs
    with generic names like triangulate, surface, etc.) all GMT 5 modules are
    launched from the gmt executable via "gmt module" calls (e.g, gmt pscoast).
    For backwards compatibility (see below) we also offer symbolic links with
    the old executable names that simply point to the gmt program, which then
    can start the correct module.  Any module whose name starts with "gmt" can
    be abbreviated, e.g., "gmt gmtconvert" may be called as "gmt convert".

:doc:`gmt2kml`
    A :doc:`psxy` -like tool to produce KML overlays for Google Earth. Previously
    found in the misc supplement.

:doc:`gmtconnect`
    Connect individual lines whose end points match within given tolerance.
    Previously known as gmtstitch in the misc supplement (this name is recognized
    when GMT is running in compatibility mode).

:doc:`gmtget`
    Return the values of the specified GMT defaults.  Previously only
    implemented as a shell script and thus not available on all platforms.

:doc:`gmtinfo`
    Report information about data tables. Previously known by the name minmax
    (this name is still recognized when GMT is running in compatibility mode).

:doc:`gmtsimplify`
    A line-reduction tool for coastlines and similar lines. Previously found
    in the misc supplement under the name gmtdp (this name is recognized when
    GMT is running in compatibility mode).

:doc:`gmtspatial`
    Perform various geospatial operations on lines and polygons.

:doc:`gmtvector`
    Perform basic vector manipulation in 2-D and 3-D.

:doc:`gmtwhich`
    Return the full path to specified data files.

:doc:`grdraster`
    Extracts subsets from large global grids.  Previously
    found in the dbase supplement.

:doc:`kml2gmt`
    Extract GMT data tables from Google Earth KML files.  Previously
    found in the misc supplement.

:doc:`sph2grd`
    Compute grid from list of spherical harmonic coefficients [We will add its
    natural complement grd2sph at a later date].

:doc:`sphdistance`
    Make grid of distances to nearest points on a sphere.  Previously
    found in the sph supplement.

:doc:`sphinterpolate`
    Spherical gridding in tension of data on a sphere.  Previously
    found in the sph supplement.

:doc:`sphtriangulate`
    Delaunay or Voronoi construction of spherical lon,lat data.  Previously
    found in the sph supplement.

We have also added a new supplement called potential that contains these five modules:

:doc:`gmtgravmag3d <supplements/potential/gmtgravmag3d>`:
    Compute the gravity/magnetic anomaly of a body by the method of Okabe.

:doc:`gravfft <supplements/potential/gravfft>`:
    Compute gravitational attraction of 3-D surfaces and a little more by the method of Parker.

:doc:`grdgravmag3d <supplements/potential/grdgravmag3d>`:
    Computes the gravity effect of one (or two) grids by the method of Okabe.

:doc:`grdredpol <supplements/potential/grdredpol>`:
    Compute the Continuous Reduction To the Pole, also known as differential RTP.

:doc:`grdseamount <supplements/potential/grdseamount>`:
    Compute synthetic seamount (Gaussian or cone, circular or elliptical) bathymetry.

Finally, the spotter supplement has added one new module:

:doc:`grdpmodeler <supplements/spotter/grdpmodeler>`:
    Evaluate a plate model on a geographic grid.

New common options
------------------

First we discuss changes that have been
implemented by a series of new lower-case GMT common options:

*  Programs that read data tables can now process the aspatial metadata
   in OGR/GMT files with the new **-a** option. These can be produced by
   **ogr2ogr** (a GDAL tool) when selecting the -f "GMT" output
   format. See Appendix `P. The GMT Vector Data Format for OGR Compatibility`_
   for an explanation of the OGR/GMT file format.
   Because all GIS information is encoded via GMT comment lines these
   files can also be used in GMT 4 (the GIS metadata is simply
   skipped).

*  Programs that read or write data tables can specify a custom binary format
   using the enhanced **-b** option.

*  Programs that read data tables can control which columns to read and
   in what order (and optionally supply scaling relations) with the new **-i** option

*  Programs that read grids can use new common option **-n** to control
   grid interpolation settings and boundary conditions.

*  Programs that write data tables can control which columns to write
   and in what order (and optionally supply scaling relations) with the new **-o** option.

*  All plot programs can take a new **-p** option for perspective view
   from infinity. In GMT 4, only some programs could do this (e.g.,
   :doc:`pscoast`) and it took a
   program-specific option, typically **-E** and sometimes an option
   **-Z** would be needed as well. This information is now all passed
   via **-p** and applies across all GMT plotting programs.

*  Programs that read data tables can control how records with NaNs are
   handled with the new **-s** option.

*  All plot programs can take a new **-t** option to modify the PDF
   transparency level for that layer. However, as PostScript has no provision for
   transparency you can only see the effect if you convert it to PDF.

Updated common options
----------------------

Some of the established GMT common options have seen significant
improvements; these include:

*  The completely revised **-B** option can now handle irregular and custom annotations
   (see Section `Custom axes`_). It also has a new automatic mode which
   will select optimal intervals given data range and plot size. The 3-D base maps can now have
   horizontal gridlines on xz and yz back walls.

*  The **-R** option may now accept a leading unit which implies the
   given coordinates are projected map coordinates and should be
   replaced with the corresponding geographic coordinates given the
   specified map projection. For linear projections such units imply a
   simple unit conversion for the given coordinates (e.g., km to meter).

*  Introduced **-fp**\ [*unit*] which allows data input to be in
   projected values, e.g., UTM coordinates while **-Ju** is given.

While just giving - (the hyphen) as argument presents just the synopsis of the command
line arguments, we now also support giving + which in addition will list
the explanations for all options that are not among the GMT common set.

New default parameters
----------------------

Most of the GMT default parameters have changed names in order to
group parameters into logical groups and to use more consistent naming.
However, under compatibility mode (see below) the old names are still recognized.
New capabilities have been implemented by introducing new GMT default settings:

*  :ref:`DIR_DCW <DIR Parameters>` specifies where to look for the optional
   Digital Charts of the World database (for country coloring or selections).

*  :ref:`DIR_GSHHG <DIR Parameters>` specifies where to look for the required
   Global Self-consistent Hierarchical High-resolution Geography database.

*  :ref:`GMT_COMPATIBILITY <GMT_COMPATIBILITY>` can be set to 4 to allow
   backwards compatibility with GMT 4 command-line syntax or 5 to impose
   strict GMT5 syntax checking.

*  :ref:`IO_NC4_CHUNK_SIZE <IO_NC4_CHUNK_SIZE>` is used to set the default
   chunk size for the **lat** and **lon** dimension of the **z** variable of
   netCDF version 4 files.

*  :ref:`IO_NC4_DEFLATION_LEVEL <IO_NC4_DEFLATION_LEVEL>` is used to set
   the compression level for netCDF4 files upon output.

*  :ref:`IO_SEGMENT_MARKER <IO_SEGMENT_MARKER>` can be used to change the
   character that GMT uses to identify new segment header records [>].

*  :ref:`MAP_ANNOT_ORTHO <MAP_ANNOT_ORTHO>` controls whether axes annotations
   for Cartesian plots are horizontal or orthogonal to the individual axes.

*  :ref:`GMT_FFT <GMT_FFT>` controls which algorithms to use for Fourier
   transforms.

*  :ref:`GMT_TRIANGULATE <GMT_TRIANGULATE>` controls which algorithm to use
   for Delaunay triangulation.

*  Great circle distance approximations can now be fine-tuned via new GMT default parameters
   :ref:`PROJ_MEAN_RADIUS <PROJ_MEAN_RADIUS>` and :ref:`PROJ_AUX_LATITUDE <PROJ_AUX_LATITUDE>`.
   Geodesics are now even more accurate by using the Vincenty [1975] algorithm instead of
   Rudoe's method.

*  :ref:`GMT_EXTRAPOLATE_VAL <GMT_EXTRAPOLATE_VAL>` controls what splines should do if
   requested to extrapolate beyond the given data domain.

*  :ref:`PS_TRANSPARENCY <PS_TRANSPARENCY>` allows users to modify how transparency will be
   processed when converted to PDF [Normal].

General improvements
--------------------

Other wide-ranging changes have been implemented in different
ways, such as

*  All programs now use consistent, standardized choices for plot
   dimension units (**c**\ m, **i**\ nch, or **p**\ oint; we no longer
   consider **m**\ eter a plot length unit), and actual distances
   (choose spherical arc lengths in **d**\ egree, **m**\ inute, and
   **s**\ econd [was **c**], or distances in m\ **e**\ ter [Default],
   **f**\ oot [new], **k**\ m, **M**\ ile [was sometimes **i** or
   **m**], **n**\ autical mile, and s\ **u**\ rvey foot [new]).

*  Programs that read data tables can now process multi-segment tables
   automatically. This means programs that did not have this capability
   (e.g., :doc:`filter1d`) can now all operate on the
   segments separately; consequently, there is no longer a **-m**
   option.

*  While we support the scaling of z-values in grids via the filename convention
   name[=\ *ID*\ [/*scale*/*offset*\ [/*nan*]]] mechanism, there are times
   when we wish to scale the x,y domain as well. Users can now
   append **+u**\ *unit* to their gridfile names, where *unit* is one of non-arc units listed
   in Table :ref:`distunits <tbl-distunits>`.  This will convert your Cartesian
   x and y coordinates *from* the given unit *to* meters.  We also support the inverse
   option **+U**\ *unit*, which can be used to convert your grid
   coordinates *from* meters *to* the specified unit.

*  CPT files also support the **+u**\ \|\ **U**\ *unit* mechanism.  Here, the scaling
   applies to the z values.  By appending these modifiers to your CPT filenames you
   can avoid having two CPT files (one for meter and one for km) since only one is needed.

*  Programs that read grids can now directly handle Arc/Info float binary
   files (GRIDFLOAT) and ESRI .hdr formats.

*  Programs that read grids now set boundary conditions to aid further
   processing. If a subset then the boundary conditions are taken from
   the surrounding grid values.

*  All text can now optionally be filled with patterns and/or drawn with
   outline pens. In the past, only :doc:`pstext` could plot outline fonts via
   **-S**\ *pen*. Now, any text can be an outline text by manipulating
   the corresponding FONT defaults (e.g., :ref:`FONT_TITLE <FONT_TITLE>`).

*  All color or fill specifications may append @\ *transparency* to
   change the PDF transparency level for that item. See **-t** for
   limitations on how to visualize this transparency.

*  GMT now ships with 36 standard color palette tables (CPT), up from 24.

Program-specific improvements
-----------------------------

Finally, here is a list of numerous enhancements to individual programs:

*  :doc:`blockmean` added **-Ep** for error propagation and
   **-Sn** to report the number of data points per block.

*  :doc:`blockmedian` added **-Er**\ [-]
   to return as last column the record number that gave the median
   value. For ties, we return the record number of the higher data value
   unless **-Er**- is given (return lower). Added **-Es** to read and
   output source id for median value.

*  :doc:`blockmode` added **-Er**\ [-] but
   for modal value. Added **-Es** to read and output source id for modal
   value.

*  :doc:`gmtconvert` now has optional PCRE (regular expression) support,
   as well as a new option to select a subset of segments specified by
   segment running numbers (**-Q**) and improved options to extract a
   subset of records (**-E**) and support for a list of search strings
   via **-S+f**\ *patternfile*.

*  :doc:`gmtinfo` has new option **-A** to
   select what group to report on (all input, per file, or per segment).
   Also, use **-If** to report an extended region optimized for fastest results in FFTs.
   and **-Is** to report an extended region optimized for fastest results in :doc:`surface`.
   Finally, new option **-D**\ [*inc*] to align regions found via **-I** with the center
   of the data.

*  :doc:`gmtmath` with **-N**\ *ncol* and input
   files will add extra blank columns, if needed.  A new option **-E**
   sets the minimum eigenvalue used by operators LSQFIT and SVDFIT.
   Option **-L** applies operators on a per-segment basis instead of
   accumulating operations across the entire file.  Many new
   operators have been added (BITAND, BITLEFT, BITNOT, BITOR, BITRIGHT,
   BITTEST, BITXOR, DIFF, IFELSE, ISFINITE, SVDFIT, TAPER).  Finally,
   we have implemented user macros for long or commonly used expressions,
   as well as ability to store and recall using named variables.

*  :doc:`gmtselect` Takes **-E** to indicate if points exactly on a polygon
   boundary are inside or outside, and **-Z** can now be extended to apply
   to other columns than the third.

*  :doc:`grd2cpt` takes **-F** to specify output color model and **-G** to
   truncate incoming CPT to be limited to a given range.

*  :doc:`grd2xyz` takes **-C** to write row, col instead of x,y.  Append **f**
   to start at 1, else start at 0.  Alternatively, use **-Ci** to write just
   the two columns *index* and *z*, where *index*
   is the 1-D indexing that GMT uses when referring to grid nodes.

*  :doc:`grdblend` can now take list of grids on
   the command line and blend, and now has more blend choices (see **-C**). Grids no
   longer have to have same registration or spacing.

*  :doc:`grdclip` has new option **-Si** to set all data >= low and <= high
   to the *between* value, and **-Sr** to set all data == old to the *new* value.

*  :doc:`grdcontour` can specify a single contour with **-C+**\ *contour* and
   **-A+**\ *contour*.

*  :doc:`grdcut` can use **-S** to specify an origin and radius and cut the
   corresponding rectangular area, and **-N** to extend the region if the new
   **-R** domain exceeds existing boundaries.

*  :doc:`grdfft` can now accept two grids and let **-E** compute the cross-spectra.
   The **-N** option allows for many new and special settings, including ability
   to control data mirroring, detrending, tapering, and output of intermediate
   results.

*  :doc:`grdfilter` can now do spherical
   filtering (with wrap around longitudes and over poles) for non-global
   grids. We have also begun implementing Open MP threads to speed up
   calculations on multi-core machines. We have added rectangular
   filtering and automatic resampling to input resolution for high-pass
   filters. There is also **-Ff**\ *weightgrd* which reads the gridfile
   *weightgrd* for a custom Cartesian grid convolution. The *weightgrd*
   must have odd dimensions. Similarly added **-Fo**\ *opgrd* for
   operators (via coefficients in the grdfile *opgrd*) whose weight sum
   is zero (hence we do not sum and divide the convolution by the weight
   sum).

*  :doc:`grdgradient` now has **-Em** that gives results close to ESRI's
   "hillshade"'" (but faster).

*  :doc:`grdinfo` now has modifier
   **-Ts**\ *dz* which returns a symmetrical range about zero.  Also,
   if **-Ib** is given then the grid's bounding box polygon is written.

*  :doc:`grdimage` with GDAL support can write a raster image directly to
   a raster file (**-A**) and may plot raster images as well (**-Dr**).
   It also automatically assigns a color table if none is given and can use
   any of the 36 GMT color tables and scale them to fit the grid range.

*  :doc:`grdmask` has new option
   **-Ni**\ \|\ I\ \|\ p\ \|\ P to set inside of
   polygons to the polygon IDs. These may come from OGR aspatial values,
   segment head **-L**\ ID, or a running number, starting at a specified
   origin [0]. Now correctly handles polygons with perimeters and holes.
   Added z as possible radius value in **-S** which means read radii
   from 3rd input column.

*  :doc:`grdmath` added many new operators such as BITAND, BITLEFT, BITNOT, BITOR, BITRIGHT,
   BITTEST, BITXOR, DEG2KM, IFELSE, ISFINITE, KM2DEG, and TAPER.  Finally,
   we have implemented user macros for long or commonly used expressions,
   as well as ability to store and recall using named variables.

*  :doc:`grdtrack` has many new options.  The **-A** option controls how the
   input track is resampled when **-C** is selected, the new **-C**, **-D**
   options automatically create an equidistant set of cross-sectional
   profiles given input line segments; one or more grids can then be
   sampled at these locations.  The **-E** option allows users to quickly specify
   tracks for sampling without needed input tracks.  Also added **-S** which stack
   cross-profiles generated with **-C**. The **-N** will not skip
   points that are outside the grid domain but return NaN as sampled
   value.  Finally, **-T** will return the nearest non-NaN node if the initial
   location only finds a NaN value.

*  :doc:`grdvector` can now take **-Si**\ *scale* to give the reciprocal scale,
   i.e., cm/ unit or km/unit.  Also, the vector heads in GMT have completely been overhauled
   and includes geo-vector heads that follow great or small circles.

*  :doc:`grdview` will automatically assigns a color table if none is given and can use
   any of the 36 GMT color tables and scale them to fit the grid range.

*  :doc:`grdvolume` can let **-S** accept more distance units than just km. It also
   has a modified **-T**\ [**c**\ \|\ **h**] for ORS estimates based on max
   curvature or height. **-Cr** to compute the *outside* volume between two contours
   (for instances, the volume of water from a bathymetry grid).

*  :doc:`greenspline` has an improved **-C** option to control how many eigenvalues are used
   in the fitting, and **--Sl** adds a linear (or bilinear) spline.

*  :doc:`makecpt` has a new **-F** option to
   specify output color representation, e.g., to output the CPT table in
   h-s-v format despite originally being given in r/g/b, and **-G** to
   truncate incoming CPT to be limited to a given range.  It also adds **Di**
   to match the bottom/top values in the input cpt file.

*  :doc:`mapproject` has a new **-N**
   option to do geodetic/geocentric conversions; it combines with **-I**
   for inverse conversions. Also, we have extended **-A** to accept
   **-A**\ **o**\ \| \ **O** to compute line orientations (-90/90).
   In **-G**, prepend - to the unit for (fast) flat Earth or + for (slow) geodesic calculations.

*  :doc:`project` has added **-G**...[+] so
   if + is appended we get a segment header with information about the
   pole for the circle. Optionally, append /*colat* in **-G** for a small circle path.

*  :doc:`ps2raster` has added a **-TF** option to create multi-page PDF files. There is
   also modification to **-A** to add user-specified margins, and it automatically detects
   if transparent elements have been included (and a detour via PDF might be needed).

*  :doc:`psbasemap` has added a **-D** option to place a map-insert box.

*  :doc:`psclip` has added an extended **-C** option to close different types of clip paths.

*  :doc:`pscoast` has added a new option **-F** which lets users specify one or more countries
   to paint, fill, extract, or use as plot domain (requires DCW to be installed).

*  :doc:`pscontour` is now similar to :doc:`grdcontour` in the options it
   takes, e.g., **-C** in particular. In GMT 4, the program could only
   read a CPT file and not take a specific contour interval.

*  :doc:`pshistogram` now takes **-D** to place histogram count labels on top of each bar
   and **-N** to draw the equivalent normal distributions.

*  :doc:`pslegend` no longer uses system calls to do the plotting.  The modified **-D**
   allows for minor offsets, while **-F** offers more control over the frame and fill.

*  :doc:`psrose` has added **-Wv**\ *pen* to
   specify pen for vector (specified in **-C**). Added **-Zu** to set all radii to
   unity (i.e., for analysis of angles only).

*  :doc:`psscale` has a new option **-T** that paints a rectangle behind the color bar.
   The **+n** modifier to **-E** draws a rectangle with NaN color and adds a label.
   The **-G** option will truncate incoming CPT to be limited to the specified z-range.
   Modification **-Np** indicates a preference to use polygons to draw the color bar.

*  :doc:`pstext` can take simplified input
   via new option **-F** to set fixed font (including size), angle, and
   justification. If these parameters are fixed for all the text strings
   then the input can simply be *x y text*.  It also has enhanced **-DJ** option
   to shorten diagonal offsets by :math:`\sqrt{2}` to maintain the same
   radial distance from point to annotation. Change all text to upper or
   lower case with **-Q**.

*  :doc:`psxy` and :doc:`psxyz` both support the revised custom symbol macro
   language which has been expanded considerably to allow for complicated,
   multi-parameter symbols; see Appendix `M. Custom Plot Symbols`_
   for details. Finally, we allow the base for bars and columns optionally to be
   read from data file by not specifying the base value.

*  :doc:`sample1d` offers **-A** to control resampling of spatial curves (with **-I**).

*  :doc:`spectrum1d` has added **-L** to control removal of trend, mean value or mid value.

*  :doc:`surface` has added **-r** to create pixel-registered grids and knows about
   periodicity in longitude (given **-fg**).  There is also **-D** to supply a "sort" break line.

*  :doc:`triangulate` now offers **-S**
   to write triangle polygons and can handle 2-column input if **-Z** is given.
   Can also write triangle edges as line with **-M**.

*  :doc:`xyz2grd` now also offers **-Af** (first value encountered),
   **-Am** (mean, the default), **-Ar** (rms), and **-As** (last value encountered).

Several supplements have new features as well:

*  :doc:`img2grd <supplements/img/img2grd>`
   used to be a shell script but is now a C program and can be used on all platforms.

*  :doc:`mgd77convert <supplements/mgd77/mgd77convert>`
   added **-C** option to assemble \*.mgd77 files from \*.h77/\*.a77 pairs.

*  The spotter programs can now read GPlates rotation files directly as well
   as write this format. Also,
   :doc:`rotconverter <supplements/spotter/rotconverter>` can extract plate
   circuit rotations on-the-fly from the GPlates rotation file.

Note: GMT 5 only produces PostScript and no longer has a setting for
Encapsulated PostScript (EPS). We made this decision since (a) our EPS determination
was always very approximate (no consideration of font metrics, etc.) and quite often wrong,
and (b) :doc:`ps2raster` handles it exactly.  Hence, users who need EPS plots should
simply process their PostScript files via :doc:`ps2raster`.

Incompatibilities between GMT 5 and GMT 4
-----------------------------------------

As features are added and bugs are discovered, it is occasionally
necessary to break the established syntax of a GMT program option, such as when
the intent of the option is non-unique due to a modifier key being the
same as a distance unit indicator. Other times we see a greatly improved
commonality across similar options by making minor adjustments. However,
we are aware that such changes may cause grief and trouble with
established scripts and the habits of many GMT users. To alleviate this
situation we have introduced a configuration that allows GMT to tolerate and process
obsolete program syntax (to the extent possible). To activate you must
make sure :ref:`GMT_COMPATIBILITY <GMT_COMPATIBILITY>` is set to 4 in your gmt.conf file.
When not running in compatibility mode any obsolete syntax will be considered as
errors. We recommend that users with prior GMT 4 experience run
GMT 5 in compatibility mode, heed the warnings about obsolete syntax, and
correct their scripts or habits accordingly. When this transition has been
successfully navigated it is better to turn compatibility mode off and leave
the past behind. Occasionally, users will supply an ancient
GMT 3 syntax which may have worked in GMT 4 but is not honored in GMT 5.

Here are a list of known incompatibilities that are correctly processed
with a warning under compatibility mode:

*  GMT **default names**: We have
   organized the default parameters logically by group and renamed several to
   be easier to remember and to group. Old and new names can be found in
   Table :ref:`obsolete <tbl-obsoletedefs>`.
   In addition, a few defaults are no longer recognized,
   such as N_COPIES, PS_COPIES, DOTS_PR_INCH, GMT_CPTDIR, PS_DPI, and PS_EPS,
   TRANSPARENCY. This also means the old common option **-c** for specifying
   PostScript copies is no longer available.

*  **Units**: The unit abbreviation for arc seconds is finally **s**
   instead of **c**, with the same change for upper case in some clock
   format statements.

*  **Contour labels**: The modifiers **+k**\ *fontcolor* and
   **+s**\ *fontsize* are obsolete, now being part of **+f**\ *font*.

*  **Ellipsoids**: Assigning :ref:`PROJ_ELLIPSOID <PROJ_ELLIPSOID>` a file name is
   deprecated, use comma-separated parameters :math:`a, f^{-1}` instead.

*  **Custom symbol macros:** Circle macro symbol **C** is deprecated; use **c** instead.

*  **Map scale**: Used by :doc:`psbasemap`
   and others. Here, the unit **m** is deprecated; use **M** for statute miles.

*  **3-D perspective**: Some programs used a combination of **-E**,
   **-Z** to set up a 3-D perspective view, but these options were not
   universal. The new 3-D perspective in
   GMT 5 means you instead use the
   common option **-p** to configure the 3-D projection.

*  **Pixel vs. gridline registration:** Some programs used to have a
   local **-F** to turn on pixel registration; now this is a common
   option **-r**.

*  **Table file headers**: For consistency with other common i/o options
   we now use **-h** instead of **-H**.

*  **Segment headers**: These are now automatically detected and hence
   there is no longer a **-m** (or the older **-M** option).

*  **Front symbol**: The syntax for the front symbol has changed from
   **-Sf**\ *spacing/size*\ [**+d**][**+t**][:\ *offset*] to
   **-Sf**\ *spacing*\ [/*size*][**+r+l**][**+f+t+s+c+b**][\ **+o**\ *offset*].

*  **Vector symbol**: With the introduction of geo-vectors there are
   three kinds of vectors that can be drawn: Cartesian (straight)
   vectors with **-Sv** or **-SV**, geo-vectors (great circles) with
   **-S=**, and circular vectors with **-Sm**. These are all composed of
   a line (controlled by pen settings) and 0--2 arrow heads (control by
   fill and outline settings). Many modifiers common to all arrows have
   been introduced using the **+key**\ [*arg*] format. The *size* of a
   vector refers to the length of its head; all other quantities are
   given via modifiers (which have sensible default values). In
   particular, giving size as *vectorwidth/headlength/headwidth* is
   deprecated. See the :doc:`psxy` man page for
   a clear description of all modifiers.

*  :doc:`blockmean`: The **-S** and **-Sz**
   options are deprecated; use **-Ss** instead.

*  :doc:`filter1d`: The **-N**\ *ncol/tcol*
   option is deprecated; use **-N**\ *tcol* instead as we automatically
   determine the number of columns in the file.

*  :doc:`gmtconvert`: **-F** is
   deprecated; use common option **-o** instead.

*  :doc:`gmtdefaults`: **-L** is
   deprecated; this is now the default behavior.

*  :doc:`gmtmath`: **-F** is deprecated; use
   common option **-o** instead.

*  :doc:`gmtselect`: **-Cf** is deprecated;
   use common specification format **-C-** instead. Also,
   **-N**...\ **o** is deprecated; use **-E** instead.

*  :doc:`grd2xyz`: **-E** is deprecated as
   the ESRI ASCII exchange format is now detected automatically.

*  :doc:`grdcontour`: **-m** is deprecated
   as segment headers are handled automatically.

*  :doc:`grdfft`: **-M** is deprecated; use
   common option **-fg** instead.

*  :doc:`grdgradient`: **-L** is
   deprecated; use common option **-n** instead. Also, **-M** is
   deprecated; use common option **-fg** instead.

*  :doc:`grdlandmask`: **-N**...\ **o**
   is deprecated; use **-E** instead.

*  :doc:`grdimage`: **-S** is deprecated;
   use **-n**\ *mode*\ [**+a**][\ **+t**\ *threshold*] instead.

*  :doc:`grdmath`: LDIST and PDIST now return
   distances in spherical degrees; while in
   GMT 4 it returned km; use
   DEG2KM for conversion, if needed.

*  :doc:`grdproject`: **-S** is
   deprecated; use **-n**\ *mode*\ [**+a**\ ][\ **+t**\ *threshold*]
   instead. Also, **-N** is deprecated; use **-D** instead.

*  :doc:`grdsample`: **-Q** is deprecated;
   use **-n**\ *mode*\ [**+a**][\ **+t**\ *threshold*] instead. Also,
   **-L** is deprecated; use common option **-n** instead, and
   **-N**\ *nx>/<ny* is deprecated; use **-I**\ *nx+>/<ny+* instead.

*  :doc:`grdtrack`: **-Q** is deprecated;
   use **-n**\ *mode*\ [**+a**][\ **+t**\ *threshold*] instead. Also,
   **-L** is deprecated; use common option **-n** instead, and **-S** is
   deprecated; use common option **-sa** instead.

*  :doc:`grdvector`: **-E** is deprecated;
   use the vector modifier **+jc** as well as the general vector
   specifications discussed earlier.

*  :doc:`grdview`: **-L** is deprecated; use common option **-n** instead.

*  :doc:`nearneighbor`: **-L** is
   deprecated; use common option **-n** instead.

*  :doc:`project`: **-D** is deprecated; use **-**\ **-**\ :ref:`FORMAT_GEO_OUT <FORMAT_GEO_OUT>` instead.

*  :doc:`psbasemap`: **-G** is deprecated;
   specify canvas color via **-B** modifier **+g**\ *color*.

*  :doc:`pscoast`: **-m** is deprecated and
   have reverted to **-M** for selecting data output instead of plotting.

*  :doc:`pscontour`: **-T**\ *indexfile* is deprecated; use **-Q**\ *indexfile*.

*  :doc:`pshistogram`: **-T**\ *col* is
   deprecated; use common option **-i** instead.

*  :doc:`pslegend`: Paragraph text header flag > is deprecated; use P instead.

*  :doc:`psmask`: **-D**...\ **+n**\ *min* is deprecated; use **-Q** instead.

*  :doc:`psrose`: Old vector specifications in
   Option **-M** are deprecated; see new explanations.

*  :doc:`pstext`: **-m** is deprecated; use
   **-M** to indicate paragraph mode. Also, **-S** is deprecated as
   fonts attributes are now specified via the font itself.

*  :doc:`pswiggle`: **-D** is deprecated;
   use common option **-g** to indicate data gaps. Also, **-N** is
   deprecated as all fills are set via the **-G** option.

*  :doc:`psxy`: Old vector specifications in
   Option **-S** are deprecated; see new explanations.

*  :doc:`psxyz`: Old vector specifications in
   Option **-S** are deprecated; see new explanations.

*  :doc:`splitxyz`: **-G** is deprecated;
   use common option **-g** to indicate data gaps. Also, **-M** is
   deprecated; use common option **-fg** instead.

*  :doc:`triangulate`: **-m** is
   deprecated; use **-M** to output triangle vertices.

*  :doc:`xyz2grd`: **-E** is deprecated as
   the ESRI ASCII exchange format is one of our recognized formats.
   Also, **-A** (no arguments) is deprecated; use **-Az** instead.

*  :doc:`grdraster`: Now in the main GMT core.  The
   **H**\ *skip* field in ``grdraster.info`` is no longer expected as we automatically
   determine if a raster has a GMT header. Also, to output
   *x,y,z* triplets instead of writing a grid now requires **-T**.

*  :doc:`img2grd <supplements/img/img2grd>`: **-m**\ *inc* is
   deprecated; use **-I**\ *inc* instead.

*  :doc:`psvelo <supplements/meca/psvelo>`: Old vector
   specifications are deprecated; see new explanations.

*  :doc:`mgd77convert <supplements/mgd77/mgd77convert>`:
   **-4** is deprecated; use **-D** instead.

*  :doc:`mgd77list <supplements/mgd77/mgd77list>`: The unit
   **m** is deprecated; use **M** for statute miles.

*  :doc:`mgd77manage <supplements/mgd77/mgd77manage>`: The
   unit **m** is deprecated; use **M** for statute miles. The **-Q** is
   deprecated; use **-n**\ *mode*\ [**+a**][\ **+t**\ *threshold*] instead

*  :doc:`mgd77path <supplements/mgd77/mgd77path>`: **-P** is
   deprecated (clashes with
   GMT common options); use **-A** instead.

*  :doc:`backtracker <supplements/spotter/backtracker>`:
   **-C** is deprecated as stage vs. finite rotations are detected
   automatically.

*  :doc:`grdrotater <supplements/spotter/grdrotater>`:
   **-C** is deprecated as stage vs. finite rotations are detected
   automatically. Also, **-T**\ *lon/lat/angle* is now set via
   **-e**\ *lon/lat/angle*.

*  :doc:`grdspotter <supplements/spotter/grdspotter>`:
   **-C** is deprecated as stage vs. finite rotations are detected
   automatically.

*  :doc:`hotspotter <supplements/spotter/hotspotter>`: **-C**
   is deprecated as stage vs. finite rotations are detected
   automatically.

*  :doc:`originator <supplements/spotter/originator>`:
   **-C** is deprecated as stage vs. finite rotations are detected
   automatically.

*  :doc:`rotconverter <supplements/spotter/rotconverter>`:
   **-Ff** selection is deprecated, use **-Ft** instead.

*  :doc:`x2sys_datalist <supplements/x2sys/x2sys_datalist>`:
   The unit **m** is deprecated; use **M** for statute miles.

.. _tbl-obsoletedefs:

+---------------------------------+-----------------------------------------+
| **Old Name**                    | **New Name**                            |
+=================================+=========================================+
| **ANNOT_FONT_PRIMARY**          | **FONT_ANNOT_PRIMARY**                  |
+---------------------------------+-----------------------------------------+
| **ANNOT_FONT_SECONDARY**        | **FONT_ANNOT_SECONDARY**                |
+---------------------------------+-----------------------------------------+
| **ANNOT_FONT_SIZE_PRIMARY**     | **FONT_ANNOT_PRIMARY**                  |
+---------------------------------+-----------------------------------------+
| **ANNOT_FONT_SIZE_SECONDARY**   | **FONT_ANNOT_SECONDARY**                |
+---------------------------------+-----------------------------------------+
| **ANNOT_MIN_ANGLE**             | **MAP_ANNOT_MIN_SPACING**               |
+---------------------------------+-----------------------------------------+
| **ANNOT_OFFSET_PRIMARY**        | **MAP_ANNOT_OFFSET_PRIMARY**            |
+---------------------------------+-----------------------------------------+
| **ANNOT_OFFSET_SECONDARY**      | **MAP_ANNOT_OFFSET_SECONDARY**          |
+---------------------------------+-----------------------------------------+
| **BASEMAP_AXES**                | **MAP_FRAME_AXES**                      |
+---------------------------------+-----------------------------------------+
| **BASEMAP_FRAME_RGB**           | **MAP_DEFAULT_PEN**                     |
+---------------------------------+-----------------------------------------+
| **BASEMAP_TYPE**                | **MAP_FRAME_TYPE**                      |
+---------------------------------+-----------------------------------------+
| **CHAR_ENCODING**               | **PS_CHAR_ENCODING**                    |
+---------------------------------+-----------------------------------------+
| **D_FORMAT**                    | **FORMAT_FLOAT_OUT**                    |
+---------------------------------+-----------------------------------------+
| **DEGREE_SYMBOL**               | **MAP_DEGREE_SYMBOL**                   |
+---------------------------------+-----------------------------------------+
| **ELLIPSOID**                   | **PROJ_ELLIPSOID**                      |
+---------------------------------+-----------------------------------------+
| **FIELD_DELIMITER**             | **IO_COL_SEPARATOR**                    |
+---------------------------------+-----------------------------------------+
| **FRAME_PEN**                   | **MAP_FRAME_PEN**                       |
+---------------------------------+-----------------------------------------+
| **FRAME_WIDTH**                 | **MAP_FRAME_WIDTH**                     |
+---------------------------------+-----------------------------------------+
| **GLOBAL_Y_SCALE**              | **PS_SCALE_X**                          |
+---------------------------------+-----------------------------------------+
| **GLOBAL\_X_SCALE**             | **PS_SCALE_X**                          |
+---------------------------------+-----------------------------------------+
| **GRID_CROSS_SIZE_PRIMARY**     | **MAP_GRID_CROSS_SIZE_PRIMARY**         |
+---------------------------------+-----------------------------------------+
| **GRID_CROSS_SIZE_SECONDARY**   | **MAP_GRID_CROSS_SIZE_SECONDARY**       |
+---------------------------------+-----------------------------------------+
| **GRID_PEN_PRIMARY**            | **MAP_GRID_PEN_PRIMARY**                |
+---------------------------------+-----------------------------------------+
| **GRID_PEN_SECONDARY**          | **MAP_GRID_PEN_SECONDARY**              |
+---------------------------------+-----------------------------------------+
| **GRIDFILE_FORMAT**             | **IO_GRIDFILE_FORMAT**                  |
+---------------------------------+-----------------------------------------+
| **GRIDFILE_SHORTHAND**          | **IO_GRIDFILE_SHORTHAND**               |
+---------------------------------+-----------------------------------------+
| **HEADER_FONT_SIZE**            | **FONT_TITLE**                          |
+---------------------------------+-----------------------------------------+
| **HEADER_FONT**                 | **FONT_TITLE**                          |
+---------------------------------+-----------------------------------------+
| **HEADER_OFFSET**               | **MAP_TITLE_OFFSET**                    |
+---------------------------------+-----------------------------------------+
| **HISTORY**                     | **GMT_HISTORY**                         |
+---------------------------------+-----------------------------------------+
| **HSV_MAX_SATURATION**          | **COLOR_HSV_MAX_S**                     |
+---------------------------------+-----------------------------------------+
| **HSV_MAX_VALUE**               | **COLOR_HSV_MAX_V**                     |
+---------------------------------+-----------------------------------------+
| **HSV_MIN_SATURATION**          | **COLOR_HSV_MIN_S**                     |
+---------------------------------+-----------------------------------------+
| **HSV_MIN_VALUE**               | **COLOR_HSV_MIN_V**                     |
+---------------------------------+-----------------------------------------+
| **INPUT_CLOCK_FORMAT**          | **FORMAT_CLOCK_IN**                     |
+---------------------------------+-----------------------------------------+
| **INPUT_DATE_FORMAT**           | **FORMAT_DATE_IN**                      |
+---------------------------------+-----------------------------------------+
| **INTERPOLANT**                 | **GMT_INTERPOLANT**                     |
+---------------------------------+-----------------------------------------+
| **INTERPOLANT**                 | **GMT_INTERPOLANT**                     |
+---------------------------------+-----------------------------------------+
| **INTERPOLANT**                 | **GMT_INTERPOLANT**                     |
+---------------------------------+-----------------------------------------+
| **LABEL_FONT**                  | **FONT_LABEL**                          |
+---------------------------------+-----------------------------------------+
| **LABEL_OFFSET**                | **MAP_LABEL_OFFSET**                    |
+---------------------------------+-----------------------------------------+
| **LINE_STEP**                   | **MAP_LINE_STEP**                       |
+---------------------------------+-----------------------------------------+
| **MAP_SCALE_FACTOR**            | **PROJ_SCALE_FACTOR**                   |
+---------------------------------+-----------------------------------------+
| **MEASURE_UNIT**                | **PROJ_LENGTH_UNIT**                    |
+---------------------------------+-----------------------------------------+
| **NAN_RECORDS**                 | **IO_NAN_RECORDS**                      |
+---------------------------------+-----------------------------------------+
| **OBLIQUE_ANNOTATION**          | **MAP_ANNOT_OBLIQUE**                   |
+---------------------------------+-----------------------------------------+
| **OUTPUT_CLOCK_FORMAT**         | **FORMAT_CLOCK_OUT**                    |
+---------------------------------+-----------------------------------------+
| **OUTPUT_CLOCK_FORMAT**         | **FORMAT_CLOCK_OUT**                    |
+---------------------------------+-----------------------------------------+
| **OUTPUT_DATE_FORMAT**          | **FORMAT_DATE_OUT**                     |
+---------------------------------+-----------------------------------------+
| **OUTPUT_DEGREE_FORMAT**        | **FORMAT_GEO_OUT**                      |
+---------------------------------+-----------------------------------------+
| **PAGE_COLOR**                  | **PS_PAGE_COLOR**                       |
+---------------------------------+-----------------------------------------+
| **PAGE_ORIENTATION**            | **PS_PAGE_ORIENTATION**                 |
+---------------------------------+-----------------------------------------+
| **PAPER_MEDIA**                 | **PS_MEDIA**                            |
+---------------------------------+-----------------------------------------+
| **PLOT_CLOCK_FORMAT**           | **FORMAT_CLOCK_MAP**                    |
+---------------------------------+-----------------------------------------+
| **PLOT_DATE_FORMAT**            | **FORMAT_DATE_MAP**                     |
+---------------------------------+-----------------------------------------+
| **PLOT_DEGREE_FORMAT**          | **FORMAT_GEO_MAP**                      |
+---------------------------------+-----------------------------------------+
| **POLAR_CAP**                   | **MAP_POLAR_CAP**                       |
+---------------------------------+-----------------------------------------+
| **PS_COLOR**                    | **COLOR_HSV_MAX_V**                     |
+---------------------------------+-----------------------------------------+
| **TICK_LENGTH**                 | **MAP_TICK_LENGTH_PRIMARY\|SECONDARY**  |
+---------------------------------+-----------------------------------------+
| **TICK_PEN**                    | **MAP_TICK_PEN_PRIMARY\|SECONDARY**     |
+---------------------------------+-----------------------------------------+
| **TIME_FORMAT_PRIMARY**         | **FORMAT_TIME_PRIMARY_MAP**             |
+---------------------------------+-----------------------------------------+
| **TIME_FORMAT_SECONDARY**       | **FORMAT_TIME_SECONDARY_MAP**           |
+---------------------------------+-----------------------------------------+
| **UNIX_TIME_FORMAT**            | **FORMAT_TIME_STAMP**                   |
+---------------------------------+-----------------------------------------+
| **UNIX_TIME_POS**               | **MAP_LOGO_POS**                        |
+---------------------------------+-----------------------------------------+
| **UNIX_TIME**                   | **MAP_LOGO**                            |
+---------------------------------+-----------------------------------------+
| **VECTOR_SHAPE**                | **MAP_VECTOR_SHAPE**                    |
+---------------------------------+-----------------------------------------+
| **VERBOSE**                     | **GMT_VERBOSE**                         |
+---------------------------------+-----------------------------------------+
| **WANT_LEAP_SECONDS**           | **TIME_LEAP_SECONDS**                   |
+---------------------------------+-----------------------------------------+
| **X_ORIGIN**                    | **MAP_ORIGIN_X**                        |
+---------------------------------+-----------------------------------------+
| **XY_TOGGLE**                   | **IO_LONLAT_TOGGLE**                    |
+---------------------------------+-----------------------------------------+
| **Y_AXIS_TYPE**                 | **MAP_ANNOT_ORTHO**                     |
+---------------------------------+-----------------------------------------+
| **Y_ORIGIN**                    | **MAP_ORIGIN_Y**                        |
+---------------------------------+-----------------------------------------+
| **Y2K_OFFSET_YEAR**             | **TIME_Y2K_OFFSET_YEAR**                |
+---------------------------------+-----------------------------------------+

Note: While **TIME_LEAP_SECONDS** is a recognized keyword it is
currently not implemented and has no effect.  We reserve the right
to enable this feature in the future.


Switching between different GMT versions
========================================

We encourage all GMT users to start using version 5 immediately; it
has been tested extensively by the GMT team and has benefitted from
bug reports for the 4.5.x versions. Users who still worry about the new
version breaking things may install GMT 4.5.x and 5 side by side.

Because GMT 5 is backwards compatible with the 4.5.x series
(provided you configured it that way) yet maintains its parameters and
history in separate files (e.g. ``.gmtdefaults4``, versus ``gmt.conf``)
it is possible to install and use both versions on the same workstation.
Switching between different GMT versions can be accomplished in several
ways, two of which will be addressed here:

#. By using the :doc:`gmtswitch` utility to select the current working version.
   **Pro:** easy, interactive way to switch versions on the command line; works with
   previous GMT syntax. **Con:** editing of shell startup files required; needs write
   access in ``$HOME``-directory; manual intervention necessary if symlink
   ``$HOME/this_gmt`` is broken.

#. By using the recommended ``gmt <module>``-syntax in conjunction with a shell
   helper function that points to the desired GMT executable. **Pro:** no need to
   create symlinks and edit shell startup files; scripts are more portable.
   **Con:** different syntax required.

Setup of gmtswitch
------------------

Run :doc:`gmtswitch` after you have finished installing all
GMT versions of interest. The first time you run :doc:`gmtswitch` it
will try to find all the available versions installed on your file
system. The versions found will be listed in the file ``.gmtversions`` in your home
directory; each line is the full path to a GMT root directory (e.g.,
/usr/local/GMT4.5.9). You may manually add or remove entries there at
any time. You are then instructed to make two changes to your
environment (the details are shell-dependent but explained by :doc:`gmtswitch`):

#. :doc:`gmtswitch` creates and maintains a symbolic link ``this_gmt`` in your home
   directory that will point to a directory with one of the installed
   GMT versions.

#. Make sure ``$HOME/this_gmt/bin`` is in your executable PATH.

Make those edits, logout, and login again. The next time you
run :doc:`gmtswitch` you will be able to switch between versions. Typing
:doc:`gmtswitch` with no argument will list the available versions in a
numerical menu and prompt you to choose one, whereas :doc:`gmtswitch`
*version* will immediately switch to that version (*version* must be a
piece of unique text making up the full path to a version, e.g., 4.5.9).
If you use **bash**, **tcsh**, or **csh** you may have to type ``hash -r`` or
``rehash`` to initiate the path changes.

On Windows, the process is somewhat similar. The GMT bin directory has one batch file
``gmtswitch.bat`` that works by changing the Windows PATH variable so that the BIN
directory of the preferred version always comes first. To do that the batch works in two
alternative modes.

#. Permanent mode

#. Temporary mode

The permanent mode makes use of the free executable program `EditPath
<http://www.softpedia.com/get/Tweak/Registry-Tweak/EditPath.shtml>`_
to change the user path in the registry. It's called permanent because the changes
remains until ... next change.
Off course the editpath.exe binary must be in your system's path as well.
WARNING: The path change will not be visible on the shell cmd where it was executed.
For the change to be active you will need to open a new cmd window.

The second mode is temporary because the path to the selected GMT binary dir is
prepended to previous path via a shell command line. This modification disappears
when the shell cmd window where it was executes is deleted.

For further details the user should read the entire help section at the header of the
``gmtswitch.bat``.

The ``gmtswitch.bat`` solution, however, has the drawback that the batch file must be located
elsewhere and in a directory that is on the user's PATH, otherwise it wont be located after
first use unless the other GMT bin directory has a similar batch file. A better solution is to install the
`Windows console enhancement <http://sourceforge.net/projects/console>`_
that includes multiple tabs and configure the different tabs to start the different GMT versions.
All it takes is in the Tab setting to call a batch that modifies the PATH locally. That PATH
modifying batch will have a single line with something like:

   ::

    set path=C:\programs\gmt5\bin;%PATH%


Version selection with helper function
--------------------------------------

A shell function can be used as a wrapper around the gmt executable. This even
works when a gmt application is in the search PATH as it would shadow the real
command. This method can easily be applied on the command line or in scripts
when the recommended ``gmt <module>``-syntax is used. Shell scripts using
old-style GMT commands would have to be converted first. The syntax conversion
can be accomplished with the :doc:`gmt5syntax` utility. A suitable bash wrapper
function for GMT 5 would look like this:

.. code-block:: bash

   function gmt() { /path/to/gmt5/bin/gmt "$@"; }
   export -f gmt

Exporting the function is necessary to make it available to subshells and
scripts. This wrapper function can be either set in your working shell or
inside a GMT shell script. The latter is useful to switch to a certain GMT
version on a per-script basis.

For GMT releases prior to GMT 5 which only provide the module commands,
we need a slightly modified version of the wrapper script:

.. code-block:: bash

   function gmt() { module=$1; shift; /path/to/gmt4/bin/${module} "$@"; }
   export -f gmt

On the command line this might be too much typing to switch between
versions. So we might as well put everything together in a script file
``gmtfun``:

.. code-block:: bash

   case $1 in
     4)
     function gmt() {
       module=$1; shift; /path/to/gmt4/bin/${module} "$@"
     }
     ;;
     5)
     function gmt() {
       /path/to/gmt5/bin/gmt "$@"
     }
     ;;
     *)
     return
     ;;
   esac
   export -f gmt

Source the file with either ``. gmtfun 4`` or ``. gmtfun 5`` to switch
between versions.


Introduction
============

Most scientists are familiar with the sequence: *raw data* →
*processing* → *final illustration*.
In order to finalize papers for submission to scientific journals,
prepare proposals, and create overheads and slides for various
presentations, many scientists spend large amounts of time and money to
create camera-ready figures. This process can be tedious and is often
done manually, since available commercial or in-house software usually
can do only part of the job. To expedite this process we introduce the
Generic Mapping Tools (GMT for short), which is a free [2]_, software
package that can be used to manipulate columns of tabular data,
time-series, and gridded data sets, and display these data in a variety
of forms ranging from simple *x*--*y* plots to maps and color,
perspective, and shaded-relief illustrations. GMT uses the
PostScript page description language [*Adobe Systems Inc.*, 1990].
With PostScript, multiple plot files can easily be superimposed to
create arbitrarily complex images in gray tones or 24-bit true color.
Line drawings, bitmapped images, and text can be easily combined in one
illustration. PostScript plot files are device-independent: The same
file can be printed at 300 dots per inch (dpi) on an ordinary
laserwriter or at 2470 dpi on a phototypesetter when ultimate quality is
needed. GMT software is written as a set of UNIX tools [3]_ and is
totally self-contained and fully documented. The system is offered free
of charge and is distributed over the computer network (Internet)
[*Wessel and Smith, 1991; 1995; 1998*].

The original version 1.0 of GMT was released in the summer of 1988
when the authors were graduate students at Lamont-Doherty Earth
Observatory of Columbia University. During our tenure as graduate
students, L-DEO changed its computing environment to a distributed
network of UNIX workstations, and we wrote GMT to run in this
environment. It became a success at L-DEO, and soon spread to numerous
other institutions in the US, Canada, Europe, and Japan. The current
version benefits from the many suggestions contributed by users of the
earlier versions, and now includes more than 50 tools, more than 30
projections, and many other new, more flexible features. GMT provides
scientists with a variety of tools for data manipulation and display,
including routines to sample, filter, compute spectral estimates, and
determine trends in time series, grid or triangulate arbitrarily spaced
data, perform mathematical operations (including filtering) on 2-D data
sets both in the space and frequency domain, sample surfaces along
arbitrary tracks or onto a new grid, calculate volumes, and find trend
surfaces. The plotting programs will let the user make linear,
log\ :math:`_{10}`, and :math:`x^a - y^b` diagrams, polar
and rectangular histograms, maps with filled continents and coastlines
choosing from many common map projections, contour plots, mesh plots,
monochrome or color images, and artificially illuminated shaded-relief
and 3-D perspective illustrations.

GMT is written in the highly portable ANSI C programming language
[*Kernighan and Ritchie*, 1988], is fully POSIX compliant [*Lewine*,
1991], has no Year 2000 problems, and may be used with any hardware
running some flavor of UNIX, possibly with minor modifications. In
writing GMT, we have followed the modular design philosophy of UNIX:
The *raw data* → *processing* → *final illustration* flow is broken
down to a series of elementary steps; each
step is accomplished by a separate GMT or UNIX tool. This modular
approach brings several benefits: (1) only a few programs are needed,
(2) each program is small and easy to update and maintain, (3) each step
is independent of the previous step and the data type and can therefore
be used in a variety of applications, and (4) the programs can be
chained together in shell scripts or with pipes, thereby creating a
process tailored to do a user-specific task. The decoupling of the data
retrieval step from the subsequent massage and plotting is particularly
important, since each institution will typically have its own data base
formats. To use GMT with custom data bases, one has only to write a
data extraction tool which will put out data in a form readable by
GMT (discussed below). After writing the extractor, all other
GMT modules will work as they are.

GMT makes full use of the PostScript page description language, and
can produce color illustrations if a color PostScript device is
available. One does not necessarily have to have access to a
top-of-the-line color printer to take advantage of the color
capabilities offered by GMT: Several companies offer imaging services
where the customer provides a PostScript plot file and gets color
slides or hardcopies in return. Furthermore, general-purpose
PostScript raster image processors (RIPs) are now becoming available,
letting the user create raster images from PostScript and plot these
bitmaps on raster devices like computer screens, dot-matrix printers,
large format raster plotters, and film writers [4]_. Because the
publication costs of color illustrations are high, GMT offers 90
common bit and hachure patterns, including many geologic map symbol
types, as well as complete graytone shading operations. Additional bit
and hachure patterns may also be designed by the user. With these tools,
it is possible to generate publication-ready monochrome originals on a
common laserwriter.

GMT is thoroughly documented and comes with a technical reference and
cookbook which explains the purpose of the package and its many
features, and provides numerous examples to help new users quickly
become familiar with the operation and philosophy of the system. The
cookbook contains the shell scripts that were used for each example;
PostScript\ files of each illustration are also provided. All programs
have individual manual pages which can be installed as part of the
on-line documentation under the UNIX **man** utility or as web
pages. In addition, the programs offer friendly help messages which make
them essentially self-teaching -- if a user enters invalid or ambiguous
command arguments, the program will print a warning to the screen with a
synopsis of the valid arguments. All the documentation is available for
web browsing and may be installed at the user's site.

The processing and display routines within GMT are completely general
and will handle any (*x,y*) or (*x,y,z*) data as input. For many
purposes the (*x,y*) coordinates will be (longitude, latitude) but in
most cases they could equally well be any other variables (e.g.,
wavelength, power spectral density). Since the GMT\ plot tools will
map these (*x,y*) coordinates to positions on a plot or map using a
variety of transformations (linear, log-log, and several map
projections), they can be used with any data that are given by two or
three coordinates. In order to simplify and standardize input and
output, GMT uses two file formats only. Arbitrary sequences of (*x,y*)
or (*x,y,z*) data are read from multi-column ASCII tables, i.e., each
file consists of several records, in which each coordinate is confined
to a separate column [5]_. This format is straightforward and allows the
user to perform almost any simple (or complicated) reformatting or
processing task using standard UNIX utilities such as **cut**,
**paste**, **grep**, **sed** and **awk**. Two-dimensional data
that have been sampled on an equidistant grid are read and written by
GMT in a binary grid file using the functions provided with the netCDF
library (a free, public-domain software library available separately
from UCAR, the University Corporation of Atmospheric Research [*Treinish
and Gough*, 1987]). This XDR (External Data Representation) based format
is architecture independent, which allows the user to transfer the
binary data files from one computer system to another [6]_.
GMT contains programs that will read ASCII (*x,y,z*) files and produce
grid files. One such program, :doc:`surface`,
includes new modifications to the gridding algorithm developed by *Smith
and Wessel* [1990] using continuous splines in tension.

Most of the programs will produce some form of output, which falls into
four categories. Several of the programs may produce more than one of
these types of output:

*  1-D ASCII Tables — For example, a (*x,y*) series may be
   filtered and the filtered values output. ASCII output is written to
   the standard output stream.

*  2-D binary (netCDF or user-defined) grid files -- Programs that grid
   ASCII (*x,y,z*) data or operate on existing grid files produce
   this type of output.

*  PostScript -- The plotting programs all use the PostScript\ page
   description language to define plots. These commands are stored as
   ASCII text and can be edited should you want to customize the plot
   beyond the options available in the programs themselves.

*  Reports -- Several GMT programs read input files and report
   statistics and other information. Nearly all programs have an
   optional "verbose" operation, which reports on the progress of
   computation. All programs feature usage messages, which prompt the
   user if incorrect commands have been given. Such text is written to
   the standard error stream and can therefore be separated from ASCII
   table output.

GMT is available over the Internet at no charge. To obtain a copy,
goto GMT home page http://gmt.soest.hawaii.edu/ and follow instructions.
We also maintain two electronic
mailing lists you may subscribe to in order to stay informed about bug
fixes and upgrades.

For those without net-access that need to obtain GMT: Geoware makes
and distributes CD-R and DVD-R media with the GMT package, compatible
supplements, and several Gb of useful Earth and ocean science data sets.
For more information send e-mail to geoware@geoware-online.com.

GMT has served a multitude of scientists very well, and their
responses have prompted us to develop these programs even further. It is
our hope that the new version will satisfy these users and attract new
users as well. We present this system to the community in order to
promote sharing of research software among investigators in the US and abroad.

References
----------

*  Adobe Systems Inc., *PostScript Language Reference Manual*, 2nd
   edition, 764, Addison-Wesley, Reading, Massachusetts, 1990.

*  Kernighan, B. W., and D. M. Ritchie, *The C programming language*,
   2nd edition, 272, Prentice-Hall, Englewood Cliffs, New Jersey, 1988.

*  Lewine, D., POSIX programmer's guide, 1st edition, 607, O'Reilly &
   Associates, Sebastopol, California, 1991.

*  Treinish, L. A., and M. L. Gough, A software package for the
   data-independent management of multidimensional data, *EOS Trans.
   AGU*, 68(28), 633--635, 1987. `doi:10.1029/EO068i028p00633 <http://dx.doi.org/10.1029/EO068i028p00633>`_.

GMT Overview and Quick Reference
================================

GMT summary
-----------

The following is a summary of all the programs supplied with GMT and
a very short description of their purpose. For more details, see the
individual UNIX manual pages or the online web documentation. For a
listing sorted by program purpose, see Section `GMT quick reference`_.

+-----------------------+---------------------------------------------------------------------+
|                       |                                                                     |
+=======================+=====================================================================+
| :doc:`blockmean`      | L\ :math:`_2` (*x*,\ *y*,\ *z*) table data filter/decimator         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`blockmedian`    | L\ :math:`_1` (*x*,\ *y*,\ *z*) table data filter/decimator         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`blockmode`      | Mode estimate (*x*,\ *y*,\ *z*) table data filter/decimator         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`filter1d`       | Filter 1-D table data sets (time series)                            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`fitcircle`      | Finds the best-fitting great or small circle for a set of points    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmt2kml`        | Like :doc:`psxy` but plots KML for use in Google Earth              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtconnect`     | Connect segments into more complete lines or polygons               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtconvert`     | Convert data tables from one format to another                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtdefaults`    | List the current default settings                                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtget`         | Retrieve selected parameters in current ``gmt.conf`` file           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtinfo`        | Get information about table data files                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtmath`        | Mathematical operations on table data                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtselect`      | Select subsets of table data based on multiple spatial criteria     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtset`         | Change selected parameters in current ``gmt.conf`` file             |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtsimplify`    | Line reduction using the Douglas-Peucker algorithm                  |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtspatial`     | Geospatial operations on lines and polygons                         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtvector`      | Basic operations on vectors in 2-D and 3-D                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtwhich`       | Find full path to specified data files                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grd2cpt`        | Make color palette table from a grid files                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grd2rgb`        | Convert Sun raster or grid file to red, green, blue component grids |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grd2xyz`        | Conversion from 2-D grid file to table data                         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdblend`       | Blend several partially over-lapping grid files onto one grid       |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdclip`        | Limit the *z*-range in gridded data sets                            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdcontour`     | Contouring of 2-D gridded data sets                                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdcut`         | Cut a sub-region from a grid file                                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdedit`        | Modify header information in a 2-D grid file                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdfft`         | Perform operations on grid files in the frequency domain            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdfilter`      | Filter 2-D gridded data sets in the space domain                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdgradient`    | Compute directional gradient from grid files                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdhisteq`      | Histogram equalization for grid files                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdimage`       | Produce images from 2-D gridded data sets                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdinfo`        | Get information about grid files                                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdlandmask`    | Create masking grid files from shoreline data base                  |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdmask`        | Reset grid nodes in/outside a clip path to constants                |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdmath`        | Mathematical operations on grid files                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdpaste`       | Paste together grid files along a common edge                       |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdproject`     | Project gridded data sets onto a new coordinate system              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdreformat`    | Converts grid files into other grid formats                         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdsample`      | Resample a 2-D gridded data set onto a new grid                     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdtrack`       | Sampling of 2-D gridded data set(s) along 1-D track                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdtrend`       | Fits polynomial trends to grid files                                |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdvector`      | Plotting of 2-D gridded vector fields                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdview`        | 3-D perspective imaging of 2-D gridded data sets                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdvolume`      | Calculate volumes under a surface within specified contour          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`greenspline`    | Interpolation with Green's functions for splines in 1--3 D          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`kml2gmt`        | Extracts coordinates from Google Earth KML files                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`makecpt`        | Make color palette tables                                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`mapproject`     | Transformation of coordinate systems for table data                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`nearneighbor`   | Nearest-neighbor gridding scheme                                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`project`        | Project table data onto lines or great circles                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`ps2raster`      | Crop and convert PostScript files to raster images, EPS, and PDF    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psbasemap`      | Create a basemap plot                                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psclip`         | Use polygon files to define clipping paths                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pscoast`        | Plot (and fill) coastlines, borders, and rivers on maps             |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pscontour`      | Contour or image raw table data by triangulation                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pshistogram`    | Plot a histogram                                                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psimage`        | Plot Sun raster files on a map                                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pslegend`       | Plot a legend on a map                                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psmask`         | Create overlay to mask out regions on maps                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psrose`         | Plot sector or rose diagrams                                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psscale`        | Plot gray scale or color scale on maps                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pstext`         | Plot text strings on maps                                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pswiggle`       | Draw table data time-series along track on maps                     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psxy`           | Plot symbols, polygons, and lines on maps                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psxyz`          | Plot symbols, polygons, and lines in 3-D                            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sample1d`       | Resampling of 1-D table data sets                                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`spectrum1d`     | Compute various spectral estimates from time-series                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sph2grd`        | Compute grid from spherical harmonic coefficients                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sphdistance`    | Make grid of distances to nearest points on a sphere                |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sphinterpolate` | Spherical gridding in tension of data on a sphere                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sphtriangulate` | Delaunay or Voronoi construction of spherical lon,lat data          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`splitxyz`       | Split *xyz* files into several segments                             |
+-----------------------+---------------------------------------------------------------------+
| :doc:`surface`        | A continuous curvature gridding algorithm                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`trend1d`        | Fits polynomial or Fourier trends to :math:`y = f(x)` series        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`trend2d`        | Fits polynomial trends to :math:`z = f(x,y)` series                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`triangulate`    | Perform optimal Delauney triangulation and gridding                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`xyz2grd`        | Convert an equidistant table *xyz* file to a 2-D grid file          |
+-----------------------+---------------------------------------------------------------------+

GMT quick reference
-------------------

Instead of an alphabetical listing, this section
contains a summary sorted by program purpose. Also included is a quick
summary of the standard command line options and a breakdown of the
**-J** option for each of the over 30 projections available in GMT.

+-----------------------+---------------------------------------------------------------------+
|                       | *Filtering of 1-D and 2-D Data*                                     |
+=======================+=====================================================================+
| :doc:`blockmean`      | L\ :math:`_2` (*x*,\ *y*,\ *z*) table data filter/decimator         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`blockmedian`    | L\ :math:`_1` (*x*,\ *y*,\ *z*) table data filter/decimator         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`blockmode`      | Mode estimate (*x*,\ *y*,\ *z*) table data filter/decimator         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`filter1d`       | Filter 1-D table data sets (time series)                            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdfilter`      | Filter 2-D gridded data sets in the space domain                    |
+-----------------------+---------------------------------------------------------------------+
|                       | *Plotting of 1-D and 2-D Data*                                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdcontour`     | Contouring of 2-D gridded data sets                                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdimage`       | Produce images from 2-D gridded data sets                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdvector`      | Plotting of 2-D gridded vector fields                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdview`        | 3-D perspective imaging of 2-D gridded data sets                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psbasemap`      | Create a basemap plot                                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psclip`         | Use polygon files to define clipping paths                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pscoast`        | Plot (and fill) coastlines, borders, and rivers on maps             |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pscontour`      | Contour or image raw table data by triangulation                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pshistogram`    | Plot a histogram                                                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psimage`        | Plot Sun raster files on a map                                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pslegend`       | Plot a legend on a map                                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psmask`         | Create overlay to mask out regions on maps                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psrose`         | Plot sector or rose diagrams                                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psscale`        | Plot gray scale or color scale on maps                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pstext`         | Plot text strings on maps                                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`pswiggle`       | Draw table data time-series along track on maps                     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psxy`           | Plot symbols, polygons, and lines on maps                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`psxyz`          | Plot symbols, polygons, and lines in 3-D                            |
+-----------------------+---------------------------------------------------------------------+
|                       | *Gridding of Data Tables*                                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`greenspline`    | Interpolation with Green's functions for splines in 1--3 D          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`nearneighbor`   | Nearest-neighbor gridding scheme                                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sphinterpolate` | Spherical gridding in tension of data on a sphere                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`surface`        | A continuous curvature gridding algorithm                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`triangulate`    | Perform optimal Delauney triangulation and gridding                 |
+-----------------------+---------------------------------------------------------------------+
|                       | *Sampling of 1-D and 2-D Data*                                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtsimplify`    | Line reduction using the Douglas-Peucker algorithm                  |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdsample`      | Resample a 2-D gridded data set onto a new grid                     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdtrack`       | Sampling of 2-D gridded data set(s) along 1-D track                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sample1d`       | Resampling of 1-D table data sets                                   |
+-----------------------+---------------------------------------------------------------------+
|                       | *Projection and Map-transformation*                                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdproject`     | Project gridded data sets onto a new coordinate system              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`mapproject`     | Transformation of coordinate systems for table data                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`project`        | Project table data onto lines or great circles                      |
+-----------------------+---------------------------------------------------------------------+
|                       | *Retrieve Information*                                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtdefaults`    | List the current default settings                                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtget`         | Retrieve selected parameters in current file                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtinfo`        | Get information about table data files                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtset`         | Change selected parameters in current file                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdinfo`        | Get information about grid files                                    |
+-----------------------+---------------------------------------------------------------------+
|                       | *Mathematical Operations on Tables or Grids*                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtmath`        | Mathematical operations on table data                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`makecpt`        | Make color palette tables                                           |
+-----------------------+---------------------------------------------------------------------+
| :doc:`spectrum1d`     | Compute various spectral estimates from time-series                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sph2grd`        | Compute grid from spherical harmonic coefficients                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sphdistance`    | Make grid of distances to nearest points on a sphere                |
+-----------------------+---------------------------------------------------------------------+
| :doc:`sphtriangulate` | Delaunay or Voronoi construction of spherical lon,lat data          |
+-----------------------+---------------------------------------------------------------------+
|                       | *Convert or Extract Subsets of Data*                                |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtconnect`     | Connect segments into more complete lines or polygons               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtconvert`     | Convert data tables from one format to another                      |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtselect`      | Select subsets of table data based on multiple spatial criteria     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtspatial`     | Geospatial operations on lines and polygons                         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmtvector`      | Basic operations on vectors in 2-D and 3-D                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grd2rgb`        | Convert Sun raster or grid file to red, green, blue component grids |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grd2xyz`        | Conversion from 2-D grid file to table data                         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdblend`       | Blend several partially over-lapping grid files onto one grid       |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdcut`         | Cut a sub-region from a grid file                                   |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdpaste`       | Paste together grid files along a common edge                       |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdreformat`    | Converts grid files into other grid formats                         |
+-----------------------+---------------------------------------------------------------------+
| :doc:`splitxyz`       | Split *xyz* files into several segments                             |
+-----------------------+---------------------------------------------------------------------+
| :doc:`xyz2grd`        | Convert an equidistant table *xyz* file to a 2-D grid file          |
+-----------------------+---------------------------------------------------------------------+
|                       | *Determine Trends in 1-D and 2-D Data*                              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`fitcircle`      | Finds the best-fitting great or small circle for a set of points    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdtrack`       | Sampling of 2-D gridded data set(s) along 1-D track                 |
+-----------------------+---------------------------------------------------------------------+
| :doc:`trend1d`        | Fits polynomial or Fourier trends to :math:`y = f(x)` series        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`trend2d`        | Fits polynomial trends to :math:`z = f(x,y)` series                 |
+-----------------------+---------------------------------------------------------------------+
|                       | *Other Operations on 2-D Grids*                                     |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grd2cpt`        | Make color palette table from a grid files                          |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdclip`        | Limit the *z*-range in gridded data sets                            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdedit`        | Modify header information in a 2-D grid file                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdfft`         | Perform operations on grid files in the frequency domain            |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdgradient`    | Compute directional gradient from grid files                        |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdhisteq`      | Histogram equalization for grid files                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdlandmask`    | Create masking grid files from shoreline data base                  |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdmask`        | Reset grid nodes in/outside a clip path to constants                |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdmath`        | Mathematical operations on grid files                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`grdvolume`      | Calculate volumes under a surface within specified contour          |
+-----------------------+---------------------------------------------------------------------+
|                       | *Miscellaneous Tools*                                               |
+-----------------------+---------------------------------------------------------------------+
| :doc:`gmt2kml`        | Like :doc:`psxy` but plots KML for use in Google Earth              |
+-----------------------+---------------------------------------------------------------------+
| :doc:`kml2gmt`        | Extracts coordinates from Google Earth KML files                    |
+-----------------------+---------------------------------------------------------------------+
| :doc:`ps2raster`      | Crop and convert PostScript files to raster images, EPS, and PDF    |
+-----------------------+---------------------------------------------------------------------+


GMT offers 31 map projections. These are specified using the **-J**
common option. There are two conventions you may use: (a) GMT-style
syntax and (b) **Proj4**\ -style syntax. The projection codes for the
GMT-style are tabulated below.

.. Substitution definitions:
.. |lon0| replace:: lon\ :sub:`0`
.. |lat0| replace:: lat\ :sub:`0`
.. |lon1| replace:: lon\ :sub:`1`
.. |lat1| replace:: lat\ :sub:`1`
.. |lat2| replace:: lat\ :sub:`2`
.. |lonp| replace:: lon\ :sub:`p`
.. |latp| replace:: lat\ :sub:`p`

+----------------------------------------------------------+--------------------------------------------------------+
|         **WITH GMT PROJECTION CODES**                                                                             |
+==========================================================+========================================================+
| **-J** (upper case for *width*, lower case for *scale*) Map projection                                            |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JA**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *width*         | Lambert azimuthal equal area                           |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JB**\ |lon0|/|lat0|/|lat1|/|lat2|\ *width*            | Albers conic equal area                                |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JC**\ |lon0|/|lat0|\ *width*                          | Cassini cylindrical                                    |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JCyl_stere/**\ [|lon0|\ [/|lat0|/]]\ *width*          | Cylindrical stereographic                              |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JD**\ |lon0|/|lat0|/|lat1|/|lat2|\ *width*            | Equidistant conic                                      |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JE**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *width*         | Azimuthal equidistant                                  |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JF**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *width*         | Azimuthal gnomonic                                     |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JG**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *width*         | Azimuthal orthographic                                 |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JG**\ |lon0|/|lat0|\                                  |                                                        |
| *alt*/*azim*/*tilt*/*twist*/*W*/*H/width*                | General perspective                                    |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JH**\ |lon0|\ *width*                                 | Hammer equal area                                      |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JI**\ |lon0|\ *width*                                 | Sinusoidal equal area                                  |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JJ**\ |lon0|\ *width*                                 | Miller cylindrical                                     |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JKf**\ |lon0|\ *width*                                | Eckert IV equal area                                   |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JKs**\ |lon0|\ *width*                                | Eckert VI equal area                                   |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JL**\ |lon0|/|lat0|/|lat1|/|lat2|\ *width*            | Lambert conic conformal                                |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JM**\ [|lon0|\ [/|lat0|/]]\ *width*                   | Mercator cylindrical                                   |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JN**\ [|lon0|/]\ *width*                              | Robinson                                               |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JOa**\ |lon0|/|lat0|\ *azim*/*width*                  | Oblique Mercator, 1: origin and azimuth                |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JOb**\ |lon0|/|lat0|/|lon1|/|lat1|\ *width*           | Oblique Mercator, 2: two points                        |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JOc**\ |lon0|/|lat0|/|lonp|/|latp|\ *width*           | Oblique Mercator, 3: origin and pole                   |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JP**\ [**a**]\ *width*\ [/*origin*]                   | Polar [azimuthal] (:math:`\theta, r`) (or cylindrical) |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JPoly**\ [|lon0|\ [/|lat0|/]]\ *width*                | (American) polyconic                                   |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JQ**\ [|lon0|\ [/|lat0|/]]\ *width*                   | Equidistant cylindrical                                |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JR**\ [|lon0|/]\ *width*                              | Winkel Tripel                                          |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JS**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *width*         | General stereographic                                  |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JT**\ [|lon0|\ [/|lat0|/]]\ *width*                   | Transverse Mercator                                    |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JU**\ *zone*/*width*                                  | Universal Transverse Mercator (UTM)                    |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JV**\ [|lon0|/]\ *width*                              | Van der Grinten                                        |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JW**\ [|lon0|/]\ *width*                              | Mollweide                                              |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JX**\ *width*\ [**l**\ \|\ **p**\ *exp*\ \|\          |                                                        |
| **T**\ \|\ **t**][/\ *height*\ [**l**\ \|\ **p**\        |                                                        |
| *exp*\ \|\ **T**\ \|\ **t**]][**d**]                     | Linear, log\ :math:`_{10}`, :math:`x^a-y^b`, and time  |
+----------------------------------------------------------+--------------------------------------------------------+
| **-JY**\ |lon0|/|lat0|\ *width*                          | Cylindrical equal area                                 |
+----------------------------------------------------------+--------------------------------------------------------+

The projection codes for the **Proj4**-style are tabulated below;
these all accept a map *scale*.

+------------------------------------------------------------+-------------------------------------------------------+
|         **WITH Proj4 PROJECTION CODES**                                                                            |
+============================================================+=======================================================+
| **-J** (lower case for *scale* only) Map projection                                                                |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jaea/**\ |lon0|/|lat0|/|lat1|/|lat2|\ *scale*           | Albers conic equal area                               |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jaeqd/**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *scale*       | Azimuthal equidistant                                 |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jcass/**\ |lon0|/|lat0|\ *scale*                        | Cassini cylindrical                                   |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jcea/**\ |lon0|/|lat0|\ *scale*                         | Cylindrical equal area                                |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jcyl_stere/**\ [|lon0|\ [/|lat0|/]]\ *scale*            | Cylindrical stereographic                             |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jeqc/**\ [|lon0|\ [/|lat0|/]]\ *scale*                  | Equidistant cylindrical                               |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jeqdc/**\ |lon0|/|lat0|/|lat1|/|lat2|\ *scale*          | Equidistant conic                                     |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jgnom/**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *scale*       | Azimuthal gnomonic                                    |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jhammer/**\ |lon0|\ *scale*                             | Hammer equal area                                     |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jeck4/**\ |lon0|\ *scale*                               | Eckert IV equal area                                  |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jeck6/**\ |lon0|\ *scale*                               | Eckert VI equal area                                  |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jlaea/**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *scale*       | Lambert azimuthal equal area                          |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jlcc/**\ |lon0|/|lat0|/|lat1|/|lat2|\ *scale*           | Lambert conic conformal                               |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jmerc/**\ [|lon0|\ [/|lat0|/]]\ *scale*                 | Mercator cylindrical                                  |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jmill/**\ |lon0|\ *scale*                               | Miller cylindrical                                    |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jmoll/**\ [|lon0|/]\ *scale*                            | Mollweide                                             |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jnsper/**\ |lon0|/|lat0|\                               |                                                       |
| *alt/azim/tilt/twist/W/H/scale*                            | General perspective                                   |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jomerc/**\ |lon0|/|lat0|\ *azim*/*scale*                | Oblique Mercator, 1: origin and azimuth               |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jomerc/**\ |lon0|/|lat0|/|lon1|/|lat1|\ *scale*         | Oblique Mercator, 2: two points                       |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jomercp/**\ :|lon0|/|lat0|/|lonp|/|latp|\ *scale*       | Oblique Mercator, 3: origin and pole                  |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jortho/**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *scale*      | Azimuthal orthographic                                |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jpolar/**\ [**a**]\ *scale*\ [/*origin*]                | Polar [azimuthal] (:math:`\theta, r`) (or cylindrical)|
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jpoly/**\ [|lon0|\ [/|lat0|/]]\ *scale*                 | (American) polyconic                                  |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jrobin/**\ [|lon0|/]\ *scale*                           | Robinson                                              |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jsinu/**\ |lat0|\ *scale*                               | Sinusoidal equal area                                 |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jstere/**\ |lon0|/|lat0|\ [/\ *horizon*]/\ *scale*      | General stereographic                                 |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jtmerc/**\ [|lon0|\ [/|lat0|/]]\ *scale*                | Transverse Mercator                                   |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jutm/**\ *zone*/*scale*                                 | Universal Transverse Mercator (UTM)                   |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jvandg/**\ [|lon0|/]\ *scale*                           | Van der Grinten                                       |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jwintri/**\ [|lon0|/]\ *scale*                          | Winkel Tripel                                         |
+------------------------------------------------------------+-------------------------------------------------------+
| **-Jxy**\ *xscale*\ [**l**\ \|\ **p**\ *exp*\ \|\          |                                                       |
| **T**\ \|\ **t**][/\ *yscale*\ [**l**\ \|\ **p**\          |                                                       |
| *exp*\ \|\ **T**\ \|\ **t**]][**d**]                       | Linear, log\ :math:`_{10}`, :math:`x^a-y^b`, and time |
+------------------------------------------------------------+-------------------------------------------------------+

Finally, the rest of the GMT common options are given below:

+------------------------------------------------------------+--------------------------------------------------------+
|         **STANDARDIZED COMMAND LINE OPTIONS**                                                                       |
+============================================================+========================================================+
| **-B**\ *information*                                      | Specify map frame and axes parameters                  |
+------------------------------------------------------------+--------------------------------------------------------+
| **-K**                                                     | Append more PS later                                   |
+------------------------------------------------------------+--------------------------------------------------------+
| **-O**                                                     | This is an overlay plot                                |
+------------------------------------------------------------+--------------------------------------------------------+
| **-P**                                                     | Select Portrait orientation                            |
+------------------------------------------------------------+--------------------------------------------------------+
| **-R**\ *west/east/south/north*\ [*/zmin/zmax*][**r**]     | Specify Region of interest                             |
+------------------------------------------------------------+--------------------------------------------------------+
| **-U**\ [[*just*]/\ *dx*/*dy*/][*label*]                   | Plot time-stamp on plot                                |
+------------------------------------------------------------+--------------------------------------------------------+
| **-V**                                                     | Run in verbose mode                                    |
+------------------------------------------------------------+--------------------------------------------------------+
| **-X**\ [**a**\ \| \ **c**\ \| \ **r**]\ *off*\ [**u**]    | Shift plot origin in *x*-direction                     |
+------------------------------------------------------------+--------------------------------------------------------+
| **-Y**\ [**a**\ \| \ **c**\ \| \ **r**]\ *off*\ [**u**]    | Shift plot origin in *y*-direction                     |
+------------------------------------------------------------+--------------------------------------------------------+
| **-a**\ *name=col,...*                                     | Associates aspatial data with columns                  |
+------------------------------------------------------------+--------------------------------------------------------+
| **-b**\ [**i**\ \| \ **o**][*ncol*\ ][**t**]               | Select binary input or output                          |
+------------------------------------------------------------+--------------------------------------------------------+
| **-c**\ *copies*                                           | Set number of plot copies [1]                          |
+------------------------------------------------------------+--------------------------------------------------------+
| **-f**\ [**i**\ \| \ **o**]\ *colinfo*                     | Set formatting of ASCII input or output                |
+------------------------------------------------------------+--------------------------------------------------------+
| **-g**\ [**+**]\ **x**\ \| \ **X**\ \| \ **y**\ \| \       |                                                        |
| **Y**\ \| \ **d**\ \| \ **D**\ *gap*\ [**u**]              | Segment data by detecting gaps                         |
+------------------------------------------------------------+--------------------------------------------------------+
| **-h**\ [**i**\ \| \ **o**][*n\_headers*]                  | ASCII [input\ \| \ output] tables have header record[s]|
+------------------------------------------------------------+--------------------------------------------------------+
| **-i**\ *columns*                                          | Selection of input columns                             |
+------------------------------------------------------------+--------------------------------------------------------+
| **-o**\ *columns*                                          | Selection of output columns                            |
+------------------------------------------------------------+--------------------------------------------------------+
| **-n**\ [*type*][**+a**][\ **+b**\ *BC*]                   |                                                        |
| [\ **+c**\ ][\ **+t**\ *threshold*]                        | Set grid interpolation mode                            |
+------------------------------------------------------------+--------------------------------------------------------+
| **-p**\ *azim/elev*\ [/*zlevel*][\ **+w**\ *lon0/lat0*\    |                                                        |
| [/*z0*]][\ **+v**\ *x0/y0*]                                | Control 3-D perspective view                           |
+------------------------------------------------------------+--------------------------------------------------------+
| **-r**                                                     | Sets grid registration                                 |
+------------------------------------------------------------+--------------------------------------------------------+
| **-s**\ [**z**\ \| \ *cols*]                               | Control treatment of NaN records                       |
+------------------------------------------------------------+--------------------------------------------------------+
| **-t**\ *transparency*                                     | Set layer PDF transparency                             |
+------------------------------------------------------------+--------------------------------------------------------+
| **-:**\ [**i**\ \| \ **o**]                                | Expect *y*/*x* input rather than *x*/*y*               |
+------------------------------------------------------------+--------------------------------------------------------+

General Features
================

This section explains features common to all the programs in GMT and
summarizes the philosophy behind the system. Some of the features
described here may make more sense once you reach the cook-book section
where we present actual examples of their use.

GMT units
---------

While GMT has default units for both actual Earth distances and plot
lengths (dimensions) of maps, it is recommended that you specifically
indicate the units of your arguments by appending the unit character, as
discussed below. This will aid you in debugging, let others understand your
scripts, and remove any uncertainty as to what unit you thought you wanted.

Distance units
~~~~~~~~~~~~~~

.. _tbl-distunits:

+---------+-------------------+---------+------------------+
+=========+===================+=========+==================+
| **d**   | Degree of arc     | **M**   | Statute mile     |
+---------+-------------------+---------+------------------+
| **e**   | Meter [Default]   | **n**   | Nautical mile    |
+---------+-------------------+---------+------------------+
| **f**   | Foot              | **s**   | Second of arc    |
+---------+-------------------+---------+------------------+
| **k**   | Kilometer         | **u**   | US Survey foot   |
+---------+-------------------+---------+------------------+
| **m**   | Minute of arc     |         |                  |
+---------+-------------------+---------+------------------+

For Cartesian data and scaling the data units do not normally matter
(they could be kg or Lumens for all we know) and are never entered.
Geographic data are different as distances can be specified in a variety
of ways. GMT programs that accept actual Earth length scales like
search radii or distances can therefore handle a variety of units. These
choices are listed in Table :ref:`distunits <tbl-distunits>`; simply append the desired
unit to the distance value you supply. A value without a unit suffix
will be consider to be in meters. For example, a distance of 30 nautical
miles should be given as 30\ **n**.

Distance calculations
~~~~~~~~~~~~~~~~~~~~~

The calculation of distances on Earth (or other planetary bodies)
depends on the ellipsoidal parameters of the body (via
:ref:`PROJ_ELLIPSOID <PROJ_ELLIPSOID>`) and the method of computation. GMT offers three
alternatives that trade off accuracy and computation time.

Flat Earth distances
^^^^^^^^^^^^^^^^^^^^

Quick, but approximate "Flat Earth" calculations make a first-order
correction for the spherical nature of a planetary body by computing the
distance between two points A and B as

.. math::

	 d_f = R \sqrt{(\theta_A - \theta_B)^2 + \cos \left [ \frac{\theta_A +
	 \theta_B}{2} \right ] \Delta \lambda^2}, \label{eq:flatearth}

where *R* is the representative (or spherical) radius of the
planet, :math:`\theta` is latitude, and the difference in longitudes,
:math:`\Delta \lambda = \lambda_A - \lambda_B`, is adjusted for any
jumps that might occur across Greenwich or the Dateline. As written, the
geographic coordinates are given in radians. This approach is suitable
when the points you use to compute :math:`d_f` do not greatly differ in
latitude and computation speed is paramount. You can specify this mode
of computation by using the **-** prefix to the specified distance (or
to the unit itself in cases where no distance is required and only a
unit is expected). For instance, a search radius of 50 statute miles
using this mode of computation might be specified via **-S-**\ 50\ **M**.

Great circle distances
^^^^^^^^^^^^^^^^^^^^^^

This is the default distance calculation, which will also approximate
the planetary body by a sphere of mean radius *R*. However, we
compute an exact distance between two points A and B on such a sphere
via the Haversine equation

.. math::

	 d_g = 2R \sin^{-1}  {\sqrt{\sin^2\frac{\theta_A - \theta_B}{2} + \cos
	 \theta_A \cos \theta_B \sin^2 \frac{\lambda_A - \lambda_B}{2}} },
	 \label{eq:greatcircle}

This approach is suitable for most situations unless exact calculations
for an ellipsoid is required (typically for a limited surface area). For
instance, a search radius of 5000 feet using this mode of computation
would be specified as **-S**\ 5000\ **f**.

Note: There are two additional
GMT defaults that control how
great circle (and Flat Earth) distances are computed. One concerns the
selection of the "mean radius". This is selected by
:ref:`PROJ_MEAN_RADIUS <PROJ_MEAN_RADIUS>`, which selects one of several possible
representative radii. The second is :ref:`PROJ_AUX_LATITUDE <PROJ_AUX_LATITUDE>`, which
converts geodetic latitudes into one of several possible auxiliary
latitudes that are better suited for the spherical approximation. While
both settings have default values to best approximate geodesic distances
(*authalic* mean radius and latitudes), expert users can choose from a
range of options as detailed in the :doc:`gmt.conf` man page.

Geodesic distances
^^^^^^^^^^^^^^^^^^

For the most accurate calculations we use a full ellipsoidal
formulation. Currently, we are using Vincenty's [1975] formula [7]_. You
select this mode of computation by using the **+** prefix to the
specified distance (or to the unit itself in cases where no distance is
required). For instance, a search radius of 20 km using this mode of
computation would be set by **-S+**\ 20\ **k**.

Length units
~~~~~~~~~~~~

GMT programs can accept dimensional quantities and plot lengths in
**c**\ m, **i**\ nch, or **p**\ oint (1/72 of an inch) [8]_. There are
two ways to ensure that GMT understands which unit you intend to use:

#. Append the desired unit to the dimension you supply. This way is
   explicit and clearly communicates what you intend, e.g.,
   **-X**\ 4\ **c** means the length being passed to the **-X** switch
   is 4 cm.

#. Set the parameter :ref:`PROJ_LENGTH_UNIT <PROJ_LENGTH_UNIT>` to the desired unit. Then,
   all dimensions without explicit unit will be interpreted accordingly.

The latter method is less secure as other users may have a different
unit set and your script may not work as intended. We therefore
recommend you always supply the desired unit explicitly.

GMT defaults
------------

Overview and the gmt.conf file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are about 100 parameters which can be adjusted individually to
modify the appearance of plots or affect the manipulation of data. When
a program is run, it initializes all parameters to the
GMT\ defaults [9]_, then tries to open the file ``gmt.conf`` in the current
directory [10]_. If not found, it will look for that file in a
sub-directory ``/.gmt`` of your home directory, and finally in your home directory
itself. If successful, the program will read the contents and set the
default values to those provided in the file. By editing this file you
can affect features such as pen thicknesses used for maps, fonts and
font sizes used for annotations and labels, color of the pens,
dots-per-inch resolution of the hardcopy device, what type of spline
interpolant to use, and many other choices. A complete list of all the
parameters and their default values can be found in the
:doc:`gmt.conf` manual pages. Figures
:ref:`GMT Parameters a <gmt_defaults_a>`,
:ref:`b <gmt_defaults_b>`, and
:ref:`c <gmt_defaults_c>` show the parameters that affect
plots. You may create your own ``gmt.conf`` files by running
:doc:`gmtdefaults` and then modify those
parameters you want to change. If you want to use the parameter settings
in another file you can do so by specifying ``+<defaultfile>`` on the
command line. This makes it easy to maintain several distinct parameter
settings, corresponding perhaps to the unique styles required by
different journals or simply reflecting font changes necessary to make
readable overheads and slides. Note that any arguments given on the
command line (see below) will take precedent over the default values.
E.g., if your ``gmt.conf`` file has *x* offset = 1\ **i** as default, the
**-X**\ 1.5\ **i** option will override the default and set the offset to 1.5 inches.

.. _gmt_defaults_a:

.. figure:: /_images/GMT_Defaults_1a.*
   :width: 500 px
   :align: center

   Some GMT parameters that affect plot appearance.

.. _gmt_defaults_b:

.. figure:: /_images/GMT_Defaults_1b.*
   :width: 500 px
   :align: center

   More GMT parameters that affect plot appearance.

.. _gmt_defaults_c:

.. figure:: /_images/GMT_Defaults_1c.*
   :width: 500 px
   :align: center

   Even more GMT parameters that affect plot appearance.

There are at least two good reasons why the GMT default options are
placed in a separate parameter file:

#. It would not be practical to allow for command-line syntax covering
   so many options, many of which are rarely or never changed (such as
   the ellipsoid used for map projections).

#. It is convenient to keep separate ``gmt.conf`` files for specific projects, so
   that one may achieve a special effect simply by running
   GMT commands in the directory whose ``gmt.conf`` file has the desired settings.
   For example, when making final illustrations for a journal article
   one must often standardize on font sizes and font types, etc. Keeping
   all those settings in a separate ``gmt.conf`` file simplifies this process and
   will allow you to generate those illustrations with the same settings
   later on. Likewise, GMT scripts that make figures for PowerPoint
   presentations often use a different color scheme and font size than
   output intended for laser printers. Organizing these various
   scenarios into separate ``gmt.conf`` files will minimize headaches associated with
   micro-editing of illustrations.

Changing GMT defaults
~~~~~~~~~~~~~~~~~~~~~

As mentioned, GMT programs will attempt to open a file named  ``gmt.conf``. At
times it may be desirable to override that default. There are several
ways in which this can be accomplished.

*  One method is to start each script by saving a copy of the current  ``gmt.conf``,
   then copying the desired ``gmt.conf`` file to the current directory, and finally
   reverting the changes at the end of the script. Possible side effects
   include premature ending of the script due to user error or bugs
   which means the final resetting does not take place (unless you write
   your script very carefully.)

*  To permanently change some of the GMT parameters on the fly
   inside a script the :doc:`gmtset` utility
   can be used. E.g., to change the primary annotation font to 12 point
   Times-Bold in red we run

   ::

    gmt gmtset FONT_ANNOT_PRIMARY 12p,Times-Bold,red

   These changes will remain in effect until they are overridden.

*  If all you want to achieve is to change a few parameters during the
   execution of a single command but otherwise leave the environment
   intact, consider passing the parameter changes on the command line
   via the **-**\ **-**\ *PAR=value* mechanism. For instance, to temporarily
   set the output format for floating points to have lots of decimals,
   say, for map projection coordinate output, append
   **-**\ **-**\ :ref:`FORMAT_FLOAT_OUT <FORMAT_FLOAT_OUT>`\ =%.16lg to the command in question.

*  Finally, GMT provides to possibility to override the settings only
   during the running of a single script, reverting to the original
   settings after the script is run, as if the script was run in
   "isolation". The isolation mode is discussed in
   Section `Running GMT in isolation mode`_.

In addition to those parameters that directly affect the plot there are
numerous parameters than modify units, scales, etc. For a complete
listing, see the :doc:`gmt.conf` man pages.
We suggest that you go through all the available parameters at least
once so that you know what is available to change via one of the
described mechanisms.

Command line arguments
----------------------

Each program requires certain arguments specific to its operation. These
are explained in the manual pages and in the usage messages. Most
programs are "case-sensitive"; almost all options must start with an
upper-case letter. We have tried to choose letters of the alphabet which
stand for the argument so that they will be easy to remember. Each
argument specification begins with a hyphen (except input file names;
see below), followed by a letter, and sometimes a number or character
string immediately after the letter. *Do not* space between the hyphen,
letter, and number or string. *Do* space between options. Example:

   ::

    gmt pscoast -R0/20/0/20 -Ggray -JM6i -Wthin -B5 -V -P > map.ps

Standardized command line options
---------------------------------

Most of the programs take many of the same arguments like those related
to setting the data region, the map projection, etc. The 24 switches in
Table :ref:`switches <tbl-switches>` have the same meaning in all the programs (although
some programs may not use all of them). These options will be described
here as well as in the manual pages, as is vital that you understand how
to use these options. We will present these options in order of
importance (some are use a lot more than others).

.. _tbl-switches:

+----------+--------------------------------------------------------------------+
+==========+====================================================================+
| **-B**   | Define tickmarks, annotations, and labels for basemaps and axes    |
+----------+--------------------------------------------------------------------+
| **-J**   | Select a map projection or coordinate transformation               |
+----------+--------------------------------------------------------------------+
| **-K**   | Allow more plot code to be appended to this plot later             |
+----------+--------------------------------------------------------------------+
| **-O**   | Allow this plot code to be appended to an existing plot            |
+----------+--------------------------------------------------------------------+
| **-P**   | Select Portrait plot orientation [Default is landscape]            |
+----------+--------------------------------------------------------------------+
| **-R**   | Define the extent of the map/plot region                           |
+----------+--------------------------------------------------------------------+
| **-U**   | Plot a time-stamp, by default in the lower left corner of page     |
+----------+--------------------------------------------------------------------+
| **-V**   | Select verbose operation; reporting on progress                    |
+----------+--------------------------------------------------------------------+
| **-X**   | Set the *x*-coordinate for the plot origin on the page             |
+----------+--------------------------------------------------------------------+
| **-Y**   | Set the *y*-coordinate for the plot origin on the page             |
+----------+--------------------------------------------------------------------+
| **-a**   | Associate aspatial data from OGR/GMT files with data columns       |
+----------+--------------------------------------------------------------------+
| **-b**   | Select binary input and/or output                                  |
+----------+--------------------------------------------------------------------+
| **-c**   | Specify the number of plot copies                                  |
+----------+--------------------------------------------------------------------+
| **-f**   | Specify the data format on a per column basis                      |
+----------+--------------------------------------------------------------------+
| **-g**   | Identify data gaps based on supplied criteria                      |
+----------+--------------------------------------------------------------------+
| **-h**   | Specify that input/output tables have header record(s)             |
+----------+--------------------------------------------------------------------+
| **-i**   | Specify which input columns to read                                |
+----------+--------------------------------------------------------------------+
| **-n**   | Specify grid interpolation settings                                |
+----------+--------------------------------------------------------------------+
| **-o**   | Specify which output columns to write                              |
+----------+--------------------------------------------------------------------+
| **-p**   | Control perspective views for plots                                |
+----------+--------------------------------------------------------------------+
| **-r**   | Set the grid registration to pixel [Default is gridline]           |
+----------+--------------------------------------------------------------------+
| **-s**   | Control output of records containing one or more NaNs              |
+----------+--------------------------------------------------------------------+
| **-t**   | Change layer PDF transparency                                      |
+----------+--------------------------------------------------------------------+
| **-:**   | Assume input geographic data are (*lat,lon*) and not (*lon,lat*)   |
+----------+--------------------------------------------------------------------+

Data domain or map region: The **-R** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-R** option defines the map region or data domain of interest. It
may be specified in one of three ways, two of which are shown in Figure
:ref:`Map region <gmt_region>`:

#. **-R**\ *xmin*/*xmax*/*ymin*/*ymax*. This is the standard way to
   specify Cartesian data domains and geographical regions when using
   map projections where meridians and parallels are rectilinear.

#. **-R**\ *xlleft*/*ylleft*/*xuright*/*yuright*\ **r**. This form is
   used with map projections that are oblique, making meridians and
   parallels poor choices for map boundaries. Here, we instead specify
   the lower left corner and upper right corner geographic coordinates,
   followed by the suffix **r**. This form guarantees a rectangular map
   even though lines of equal longitude and latitude are not straight lines.

#. **-R**\ *gridfile*. This will copy the domain settings found for the
   grid in specified file. Note that depending on the nature of the
   calling program, this mechanism will also set grid spacing and
   possibly the grid registration (see
   Section `Grid registration: The -r option`_).

.. _gmt_region:

.. figure:: /_images/GMT_-R.*
   :width: 500 px
   :align: center

   The plot region can be specified in two different ways. (a) Extreme values
   for each dimension, or (b) coordinates of lower left and upper right corners.

For rectilinear projections the first two forms give identical results.
Depending on the selected map projection (or the kind of expected input
data), the boundary coordinates may take on several different formats:

Geographic coordinates:
    These are longitudes and latitudes and may be given in decimal
    degrees (e.g., -123.45417) or in the
    []*ddd*\ [:*mm*\ [:*ss*\ [*.xxx*]]][\ **W**\ \|\ **E**\ \|\ **S**\ \|\ **N**]
    format (e.g., 123:27:15W). Note that **-Rg** and **-Rd** are
    shorthands for "global domain" **-R**\ *0*/*360*/*-90*/*90* and
    **-R**\ *-180*/*180*/*-90*/*90*, respectively.

    When used in conjunction with the Cartesian Linear Transformation
    (**-Jx** or **-JX**) —which can be used to map floating point data,
    geographical coordinates, as well as time coordinates— it is prudent
    to indicate that you are using geographical coordinates in one of
    the following ways:

    -  Use **-Rg** or **-Rd** to indicate the global domain.

    -  Use **-Rg**\ *xmin*/*xmax*/*ymin*/*ymax* to indicate a limited
       geographic domain.

    -  Add **W**, **E**, **S**, or **N** to the coordinate limits or add
       the generic **D** or **G**. Example:
       **-R**\ *0*/*360G*/*-90*/*90N*.

    Alternatively, you may indicate geographical coordinates by
    supplying **-fg**; see Section `Data type selection: The -f option`_.

Projected coordinates:
    These are Cartesian projected coordinates compatible with the chosen
    projection and are given with a leading length *unit*, (e.g.,
    **k**-200/200/-300/300 for a 400 by 600 km rectangular area centered
    on the projection center (0, 0). These coordinates are internally
    converted to the corresponding geographic (longitude, latitude)
    coordinates for the lower left and upper right corners. This form is
    convenient when you want to specify a region directly in the
    projected units (e.g., UTM meters). For allowable units, see
    Table :ref:`distunits <tbl-distunits>`.

Calendar time coordinates:
    These are absolute time coordinates referring to a Gregorian or ISO
    calendar. The general format is [*date*]\ **T**\ [*clock*],
    where *date* must be in the *yyyy*\ [*-mm*\ [*-dd*]] (year, month,
    day-of-month) or *yyyy*\ [*-jjj*] (year and day-of-year) for
    Gregorian calendars and *yyyy*\ [*-*\ **W**\ *ww*\ [*-d*]] (year,
    week, and day-of-week) for the ISO calendar. If no *date* is given
    we assume the current day. The **T** flag is required if a *clock* is given.

    The optional *clock* string is a 24-hour clock in
    *hh*\ [*:mm*\ [*:ss*\ [*.xxx*]]] format. If no *clock* is given it
    implies 00:00:00, i.e., the start of the specified day. Note that
    not all of the specified entities need be present in the data. All
    calendar date-clock strings are internally represented as double
    precision seconds since proleptic Gregorian date Monday January 1
    00:00:00 0001. Proleptic means we assume that the modern calendar
    can be extrapolated forward and backward; a year zero is used, and
    Gregory's reforms [11]_ are extrapolated backward. Note that this is
    not historical.

Relative time coordinates:
    These are coordinates which count seconds, hours, days or years
    relative to a given epoch. A combination of the parameters
    :ref:`TIME_EPOCH <TIME_EPOCH>` and :ref:`TIME_UNIT <TIME_UNIT>` define the epoch and time unit.
    The parameter :ref:`TIME_SYSTEM <TIME_SYSTEM>` provides a few shorthands for common
    combinations of epoch and unit, like **j2000** for days since noon
    of 1 Jan 2000. The default relative time coordinate is that of UNIX
    computers: seconds since 1 Jan 1970. Denote relative time
    coordinates by appending the optional lower case **t** after the
    value. When it is otherwise apparent that the coordinate is relative
    time (for example by using the **-f** switch), the **t** can be omitted.

Other coordinates:
    These are simply any coordinates that are not related to geographic
    or calendar time or relative time and are expected to be simple
    floating point values such as
    []\ *xxx.xxx*\ [E: \| \ e\ \| \ D\ \| \ d[]xx], i.e.,
    regular or exponential notations, with the enhancement to understand
    FORTRAN double precision output which may use D instead of E for
    exponents. These values are simply converted as they are to internal
    representation. [12]_

Coordinate transformations and map projections: The **-J** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This option selects the coordinate transformation or map projection. The
general format is

-  **-J**\ :math:`\delta`\ [*parameters*/]\ *scale*. Here, :math:`\delta`
   is a *lower-case* letter of the alphabet that selects a particular
   map projection, the *parameters* is zero or more slash-delimited
   projection parameter, and *scale* is map scale given in distance
   units per degree or as 1:xxxxx.

-  **-J**\ :math:`\Delta`\ [*parameters*/]\ *width*. Here, :math:`\Delta`
   is an *upper-case* letter of the alphabet that selects a particular
   map projection, the *parameters* is zero or more slash-delimited
   projection parameter, and *width* is map width (map height is
   automatically computed from the implied map scale and region).

Since GMT version 4.3.0, there is an alternative way to specify the
projections: use the same abbreviation as in the mapping package
**Proj4**. The options thus either look like:

-  **-J**\ *abbrev*/[*parameters*/]\ *scale*. Here, **abbrev** is a
   *lower-case* abbreviation that selects a particular map projection,
   the *parameters* is zero or more slash-delimited projection
   parameter, and *scale* is map scale given in distance units per
   degree or as 1:xxxxx.

-  **-J**\ *Abbrev*/[*parameters*/]\ *width*. Here, **Abbrev** is an
   *capitalized* abbreviation that selects a particular map projection,
   the *parameters* is zero or more slash-delimited projection
   parameter, and *width* is map width (map height is automatically
   computed from the implied map scale and region).

The projections available in GMT are presented in Figure
:ref:`gmt_projections`. For details on all GMT projections and the required
parameters, see the :doc:`psbasemap` man page. We will also show examples of
every projection in the next Chapters, and a quick summary of projection
syntax was given in Chapter `GMT overview and quick reference`.

.. _gmt_projections:

.. figure:: /_images/GMT_-J.*
   :width: 500 px
   :align: center

   The 30+ map projections and coordinate transformations available in GMT


Map frame and axes annotations: The **-B** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is potentially the most complicated option in GMT,
but most examples of its usage are actually quite simple. We distinguish
between to sets of information: Frame settings and Axes parameters.  These
are set separately by their own **-B** invocations; hence multiple **-B**
specifications may be specified. The frame settings covers things such
as which axes should be plotted, canvas fill, plot title, and what type
of gridlines be drawn, whereas the Axes settings deal with annotation,
tick, and gridline intervals, axes labels, and annotation units.

The Frame settings are specified by

-  **-B**\ [*axes*][**+b**][**+g**\ *fill*][**+o**\ *lon/lat*][**+t**\ *title*]

Here, the optional *axes* dictates which of the axes should be drawn
and possibly annotated.  By default, all 4 map boundaries (or plot axes)
are plotted (denoted **W**, **E**, **S**, **N**). To change this selection,
append the codes for those you want (e.g., **WSn**). In this example,
the lower case **n** denotes to draw the axis and (major and minor) tick
marks on the "northern" (top) edge of the plot. The upper case **WS** will
annotate the "western" and "southern" axes with numerals and plot the
any axis labels in addition to draw axis/tick-marks.  For 3-D plots you can
also specify **Z** or **z**.  By default a single vertical axes will then be
plotted at the most suitable map corner.  You can override this by appending
any combination of corner ids **1234**, where **1** represents the lower left
corner and the order goes counter-clockwise.  Append **+b** to draw the outline
of the 3-D box defined by **-R**; this modifier is also needed to display
gridlines in the x--z, y--z planes.  You may paint the
map canvas by appending the **+g**\ *fill* modifier [Default is no fill].
If gridlines are specified via the Axes parameters (discussed below) then
by default these are referenced to the North pole.  If, however, you wish
to produce oblique gridlines about another pole you can append **+o**\ *lon/lat*
to change this behavior (the modifier is ignored if no gridlines are requested).
Finally, you may optionally add **+t**\ *title* to place a title that
will appear centered above the plot frame.

The Axes settings are specified by

-  **-B**\ [**p**\|\ **s**][**x**\|\ **x**\|\ **z**]\ *intervals*\ [\ **+l**\ *label*][**+p**\ *prefix*][**+u**\ *unit*]

but you may also split this into two separate invocations for clarity, i.e.,

-  **-B**\ [**p**\|\ **s**][**x**\|\ **y**\|\ **z**][**+l**\ *label*][**+p**\ *prefix*][**+u**\ *unit*]
-  **-B**\ [**p**\|\ **s**][**x**\|\ **y**\|\ **z**]\ *intervals*

The first optional flag following **-B** selects **p** (rimary) [Default] or
**s** (econdary) axes information (which is mostly used for time axes
annotations; see examples below). The next optional flags specifies which
axes you are providing information for.  This can be an individual axis
(e.g., just **x**) or a combination (e.g., **xz**).  If none are given then
we default to **xy**.  Thus, if you wish to give different annotation intervals
or labels for the various axes then you must repeat the **B** option for
each axis.  To add a label to an axis, just append **+l**\ *label*.  If the
axis annotation should have a leading text prefix (e.g., dollar sign for those
plots of your net worth) you can append **+p**\ *prefix*.  For geographic maps
the addition of degree symbols, etc. is automatic (and controlled by the GMT
default setting :ref:`FORMAT_GEO_MAP <FORMAT_GEO_MAP>`).  However, for other plots you can add
specific units by adding **+u**\ *unit*.  If any of these text strings contain
spaces or special UNIX characters you will need to enclose them in quotes.
The *intervals* specification is a concatenated string made up of substrings of the form

[**t**]\ *stride*\ [*phase*][**u**].

The **t** flag sets the axis
item of interest; the available items are listed in Table :ref:`inttype <tbl-inttype>`.
Normally, equidistant annotations occur at multiples of *stride*; you
can phase-shift this by appending *phase*, which can be a positive or
negative number.

.. _tbl-inttype:

+------------+-------------------------------------+
| **Flag**   | **Description**                     |
+============+=====================================+
| **a**      | Annotation and major tick spacing   |
+------------+-------------------------------------+
| **f**      | Minor tick spacing                  |
+------------+-------------------------------------+
| **g**      | Grid line spacing                   |
+------------+-------------------------------------+

Note that the appearance of certain time annotations (month-, week-, and
day-names) may be affected by the :ref:`TIME_LANGUAGE <TIME_LANGUAGE>`,
:ref:`FORMAT_TIME_PRIMARY_MAP <FORMAT_TIME_PRIMARY_MAP>`, and
:ref:`FORMAT_TIME_SECONDARY_MAP <FORMAT_TIME_SECONDARY_MAP>` settings.

For automated plots the region may not always be the same and thus it
can be difficult to determine the appropriate *stride* in advance. Here
GMT provides the opportunity to autoselect the spacing between the
major and minor ticks and the grid lines, by not specifying the *stride*
value. For example, **-Bafg** will select all three spacings
automatically for both axes. In case of longitude--latitude plots, this
will keep the spacing the same on both axes. You can also use
**-Bafg/afg** to autoselect them separately.

In the case of automatic spacing, when the *stride* argument is omitted
after **g**, the grid line spacing is chosen the same as the minor tick
spacing; unless **g** is used in consort with **a**, then the grid lines
are spaced the same as the annotations.

The unit flag **u** can take on one of 18 codes; these are listed in
Table :ref:`units <tbl-units>`. Almost all of these units are time-axis specific.
However, the **m** and **s** units will be interpreted as arc minutes
and arc seconds, respectively, when a map projection is in effect.

.. _tbl-units:

+------------+------------------+--------------------------------------------------------------------------+
| **Flag**   | **Unit**         | **Description**                                                          |
+============+==================+==========================================================================+
| **Y**      | year             | Plot using all 4 digits                                                  |
+------------+------------------+--------------------------------------------------------------------------+
| **y**      | year             | Plot using last 2 digits                                                 |
+------------+------------------+--------------------------------------------------------------------------+
| **O**      | month            | Format annotation using **FORMAT_DATE_MAP**                              |
+------------+------------------+--------------------------------------------------------------------------+
| **o**      | month            | Plot as 2-digit integer (1--12)                                          |
+------------+------------------+--------------------------------------------------------------------------+
| **U**      | ISO week         | Format annotation using **FORMAT_DATE_MAP**                              |
+------------+------------------+--------------------------------------------------------------------------+
| **u**      | ISO week         | Plot as 2-digit integer (1--53)                                          |
+------------+------------------+--------------------------------------------------------------------------+
| **r**      | Gregorian week   | 7-day stride from start of week (see **TIME_WEEK_START**)                |
+------------+------------------+--------------------------------------------------------------------------+
| **K**      | ISO weekday      | Plot name of weekday in selected language                                |
+------------+------------------+--------------------------------------------------------------------------+
| **k**      | weekday          | Plot number of day in the week (1--7) (see **TIME_WEEK_START**)          |
+------------+------------------+--------------------------------------------------------------------------+
| **D**      | date             | Format annotation using **FORMAT_DATE_MAP**                              |
+------------+------------------+--------------------------------------------------------------------------+
| **d**      | day              | Plot day of month (1--31) or day of year (1--366)                        |
+------------+------------------+--------------------------------------------------------------------------+
|            |                  | (see **FORMAT_DATE_MAP**                                                 |
+------------+------------------+--------------------------------------------------------------------------+
| **R**      | day              | Same as **d**; annotations aligned with week (see **TIME_WEEK_START**)   |
+------------+------------------+--------------------------------------------------------------------------+
| **H**      | hour             | Format annotation using **FORMAT_CLOCK_MAP**                             |
+------------+------------------+--------------------------------------------------------------------------+
| **h**      | hour             | Plot as 2-digit integer (0--24)                                          |
+------------+------------------+--------------------------------------------------------------------------+
| **M**      | minute           | Format annotation using **FORMAT_CLOCK_MAP**                             |
+------------+------------------+--------------------------------------------------------------------------+
| **m**      | minute           | Plot as 2-digit integer (0--60)                                          |
+------------+------------------+--------------------------------------------------------------------------+
| **S**      | seconds          | Format annotation using **FORMAT_CLOCK_MAP**                             |
+------------+------------------+--------------------------------------------------------------------------+
| **s**      | seconds          | Plot as 2-digit integer (0--60)                                          |
+------------+------------------+--------------------------------------------------------------------------+

As mentioned, there may be two levels of annotations. Here, "primary" refers to the
annotation that is closest to the axis (this is the primary annotation),
while "secondary" refers to the secondary annotation that is plotted
further from the axis. The examples below will clarify what is meant.
Note that the terms "primary" and "secondary" do not reflect any
hierarchical order of units: The "primary" annotation interval is
usually smaller (e.g., days) while the "secondary" annotation interval
typically is larger (e.g., months).

Geographic basemaps
^^^^^^^^^^^^^^^^^^^

Geographic basemaps may differ from regular plot axis in that some
projections support a "fancy" form of axis and is selected by the
:ref:`MAP_FRAME_TYPE <MAP_FRAME_TYPE>` setting. The annotations will be formatted
according to the :ref:`FORMAT_GEO_MAP <FORMAT_GEO_MAP>` template and
:ref:`MAP_DEGREE_SYMBOL <MAP_DEGREE_SYMBOL>` setting. A simple example of part of a basemap
is shown in Figure :ref:`Geographic map border <basemap_border>`.

.. _basemap_border:

.. figure:: /_images/GMT_-B_geo_1.*
   :width: 500 px
   :align: center

   Geographic map border using separate selections for annotation,
   frame, and grid intervals.  Formatting of the annotation is controlled by
   the parameter :ref:`FORMAT_GEO_MAP <FORMAT_GEO_MAP>` in your :doc:`gmt.conf`.


The machinery for primary and secondary annotations introduced for
time-series axes can also be utilized for geographic basemaps. This may
be used to separate degree annotations from minutes- and
seconds-annotations. For a more complicated basemap example using
several sets of intervals, including different intervals and pen
attributes for grid lines and grid crosses, see Figure :ref:`Complex basemap
<complex_basemap>`.

.. _complex_basemap:

.. figure:: /_images/GMT_-B_geo_2.*
   :width: 500 px
   :align: center

   Geographic map border with both primary (P) and secondary (S) components.

Cartesian linear axes
^^^^^^^^^^^^^^^^^^^^^

For non-geographic axes, the :ref:`MAP_FRAME_TYPE <MAP_FRAME_TYPE>` setting is implicitly
set to plain. Other than that, cartesian linear axes are very similar to
geographic axes. The annotation format may be controlled with the
:ref:`FORMAT_FLOAT_OUT <FORMAT_FLOAT_OUT>` parameter. By default, it is set to "%g", which
is a C language format statement for floating point numbers [13]_, and
with this setting the various axis routines will automatically determine
how many decimal points should be used by inspecting the *stride*
settings. If :ref:`FORMAT_FLOAT_OUT <FORMAT_FLOAT_OUT>` is set to another format it will be
used directly (.e.g, "%.2f" for a fixed, two decimals format). Note that
for these axes you may use the *unit* setting to add a unit string to
each annotation (see Figure :ref:`Axis label <axis_label_basemap>`).

.. _axis_label_basemap:

.. figure:: /_images/GMT_-B_linear.*
   :width: 500 px
   :align: center

   Linear Cartesian projection axis.  Long tickmarks accompany
   annotations, shorter ticks indicate frame interval. The axis label is
   optional. For this example we used ``-R0/12/0/0.95 -JX3i/0.3i -Ba4f2g1+lFrequency+u" %" -BS``

Cartesian log\ :sub:`10` axes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Due to the logarithmic nature of annotation spacings, the *stride*
parameter takes on specific meanings. The following concerns are
specific to log axes (see Figure :ref:`Logarithmic projection axis
<Log_projection>`):

*  *stride* must be 1, 2, 3, or a negative integer -n.
   Annotations/ticks will then occur at 1, 1-2-5, or 1,2,3,4,...,9,
   respectively, for each magnitude range. For *-n* the
   annotations will take place every *n*\ 'th magnitude.

*  Append **l** to *stride*. Then, log\ :sub:`10` of the annotation
   is plotted at every integer log\ :sub:`10` value (e.g.,
   *x = 100* will be annotated as "2") [Default annotates *x* as is].

*  Append **p** to *stride*. Then, annotations appear as 10 raised to
   log\ :sub:`10` of the value (e.g., 10\ :sup:`-5`).

.. _Log_projection:

.. figure:: /_images/GMT_-B_log.*
   :width: 500 px
   :align: center

   Logarithmic projection axis using separate values for annotation,
   frame, and grid intervals.  (top) Here, we have chosen to annotate the actual
   values.  Interval = 1 means every whole power of 10, 2 means 1, 2, 5 times
   powers of 10, and 3 means every 0.1 times powers of 10.  We used
   -R1/1000/0/1 -JX3il/0.25i -Ba1f2g3. (middle) Here, we have chosen to
   annotate :math:`\log_{10}` of the actual values, with -Ba1f2g3l.
   (bottom) We annotate every power of 10 using :math:`\log_{10}` of the actual
   values as exponents, with -Ba1f2g3p.

Cartesian exponential axes
^^^^^^^^^^^^^^^^^^^^^^^^^^

Normally, *stride* will be used to create equidistant (in the user's
unit) annotations or ticks, but because of the exponential nature of the
axis, such annotations may converge on each other at one end of the
axis. To avoid this problem, you can append **p** to *stride*, and the
annotation interval is expected to be in transformed units, yet the
annotation itself will be plotted as un-transformed units (see Figure
:ref:`Power projection axis <Pow_projection>`). E.g., if
*stride* = 1 and power = 0.5 (i.e., sqrt), then equidistant annotations
labeled 1, 4, 9, ... will appear.

.. _Pow_projection:

.. figure:: /_images/GMT_-B_pow.*
   :width: 500 px
   :align: center

   Exponential or power projection axis. (top) Using an exponent of 0.5
   yields a :math:`sqrt(x)` axis.  Here, intervals refer to actual data values,
   in -R0/100/0/0.9 -JX3ip0.5/0.25i -Ba20f10g5.
   (bottom) Here, intervals refer to projected values, although the annotation
   uses the corresponding unprojected values, as in -Ba3f2g1p.

Cartesian time axes
^^^^^^^^^^^^^^^^^^^

What sets time axis apart from the other kinds of plot axes is the
numerous ways in which we may want to tick and annotate the axis. Not
only do we have both primary and secondary annotation items but we also
have interval annotations versus tickmark annotations, numerous time
units, and several ways in which to modify the plot. We will demonstrate
this flexibility with a series of examples. While all our examples will
only show a single *x*\ -axis (south, selected via **-BS**), time-axis
annotations are supported for all axes.

Our first example shows a time period of almost two months in Spring
2000. We want to annotate the month intervals as well as the date at the start of each week:

   ::

     gmt set FORMAT_DATE_MAP=-o FONT_ANNOT_PRIMARY +9p
     gmt psbasemap -R2000-4-1T/2000-5-25T/0/1 -JX5i/0.2i -Bpa7Rf1d -Bsa1O -BS -P > GMT_-B_time1.ps

These commands result in Figure :ref:`Cartesian time axis <cartesian_axis1>`.
Note the leading hyphen in the :ref:`FORMAT_DATE_MAP <FORMAT_DATE_MAP>`
removes leading zeros from calendar items (e.g., 02 becomes 2).

.. _cartesian_axis1:

.. figure:: /_images/GMT_-B_time1.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 1

The next example shows two different ways to annotate an axis portraying 2 days in July 1969:

   ::

     gmt set FORMAT_DATE_MAP "o dd" FORMAT_CLOCK_MAP hh:mm FONT_ANNOT_PRIMARY +9p
     gmt psbasemap -R1969-7-21T/1969-7-23T/0/1 -JX5i/0.2i -Bpa6Hf1h -Bsa1K -BS -P -K > GMT_-B_time2.ps
     gmt psbasemap -R -J -Bpa6Hf1h -Bsa1D -BS -O -Y0.65i >> GMT_-B_time2.ps

The lower example (Figure :ref:`cartesian_axis2`) chooses to annotate the weekdays (by
specifying **a**\ 1\ **K**) while the upper example choses dates (by
specifying **a**\ 1\ **D**). Note how the clock format only selects
hours and minutes (no seconds) and the date format selects a month name,
followed by one space and a two-digit day-of-month number.

.. _cartesian_axis2:

.. figure:: /_images/GMT_-B_time2.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 2

The third example (Figure :ref:`cartesian_axis3`) presents two years, annotating
both the years and every 3rd month.

   ::

     gmt set FORMAT_DATE_MAP o FORMAT_TIME_PRIMARY_MAP Character FONT_ANNOT_PRIMARY +9p
     gmt psbasemap -R1997T/1999T/0/1 -JX5i/0.2i -Bpa3Of1o -Bsa1Y -BS -P > GMT_-B_time3.ps

Note that while the year annotation is centered on the 1-year interval,
the month annotations must be centered on the corresponding month and
*not* the 3-month interval. The :ref:`FORMAT_DATE_MAP <FORMAT_DATE_MAP>` selects month name
only and :ref:`FORMAT_TIME_PRIMARY_MAP <FORMAT_TIME_PRIMARY_MAP>` selects the 1-character, upper
case abbreviation of month names using the current language (selected by
:ref:`TIME_LANGUAGE <TIME_LANGUAGE>`).

.. _cartesian_axis3:

.. figure:: /_images/GMT_-B_time3.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 3

The fourth example (Figure :ref:`cartesian_axis4`) only shows a few hours of a day, using
relative time by specifying **t** in the **-R** option while the
:ref:`TIME_UNIT <TIME_UNIT>` is **d** (for days). We select both primary and secondary
annotations, ask for a 12-hour clock, and let time go from right to left:

   ::

     gmt set FORMAT_CLOCK_MAP=-hham FONT_ANNOT_PRIMARY +9p
     gmt psbasemap -R0.2t/0.35t/0/1 -JX-5i/0.2i -Bpa15mf5m -Bsa1H -BS -P > GMT_-B_time4.ps

.. _cartesian_axis4:

.. figure:: /_images/GMT_-B_time4.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 4

The fifth example shows a few weeks of time (Figure :ref:`cartesian_axis5`). The lower axis
shows ISO weeks with week numbers and abbreviated names of the weekdays.
The upper uses Gregorian weeks (which start at the day chosen by
:ref:`TIME_WEEK_START <TIME_WEEK_START>`); they do not have numbers.

   ::

    gmt set FORMAT_DATE_MAP u FORMAT_TIME_PRIMARY_MAP Character \
           FORMAT_TIME_SECONDARY_MAP full FONT_ANNOT_PRIMARY +9p
    gmt psbasemap -R1969-7-21T/1969-8-9T/0/1 -JX5i/0.2i -Bpa1K -Bsa1U -BS -P -K > GMT_-B_time5.ps
    gmt set FORMAT_DATE_MAP o TIME_WEEK_START Sunday FORMAT_TIME_SECONDARY_MAP Chararacter
    gmt psbasemap -R -J -Bpa3Kf1k -Bsa1r -BS -O -Y0.65i >> GMT_-B_time5.ps

.. _cartesian_axis5:

.. figure:: /_images/GMT_-B_time5.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 5

Our sixth example (Figure :ref:`cartesian_axis6`) shows the first five months of
1996, and we have annotated each month with an abbreviated, upper case name and
2-digit year. Only the primary axes information is specified.

   ::

    gmt set FORMAT_DATE_MAP "o yy" FORMAT_TIME_PRIMARY_MAP Abbreviated
    gmt psbasemap -R1996T/1996-6T/0/1 -JX5i/0.2i -Ba1Of1d -BS -P > GMT_-B_time6.ps

.. _cartesian_axis6:

.. figure:: /_images/GMT_-B_time6.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 6

Our seventh and final example (Figure :ref:`cartesian_axis7`) illustrates
annotation of year-days. Unless we specify the formatting with a leading hyphen
in :ref:`FORMAT_DATE_MAP <FORMAT_DATE_MAP>` we get 3-digit integer days. Note that
in order to have the two years annotated we need to allow for the annotation of
small fractional intervals; normally such truncated interval must be at
least half of a full interval.

   ::

    gmt set FORMAT_DATE_MAP jjj TIME_INTERVAL_FRACTION 0.05 FONT_ANNOT_PRIMARY +9p
    gmt psbasemap -R2000-12-15T/2001-1-15T/0/1 -JX5i/0.2i -Bpa5Df1d -Bsa1Y -BS -P > GMT_-B_time7.ps

.. _cartesian_axis7:

.. figure:: /_images/GMT_-B_time7.*
   :width: 500 px
   :align: center

   Cartesian time axis, example 7

Custom axes
^^^^^^^^^^^

Irregularly spaced annotations or annotations based on
look-up tables can be implemented using the *custom* annotation
mechanism. Here, we given the **c** (custom) type to the **-B** option
followed by a filename that contains the annotations (and
tick/grid-lines specifications) for one axis. The file can contain any
number of comments (lines starting with #) and any number of records of
the format

| *coord* *type* [*label*]

The *coord* is the location of the desired annotation, tick, or
grid-line, whereas *type* is a string composed of letters from **a**
(annotation), **i** interval annotation, **f** frame tick, and **g**
gridline. You must use either **a** or **i** within one file; no mixing
is allowed. The coordinates should be arranged in increasing order. If
*label* is given it replaces the normal annotation based on the *coord*
value. Our last example (Figure :ref:`Custom and irregular annotations
<Custom_annotations>`) shows such a custom basemap with an interval
annotations on the *x*-axis and irregular annotations on the *y*-axis.

   ::

    cat << EOF > xannots.txt
    416.0 ig Devonian
    443.7 ig Silurian
    488.3 ig Ordovician
    542 ig Cambrian
    EOF
    cat << EOF > yannots.txt
    0 a
    1 a
    2 f
    2.71828 ag e
    3 f
    3.1415926 ag @~p@~
    4 f
    5 f
    6 f
    6.2831852 ag 2@~p@~
    EOF
    gmt psbasemap -R416/542/0/6.2831852 -JX-5i/2.5i -Bpx25f5g25+u" Ma" -Bpycyannots.txt \
                  -BWS+glightblue -P -K > GMT_-B_custom.ps
    gmt psbasemap -R416/542/0/6.2831852 -JX-5i/2.5i -Bsxcxannots.txt -BWS -O \
                  --MAP_ANNOT_OFFSET_SECONDARY=10p --MAP_GRID_PEN_SECONDARY=2p >> GMT_-B_custom.ps
    rm -f [xy]annots.txt

.. _Custom_annotations:

.. figure:: /_images/GMT_-B_custom.*
   :width: 500 px
   :align: center

   Custom and irregular annotations, tick-marks, and gridlines.


Portrait plot orientation: The **-P** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-P** option selects Portrait plotting mode [14]_. In general, a
plot has an *x*-axis increasing from left to right and a *y*-axis
increasing from bottom to top. If the paper is turned so that the long
dimension of the paper is parallel to the *x*-axis then the plot is said
to have *Landscape* orientation. If the long dimension of the paper
parallels the *y*-axis the orientation is called *Portrait* (think of
taking pictures with a camera and these words make sense). The default
Landscape orientation is obtained by translating the origin in the
*x*-direction (by the width of the chosen paper :ref:`PS_MEDIA <PS_MEDIA>`) and then
rotating the coordinate system counterclockwise by 90. By default the
:ref:`PS_MEDIA <PS_MEDIA>` is set to Letter (or A4 if SI is chosen); this value must
be changed when using different media, such as 11" x 17" or large format
plotters (Figure :ref:`Plot orientation <P_option>`).

.. _P_option:

.. figure:: /_images/GMT_-P.*
   :width: 500 px
   :align: center

   Users can specify Landscape [Default] or Portrait -P) orientation.


Plot overlays: The **-K** **-O** options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-K** and **-O** options control the generation of
PostScript code for multiple overlay plots. All PostScript files
must have a header (for initializations), a body (drawing the figure),
and a trailer (printing it out) (see Figure :ref:`Multiple overlay plots
<OK_options>`). Thus, when overlaying
several GMT plots we must make sure that the first plot call omits the
trailer, that all intermediate calls omit both header and trailer, and
that the final overlay omits the header. The **-K** omits the trailer
which implies that more PostScript code will be appended later
[Default terminates the plot system]. The **-O** selects Overlay plot
mode and omits the header information [Default initializes a new plot
system]. Most unexpected results for multiple overlay plots can be
traced to the incorrect use of these options. If you run only one plot
program, ignore both the **-O** and **-K** options; they are only used
when stacking plots.

.. _OK_options:

.. figure:: /_images/GMT_-OK.*
   :width: 500 px
   :align: center

   A final PostScript file consists of any number of individual pieces.


Timestamps on plots: The **-U** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-U** option draws UNIX System time stamp. Optionally, append an
arbitrary text string (surrounded by double quotes), or the code **c**,
which will plot the current command string (Figure
:ref:`Time stamp <U_option>`).

.. _U_option:

.. figure:: /_images/GMT_-U.*
   :width: 500 px
   :align: center

   The -U option makes it easy to date a plot.


Verbose feedback: The **-V** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-V** option selects verbose mode, which will send
progress reports to standard error. Even more verbose levels are **-Vl**
(long verbose) and **-Vd** (debug). Normal verbosity level produces only
error and warning messages. This is the default or can be selected by
using **-Vn**. If compiled with backward-compatibility support, the
default is **-Vc**, which includes warnings about deprecated usage.
Finally, **-Vq** can be used to run without any warnings or errors. This
option can also be set by specifying the default :ref:`GMT_VERBOSE <GMT_VERBOSE>`, as
**quiet**, **normal**, **compat**, **verbose**, **long_verbose**, or
**debug**, in order of increased verbosity.

Plot positioning and layout: The **-X** **-Y** options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-X** and **-Y** options shift origin of plot by (*xoff*,\ *yoff*)
inches (Default is (:ref:`MAP_ORIGIN_X <MAP_ORIGIN_X>`, :ref:`MAP_ORIGIN_Y <MAP_ORIGIN_Y>`) for new
plots [15]_ and (0,0) for overlays (**-O**)). By default, all
translations are relative to the previous origin (see Figure
:ref:`Plot positioning <XY_options>`). Supply
offset as **c** to center the plot in that direction relative to the
page margin. Absolute translations (i.e., relative to a fixed point
(0,0) at the lower left corner of the paper) can be achieve by
prepending "a" to the offsets. Subsequent overlays will be co-registered
with the previous plot unless the origin is shifted using these options.
The offsets are measured in the current coordinates system (which can be
rotated using the initial **-P** option; subsequent **-P** options for
overlays are ignored).

.. _XY_options:

.. figure:: /_images/GMT_-XY.*
   :width: 300 px
   :align: center

   Plot origin can be translated freely with -X -Y.

OGR/GMT GIS i/o: The **-a** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GMT relies on external tools to translate geospatial files such as
shapefiles into a format we can read. The tool **ogr2ogr** in the GDAL
package can do such translations and preserve the aspatial metadata via
a new OGR/GMT format specification (See Appendix `P. The GMT Vector Data Format for OGR Compatibility`_).
For this to be useful we need a mechanism to associate certain metadata values with
required input and output columns expected by GMT programs. The **-a**
option allows you to supply one or more comma-separated associations
*col=name*, where *name* is the name of an aspatial attribute field in a
OGR/GMT file and whose value we wish to as data input for column *col*.
The given aspatial field thus replaces any other value already set. Note
that *col = 0* is the first data columns. Note that if no aspatial
attributes are needed then the **-a** option is not needed -- GMT will
still process and read such data files.

OGR/GMT input with **-a** option
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you need to populate GMT data columns with (constant) values
specified by aspatial attributes, use **-a** and append any number of
comma-separated *col=name* associations. E.g., *2=depth* will read the
spatial *x,y* columns from the file and add a third (*z*) column based
on the value of the aspatial field called *depth*. You can also
associate aspatial fields with other settings such as labels, fill
colors, pens, and values used to look-up colors. Do so by letting the
*col* value be one of **D**, **G**, **L**, **T**, **W**, or **Z**. This
works analogously to how standard multi-segment files can pass such
options via its segment headers (See Appendix `B. GMT file formats`_).

OGR/GMT output with **-a** option
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can also make GMT table-writing tools output the OGR/GMT format
directly. Again, specify if certain GMT data columns with constant
values should be stored as aspatial metadata using the
*col=name*\ [:*type*], where you can optionally specify what data type
it should be (double, integer, string, logical, byte, or datetime)
[double is default]. As for input, you can also use the special *col*
entries of **D**, **G**, **L**, **T**, **W**, or **Z** to have values
stored as options in segment headers be used as the source for the name
aspatial field. Finally, for output you must append
+\ **g**\ *geometry*, where *geometry* can be any of
[**M**]\ **POINT**\ \|\ **LINE**\ \|\ **POLY**; the
**M** represent the multi-versions of these three geometries. Use
upper-case +\ **G** to signal that you want to split any line or polygon
features that straddle the Dateline.

Binary table i/o: The **-b** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All GMT programs that accept table data input may read ASCII, native
binary, or netCDF data. Native binary files may have a header section
and the **-h**\ *n* option (see Section `Header data records: The -h option`_) can be used to
skip the first *n* bytes. The data record can be in any format, mixing
different data types and even containing byte-swapped items. When using
native binary data the user must be aware of the fact that GMT has no
way of determining the actual number of columns in the file. You must
therefore pass that information to GMT via the binary
**-bi** option, where *n* is the actual number of data
columns and **t** must be one of **c** (signed 1-byte character,
int8_t), **u** (unsigned 1-byte character, uint8_t), **h** (signed
2-byte int, int16_t), **H** (unsigned 2-byte int, uint16_t), **i**
(signed 4-byte int, int32_t), **I** (unsigned 4-byte int, uint32_t),
**l** (signed 8-byte int, int64_t), **L** (unsigned 8-byte int,
uint64_t), **f** (4-byte single-precision float), and **d** (8-byte
double-precision float). In addition, use **x** to skip *n* bytes
anywhere in the record. For a mixed-type data record you can concatenate
several [*n*]\ **t** combinations, separated by commas. You may append
**w** to any of the items to force byte-swapping. Alternatively, append
**+L**\ \|\ **B** to indicate that the entire data file should be
read or written as little- or big-endian, respectively. Here, *n* is the
number of each item in your binary file. Note that *n* may be larger
than *m*, the number of columns that the GMT program requires to do
its task. If *n* is not given then it defaults to *m* and all columns
are assumed to be of the single specified type **t** [**d** (double), if
not set]. If *n* < *m* an error is generated. Multiple segment
files are allowed and the segment headers are assumed to be records
where all the fields equal NaN.

For binary output, use the **-bo** option; see **-bi** for further details.

Because of its meta data, reading netCDF tables (i.e., netCDF files
containing 1-dimensional arrays) is quite a bit less complex than
reading native binary files. When feeding netCDF tables to programs like
:doc:`psxy`, the program will automatically
recognize the format and read whatever amount of columns are needed for
that program. To steer which columns are to be read, the user can append
the suffix **?**\ *var1*\ **/**\ *var2*\ **/**\ *...* to the netCDF file
name, where *var1*, *var2*, etc. are the names of the variables to be
processed. No **-bi** option is needed in this case.

Currently, netCDF tables can only be input, not output. For more
information, see Appendix [app:B].

Number of Copies: The **-c** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-c** option specifies the number of plot copies [Default is 1].
This value is embedded in the PostScript file and will make a printer
issue the chosen number of copies without respooling.

Data type selection: The **-f** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When map projections are not required we must explicitly state what kind
of data each input or output column contains. This is accomplished with
the **-f** option. Following an optional **i** (for input only) or **o**
(for output only), we append a text string with information about each
column (or range of columns) separated by commas. Each string starts
with the column number (0 is first column) followed by either **x**
(longitude), **y** (latitude), **T** (absolute calendar time) or **t**
(relative time). If several consecutive columns have the same format you
may specify a range of columns rather than a single column, i.e., 0--4
for the first 5 columns. For example, if our input file has geographic
coordinates (latitude, longitude) with absolute calendar coordinates in
the columns 3 and 4, we would specify
**fi**\ 0\ **y**,1\ **x**,3--4\ **T**. All other columns are assumed to
have the default, floating point format and need not be set
individually. The shorthand **-f**\ [**i**\ \|\ **o**]\ **g**
means **-f**\ [**i**\ \|\ **o**]0x,1y (i.e., geographic
coordinates). A special use of **-f** is to select **-fp**\ [*unit*],
which *requires* **-J** and lets you use *projected* map coordinates
(e.g., UTM meters) as data input. Such coordinates are automatically
inverted to longitude, latitude during the data import. Optionally,
append a length *unit* (see Table :ref:`distunits <tbl-distunits>`) [meter]. For more
information, see Sections `Input data formats`_ and `Output data formats`_.

Data gap detection: The **-g** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GMT has several mechanisms that can determine line
segmentation. Typically, data segments are separated by multiple segment
header records (see Appendix `B. GMT file formats`_). However, if key data columns contain a
NaN we may also use that information to break lines into multiple
segments. This behavior is modified by the parameter
**IO_NAN_RECORDS** which by default is set to *skip*, meaning such
records are considered bad and simply skipped. If you wish such records
to indicate a segment boundary then set this parameter to *pass*.
Finally, you may wish to indicate gaps based on the data values
themselves. The **-g** option is used to detect gaps based on one or
more criteria (use **-g+** if *all* the criteria must be met; otherwise
only one of the specified criteria needs to be met to signify a data
gap). Gaps can be based on excessive jumps in the *x*- or
*y*-coordinates (**-gx** or **-gy**), or on the distance between points
(**-gd**). Append the *gap* distance and optionally a unit for actual
distances. For geographic data the optional unit may be arc
**d**\ egree, **m**\ inute, and **s**\ econd, or m\ **e**\ ter
[Default], **f**\ eet, **k**\ ilometer, **M**\ iles, or **n**\ autical
miles. For programs that map data to map coordinates you can optionally
specify these criteria to apply to the projected coordinates (by using
upper-case **-gX**, **-gY** or **-gD**). In that case, choose from
**c**\ entimeter, **i**\ nch or **p**\ oint [Default unit is controlled
by **PROJ_LENGTH_UNIT**]. Note: For **-gx** or **-gy** with time data
the unit is instead controlled by :ref:`TIME_UNIT <TIME_UNIT>`.

Header data records: The **-h** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-h**\ [**i**\ \|\ **o**][*n_recs*] option
lets GMT know that input file(s) have *n_recs* header records [0]. If
there are more than one header record you must specify the number after
the **-h** option, e.g., **-h**\ 4. Note that blank lines and records
that start with the character # are automatically considered header
records and skipped. Thus, *n_recs* refers to general text lines that
do *not* start with # and thus must specifically be skipped in order for
the programs to function properly. The default number of such header
records if **-h** is used is one of the many parameters in the :doc:`gmt.conf` file
(**IO_N_HEADER_RECS**, by default 0), but can be overridden by
**-h**\ *n_header_recs*. Normally, programs that both read and write
tables will output the header records that are found on input. Use
**-hi** to suppress the writing of header records. You can use the
**-h** options modifiers to to tell programs to output extra header
records for titles, remarks or column names identifying each data column.

When **-b** is used to indicate binary data the **-h** takes on a
slightly different meaning. Now, the *n_recs* argument is taken to mean
how many *bytes* should be skipped (on input) or padded with the space
character (on output).

Input columns selection: The **-i** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-i**\ *columns* option allows you to specify which
input file data columns to use and in what order. By default, GMT will
read all the data columns in the file, starting with the first column
(0). Using **-i** modifies that process. For instance, to use the 4th,
7th, and 3rd data column as the required *x,y,z* to
:doc:`blockmean` you would specify
**-i**\ 3,6,2 (since 0 is the first column). The chosen data columns
will be used as is. Optionally, you can specify that input columns
should be transformed according to a linear or logarithmic conversion.
Do so by appending [**l**][\ **s**\ *scale*][\ **o**\ *offset*] to
each column (or range of columns). All items are optional: The **l**
implies we should first take :math:`\log_{10}` of the data [leave as
is]. Next, we may scale the result by the given *scale* [1]. Finally, we
add in the specified *offset* [0].

Grid interpolation parameters: The **-n** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-n**\ *type* option controls parameters used for
2-D grids resampling. You can select the type of spline used (**-nb**
for B-spline smoothing, **-nc** for bicubic [Default], **-nl** for
bilinear, or **-nn** for nearest-node value). For programs that support
it, antialiasing is by default on; optionally, append **+a** to switch
off antialiasing. By default, boundary conditions are set according to
the grid type and extent. Change boundary conditions by appending
**+b**\ *BC*, where *BC* is either **g** for geographic boundary
conditions or one (or both) of **n** and **p** for natural or periodic
boundary conditions, respectively. Append **x** or **y** to only apply
the condition in one dimension. E.g., **-nb+nxpy** would imply natural
boundary conditions in the *x* direction and periodic conditions in the
*y* direction. Finally, append **+t**\ *threshold* to control how close
to nodes with NaN the interpolation should go. A *threshold* of 1.0
requires all (4 or 16) nodes involved in the interpolation to be
non-NaN. 0.5 will interpolate about half way from a non-NaN value; 0.1
will go about 90% of the way, etc.

Output columns selection: The **-o** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The **-o**\ *columns* option allows you to specify which
columns to write on output and in what order. By default, GMT will
write all the data columns produced by the program. Using **-o**
modifies that process. For instance, to write just the 4th and 2nd data
column to the output you would use **-o**\ 3,1 (since 0 is the first column).

Perspective view: The **-p** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All plotting programs that normally produce a flat, two-dimensional
illustration can be told to view this flat illustration from a
particular vantage point, resulting in a perspective view. You can
select perspective view with the **-p** option by setting the azimuth
and elevation of the viewpoint [Default is 180/90]. When **-p** is used
in consort with **-Jz** or **-JZ**, a third value can be appended which
indicates at which *z*-level all 2-D material, like the plot frame, is
plotted (in perspective) [Default is at the bottom of the z-axis]. For
frames used for animation, you may want to append **+** to fix the
center of your data domain (or specify a particular world coordinate
point with **+w**\ *lon0/lat*\ [*z*\ ]) which will project to the center
of your page size (or you may specify the coordinates of the *projected*
view point with **+v**\ *x0/y0*. When **-p** is used without any further
arguments, the values from the last use of **-p** in a previous
GMT\ command will be used.

.. _grid-registration:

Grid registration: The **-r** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All 2-D grids in GMT have their nodes
organized in one of two ways, known as *gridline*- and *pixel*
registration. The GMT default is gridline registration; programs that
allow for the creation of grids can use the **-r** option to select
pixel registration instead.

Gridline registration
^^^^^^^^^^^^^^^^^^^^^

In this registration, the nodes are centered on the grid line
intersections and the data points represent the average value in a cell
of dimensions (:math:`x_{inc} \cdot y_{inc}`) centered on each node
(left side of Figure :ref:`Grid registration <Grid_registration>`).
In the case of grid line registration the number of nodes are related
to region and grid spacing by

.. math::

   \begin{array}{ccl}
   nx & =  &       (x_{max} - x_{min}) / x_{inc} + 1       \\
   ny & =  &       (y_{max} - y_{min}) / y_{inc} + 1
   \end{array}

which for the example in left side of Figure :ref:`Gridline registration
<Grid_registration>` yields nx = ny = 4.

Pixel registration
^^^^^^^^^^^^^^^^^^

Here, the nodes are centered in the grid cells, i.e., the areas
between grid lines, and the data points represent the average values
within each cell (right side of Figure :ref:`Grid registration
<Grid_registration>`). In the case of
pixel registration the number of nodes are related to region and grid
spacing by

.. _Grid_registration:

.. figure:: /_images/GMT_registration.*
   :width: 500 px
   :align: center

   Gridline- and pixel-registration of data nodes.


.. math::

   \begin{array}{ccl}
   nx & =  &       (x_{max} - x_{min}) / x_{inc}   \\
   ny & =  &       (y_{max} - y_{min}) / y_{inc}
   \end{array}

Thus, given the same region (**-R**) and grid spacing, the
pixel-registered grids have one less column and one less row than the
gridline-registered grids; here we find nx = ny = 3.

NaN-record treatment: The **-s** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We can use this option to suppress output for records whose *z*-value
equals NaN (by default we output all records). Alternatively, append
**r** to reverse the suppression, i.e., only output the records whose
*z*-value equals NaN. Use **-sa** to suppress output records where one
or more fields (and not necessarily *z*) equal NaN. Finally, you can
supply a comma-separated list of all columns or column ranges to
consider for this NaN test.

Layer PDF transparency: The **-t** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While the PostScript language does not support transparency, PDF does,
and via PostScript extensions one can manipulate the transparency
levels of objects. The **-t** option allows you to change the
transparency level for the current overlay by appending a percentage in
the 0--100 range; the default is 0, or opaque. Transparency may also be
controlled on a feature by feature basis when setting color or fill (see
section `Specifying area fill attributes`_).

Latitude/Longitude or Longitude/Latitude?: The **-:** option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For geographical data, the first column is expected to contain
longitudes and the second to contain latitudes. To reverse this
expectation you must apply the **-:** option. Optionally, append **i**
or **o** to restrict the effect to input or output only. Note that
command line arguments that may take geographic coordinates (e.g.,
**-R**) *always* expect longitude before latitude. Also, geographical
grids are expected to have the longitude as first (minor) dimension.

Command line history
--------------------

GMT programs "remember" the standardized command line options (See
Section `Standardized command line options`_) given during their previous invocations and this
provides a shorthand notation for complex options. For example, if a
basemap was created with an oblique Mercator projection, specified as

``-Joc170W/25:30S/33W/56:20N/1:500000``

then a subsequent :doc:`psxy` command to plot
symbols only needs to state **-J**\ o in order to activate the same
projection. In contrast, note that **-J** by itself will pick the most
recently used projection. Previous commands are maintained in the file ``gmt.history``,
of which there will be one in each directory you run the programs from.
This is handy if you create separate directories for separate projects
since chances are that data manipulations and plotting for each project
will share many of the same options. Note that an option spelled out on
the command line will always override the previous entry in the ``gmt.history`` file and, if
execution is successful, will replace this entry as the previous option
argument in the ``gmt.history`` file. If you call several GMT modules piped together
then GMT cannot guarantee that the ``gmt.history`` file is processed in the intended
order from left to right. The only guarantee is that the file will not
be clobbered since GMT uses advisory file locking. The uncertainty in
processing order makes the use of shorthands in pipes unreliable. We
therefore recommend that you only use shorthands in single process
command lines, and spell out the full command option when using chains
of commands connected with pipes.

Usage messages, syntax- and general error messages
--------------------------------------------------

Each program carries a usage message. If you enter the program name
without any arguments, the program will write the complete usage message
to standard error (your screen, unless you redirect it). This message
explains in detail what all the valid arguments are. If you enter the
program name followed by a *hyphen* (-) only you will get a shorter
version which only shows the command line syntax and no detailed
explanations. If you incorrectly specify an option or omit a required
option, the program will produce syntax errors and explain what the
correct syntax for these options should be. If an error occurs during
the running of a program, the program will in some cases recognize this
and give you an error message. Usually this will also terminate the run.
The error messages generally begin with the name of the program in which
the error occurred; if you have several programs piped together this
tells you where the trouble is.

Standard input or file, header records
--------------------------------------

Most of the programs which expect table data input can read either
standard input or input in one or several files. These programs will try
to read *stdin* unless you type the filename(s) on the command line
without the above hyphens. (If the program sees a hyphen, it reads the
next character as an instruction; if an argument begins without a
hyphen, it tries to open this argument as a filename). This feature
allows you to connect programs with pipes if you like. If your input is
ASCII and has one or more header records that do not begin with #, you
must use the **-h** option (see Section `Header data records: The -h
option`_). ASCII files may in many cases also contain segment-headers
separating data segments. These are called "multi-segment files". For
binary table data the **-h** option may specify how many bytes should be
skipped before the data section is reached. Binary files may also
contain segment-headers separating data segments. These segment-headers
are simply data records whose fields are all set to NaN; see Appendix
[app:B] for complete documentation.

If filenames are given for reading, GMT programs will first look for
them in the current directory. If the file is not found, the programs
will look in two other directories pointed to by the
:ref:`directory parameters <DIR Parameters>` **DIR_DATA** and
**DIR_USER** or by the environmental parameters **GMT_USERDIR** and
**GMT_DATADIR** (if set). They may be set by the user to point to
directories that contain data sets of general use, thus eliminating the
need to specify a full path to these files. Usually, the **DIR_DATA**
directory will hold data sets of a general nature (tables, grids),
whereas the **DIR_USER** directory may hold miscellaneous data sets more
specific to the user; this directory also stores GMT defaults and other
configuration files. See :ref:`directory parameters <DIR Parameters>`
for details. Program output is always written to the current directory
unless a full path has been specified.

Verbose operation
-----------------

Most of the programs take an optional **-V** argument which will run the
program in the "verbose" mode (see Section `Verbose feedback: The -V
option`_). Verbose will write to standard error information about the
progress of the operation you are running. Verbose reports things such
as counts of points read, names of data files processed, convergence of
iterative solutions, and the like. Since these messages are written to
*stderr*, the verbose talk remains separate from your data output. You
may optionally choose among five models of *verbosity*; each mode adds
more messages with an increasing level of details. The modes are

  **q** Complete silence, not even fatal error messages.

  **n** Warnings and progress messages [Default].

  **c** Warnings about deprecated usage (if compiled for compatibility).

  **l** Detailed progress messages.

  **d** Debugging messages.

The verbosity is cumulative, i.e., mode **l** means all messages of mode
**n** as well. will be reported.

Program output
--------------

Most programs write their results, including PostScript\ plots, to
standard output. The exceptions are those which may create binary netCDF
grid files such as :doc:`surface` (due to the
design of netCDF a filename must be provided; however, alternative
binary output formats allowing piping are available; see Section
`Grid file format specifications`_).
Most operating systems let you can redirect
standard output to a file or pipe it into another process. Error
messages, usage messages, and verbose comments are written to standard
error in all cases. You can usually redirect standard error as well, if
you want to create a log file of what you are doing. The syntax for
redirection differ among the main shells (Bash and C-shell) and is a bit
limited in DOS.

Input data formats
------------------

Most of the time, GMT will know what kind of *x* and *y*
coordinates it is reading because you have selected a particular
coordinate transformation or map projection. However, there may be times
when you must explicitly specify what you are providing as input using
the **-f** switch. When binary input data are expected (**-bi**) you
must specify exactly the format of the records. However, for ASCII input
there are numerous ways to encode data coordinates (which may be
separated by white-space or commas). Valid input data are generally of
the same form as the arguments to the **-R** option (see
Section `Data domain or map region: The -R option`_), with additional flexibility for calendar data.
Geographical coordinates, for example, can be given in decimal degrees
(e.g., -123.45417) or in the
[]*ddd*\ [:*mm*\ [:*ss*\ [*.xxx*\ ]]][\ **W**\ \| \ **E**\ \| \ **S**\ \| \ **N**]
format (e.g., 123:27:15W). With **-fp** you may even supply projected
data like UTM coordinates.

Because of the widespread use of incompatible and ambiguous formats, the
processing of input date components is guided by the template
:ref:`FORMAT_DATE_IN <FORMAT_DATE_IN>` in your :doc:`gmt.conf` file; it is by default set to *yyyy-mm-dd*.
Y2K-challenged input data such as 29/05/89 can be processed by setting
:ref:`FORMAT_DATE_IN <FORMAT_DATE_IN>` to dd/mm/yy. A complete description of possible
formats is given in the :doc:`gmt.conf` man
page. The *clock* string is more standardized but issues like 12- or
24-hour clocks complicate matters as well as the presence or absence of
delimiters between fields. Thus, the processing of input clock
coordinates is guided by the template :ref:`FORMAT_CLOCK_IN <FORMAT_CLOCK_IN>` which
defaults to *hh:mm:ss.xxx*.

GMT programs that require a map projection argument will implicitly
know what kind of data to expect, and the input processing is done
accordingly. However, some programs that simply report on minimum and
maximum values or just do a reformatting of the data will in general not
know what to expect, and furthermore there is no way for the programs to
know what kind of data other columns (beyond the leading *x* and
*y* columns) contain. In such instances we must explicitly tell
GMT that we are feeding it data in the specific geographic or calendar
formats (floating point data are assumed by default). We specify the
data type via the **-f** option (which sets both input and output
formats; use **-fi** and **-fo** to set input and output separately).
For instance, to specify that the the first two columns are longitude
and latitude, and that the third column (e.g., *z*) is absolute
calendar time, we add **-fi**\ 0x,1y,2T to the command line. For more
details, see the man page for the program you need to use.

Output data formats
-------------------

The numerical output from GMT programs can be binary (when **-bo** is
used) or ASCII [Default]. In the latter case the issue of formatting
becomes important. GMT provides extensive machinery for allowing just
about any imaginable format to be used on output. Analogous to the
processing of input data, several templates guide the formatting
process. These are :ref:`FORMAT_DATE_OUT <FORMAT_DATE_OUT>` and :ref:`FORMAT_CLOCK_OUT <FORMAT_CLOCK_OUT>` for
calendar-time coordinates, :ref:`FORMAT_GEO_OUT <FORMAT_GEO_OUT>` for geographical
coordinates, and :ref:`FORMAT_FLOAT_OUT <FORMAT_FLOAT_OUT>` for generic floating point data.
In addition, the user have control over how columns are separated via
the :ref:`IO_COL_SEPARATOR <IO_COL_SEPARATOR>` parameter. Thus, as an example, it is possible
to create limited FORTRAN-style card records by setting
:ref:`FORMAT_FLOAT_OUT <FORMAT_FLOAT_OUT>` to %7.3lf and :ref:`IO_COL_SEPARATOR <IO_COL_SEPARATOR>` to none
[Default is tab].

PostScript features
---------------------

PostScript is a command language for driving graphics devices such as
laser printers. It is ASCII text which you can read and edit as you wish
(assuming you have some knowledge of the syntax). We prefer this to
binary metafile plot systems since such files cannot easily be modified
after they have been created. GMT programs also write many comments to
the plot file which make it easier for users to orient themselves should
they need to edit the file (e.g., % Start of x-axis) [16]_. All
GMT programs create PostScript code by calling the :doc:`PSL <pslib>` plot
library (The user may call these functions from his/her own C or FORTRAN
plot programs. See the manual pages for :doc:`PSL <pslib>` syntax). Although
GMT programs can create very individualized plot code, there will
always be cases not covered by these programs. Some knowledge of
PostScript will enable the user to add such features directly into the
plot file. By default, GMT will produce freeform PostScript output
with embedded printer directives. To produce Encapsulated
PostScript (EPS) that can be imported into graphics programs such as
**CorelDraw**, **Illustrator** or **InkScape** for further
embellishment, simply run :doc:`ps2raster`
**-Te**. See Appendix [app:C] for an extensive discussion of converting
PostScript to other formats.

Specifying pen attributes
-------------------------

A pen in GMT has three attributes: *width*, *color*, and
*style*. Most programs will accept pen attributes in the form of an
option argument, with commas separating the given attributes, e.g.,

**-W**\ [*width*\ [**c\ \|\ i\ \|\ p**]],[*color*],[\ *style*\ [**c\ \|\ i\ \|\ p\ \|**]]

    *Width* is by default measured in points (1/72 of an inch). Append
    **c**, **i**, or **p** to specify pen width in cm, inch, or points,
    respectively. Minimum-thickness pens can be achieved by giving zero
    width, but the result is device-dependent. Finally, a few predefined
    pen names can be used: default, faint, and {thin, thick,
    fat}[er\ \|\ est], and obese. Table :ref:`pennames <tbl-pennames>` shows this
    list and the corresponding pen widths.

.. _tbl-pennames:

    +------------+---------+------------+--------+
    +============+=========+============+========+
    | faint      | 0       | thicker    | 1.5p   |
    +------------+---------+------------+--------+
    | default    | 0.25p   | thickest   | 2p     |
    +------------+---------+------------+--------+
    | thinnest   | 0.25p   | fat        | 3p     |
    +------------+---------+------------+--------+
    | thinner    | 0.50p   | fatter     | 6p     |
    +------------+---------+------------+--------+
    | thin       | 0.75p   | fattest    | 12p    |
    +------------+---------+------------+--------+
    | thick      | 1.0p    | obese      | 18p    |
    +------------+---------+------------+--------+

    The *color* can be specified in five different ways:

    #. Gray. Specify a *gray* shade in the range 0--255 (linearly going
       from black [0] to white [255]).

    #. RGB. Specify *r*/*g*/*b*, each ranging from 0--255. Here 0/0/0 is
       black, 255/255/255 is white, 255/0/0 is red, etc.

    #. HSV. Specify *hue*-*saturation*-*value*, with the former in the
       0--360 degree range while the latter two take on the range 0--1 [17]_.

    #. CMYK. Specify *cyan*/*magenta*/*yellow*/*black*, each ranging
       from 0--100%.

    #. Name. Specify one of 663 valid color names. Use **man
       gmtcolors** to list all valid names. A very small yet versatile
       subset consists of the 29 choices *white*, *black*, and
       [light:\|\ dark]{*red, orange, yellow, green, cyan, blue,
       magenta, gray\ \|\ grey, brown*\ }. The color names are
       case-insensitive, so mixed upper and lower case can be used (like
       *DarkGreen*).

    The *style* attribute controls the appearance of the line. A "."
    yields a dotted line, whereas a dashed pen is requested with "-".
    Also combinations of dots and dashes, like ".-" for a dot-dashed
    line, are allowed. The lengths of dots and dashes are scaled
    relative to the pen width (dots has a length that equals the pen
    width while dashes are 8 times as long; gaps between segments are 4
    times the pen width). For more detailed attributes including exact
    dimensions you may specify *string*:*offset*, where *string* is a
    series of numbers separated by underscores. These numbers represent
    a pattern by indicating the length of line segments and the gap
    between segments. The *offset* phase-shifts the pattern from the
    beginning the line. For example, if you want a yellow line of width
    0.1 cm that alternates between long dashes (4 points), an 8 point
    gap, then a 5 point dash, then another 8 point gap, with pattern
    offset by 2 points from the origin, specify
    **-W**\ 0.1c,yellow,4_8_5_8:2p. Just as with pen width, the
    default style units are points, but can also be explicitly specified
    in cm, inch, or points (see *width* discussion above).

Table :ref:`penex <tbl-penex>` contains additional examples of pen specifications
suitable for, say, :doc:`psxy`.

.. _tbl-penex:

+-------------------------------+-----------------------------------------------------+
+===============================+=====================================================+
| **-W**\ 0.5p                  | 0.5 point wide line of default color and style      |
+-------------------------------+-----------------------------------------------------+
| **-W**\ green                 | Green line with default width and style             |
+-------------------------------+-----------------------------------------------------+
| **-W**\ thin,red,-            | Dashed, thin red line                               |
+-------------------------------+-----------------------------------------------------+
| **-W**\ fat,.                 | Fat dotted line with default color                  |
+-------------------------------+-----------------------------------------------------+
| **-W**\ 0.1c,120-1-1          | Green (in h-s-v) pen, 1 mm thick                    |
+-------------------------------+-----------------------------------------------------+
| **-W**\ faint,100/0/0/0,..-   | Very thin, cyan (in c/m/y/k), dot-dot-dashed line   |
+-------------------------------+-----------------------------------------------------+

In addition to these pen settings there are several
PostScript settings that can affect the appearance of lines. These are
controlled via the GMT defaults settings :ref:`PS_LINE_CAP <PS_LINE_CAP>`,
:ref:`PS_LINE_JOIN <PS_LINE_JOIN>`, and :ref:`PS_MITER_LIMIT <PS_MITER_LIMIT>`. They determine how a line
segment ending is rendered, be it at the termination of a solid line or
at the end of all dashed line segments making up a line, and how a
straight lines of finite thickness should behave when joined at a common
point. By default, line segments have rectangular ends, but this can
change to give rounded ends. When :ref:`PS_LINE_CAP <PS_LINE_CAP>` is set to round the
a segment length of zero will appear as a circle. This can be used to
created circular dotted lines, and by manipulating the phase shift in
the *style* attribute and plotting the same line twice one can even
alternate the color of adjacent items.
Figure :ref:`Line appearance <Line_appearance>` shows various lines made in this
fashion. See the :doc:`gmt.conf` man page for more information.

.. _Line_appearance:

.. figure:: /_images/GMT_linecap.*
   :width: 500 px
   :align: center

   Line appearance can be varied by using :ref:`PS_LINE_CAP <PS_LINE_CAP>`

Specifying area fill attributes
-------------------------------

Many plotting programs will allow the user to draw filled polygons or
symbols. The fill specification may take two forms:

**-G**\ *fill*

**-Gp**\ *dpi/pattern*\ [:**B**\ *color*\ [**F**\ *color*]]

fill:
    In the first case we may specify a *gray* shade (0--255), RGB color
    (*r*/*g*/*b* all in the 0--255 range or in hexadecimal *#rrggbb*),
    HSV color (*hue*-*saturation*-*value* in the 0--360, 0--1, 0--1 range),
    CMYK color (*cyan*/*magenta*/*yellow*/*black*, each ranging from
    0--100%), or a valid color *name*; in that respect it is similar to
    specifying the pen color settings (see pen color discussion under
    Section `Specifying pen attributes`_).

pattern:
    The second form allows us to use a predefined bit-image pattern.
    *pattern* can either be a number in the range 1--90 or the name of a
    1-, 8-, or 24-bit Sun raster file. The former will result in one of
    the 90 predefined 64 x 64 bit-patterns provided with GMT and
    reproduced in Appendix [app:E]. The latter allows the user to create
    customized, repeating images using standard Sun raster files [18]_.
    The *dpi* parameter sets the resolution of this image on the page;
    the area fill is thus made up of a series of these "tiles".
    Specifying *dpi* as 0 will result in highest resolution obtainable
    given the present dpi setting in  ``gmt.history``. By specifying upper case **-GP**
    instead of **-Gp** the image will be bit-reversed, i.e., white and
    black areas will be interchanged (only applies to 1-bit images or
    predefined bit-image patterns). For these patterns and other 1-bit
    images one may specify alternative background and foreground colors
    (by appending :**B**\ *color*\ [**F**\ *color*]) that will replace
    the default white and black pixels, respectively. Setting one of the
    fore- or background colors to - yields a *transparent* image where
    only the back- *or* foreground pixels will be painted.

Due to PostScript implementation limitations the raster images used
with **-G** must be less than 146 x 146 pixels in size; for larger
images see :doc:`psimage`. The format of Sun
raster files is outlined in Appendix [app:B]. Note that under
PostScript Level 1 the patterns are filled by using the polygon as a
*clip path*. Complex clip paths may require more memory than the
PostScript interpreter has been assigned. There is therefore the
possibility that some PostScript interpreters (especially those
supplied with older laserwriters) will run out of memory and abort.
Should that occur we recommend that you use a regular gray-shade fill
instead of the patterns. Installing more memory in your printer *may or
may not* solve the problem!

Table :ref:`fillex <tbl-fillex>` contains a few examples of fill specifications.

.. _tbl-fillex:

+---------------------------+-----------------------------------------------------+
+===========================+=====================================================+
| **-G**\ 128               | Solid gray                                          |
+---------------------------+-----------------------------------------------------+
| **-G**\ 127/255/0         | Chartreuse, R/G/B-style                             |
+---------------------------+-----------------------------------------------------+
| **-G**\ #00ff00           | Green, hexadecimal RGB code                         |
+---------------------------+-----------------------------------------------------+
| **-G**\ 25-0.86-0.82      | Chocolate, h-s-v-style                              |
+---------------------------+-----------------------------------------------------+
| **-G**\ DarkOliveGreen1   | One of the named colors                             |
+---------------------------+-----------------------------------------------------+
| **-Gp**\ 300/7            | Simple diagonal hachure pattern in b/w at 300 dpi   |
+---------------------------+-----------------------------------------------------+
| **-Gp**\ 300/7:Bred       | Same, but with red lines on white                   |
+---------------------------+-----------------------------------------------------+
| **-Gp**\ 300/7:BredF-     | Now the gaps between red lines are transparent      |
+---------------------------+-----------------------------------------------------+
| **-Gp**\ 100/marble.ras   | Using user image of marble as the fill at 100 dpi   |
+---------------------------+-----------------------------------------------------+

Specifying Fonts
----------------

The fonts used by GMT are typically set indirectly via the
GMT defaults parameters. However, some programs, like
:doc:`pstext` may wish to have this
information passed directly. A font is specified by a comma-delimited
attribute list of *size*, *fonttype* and *fill*, each of which is
optional. The *size* is the font size (usually in points) but **c**,
**i** or **p** can be added to indicate a specific unit. The *fonttype*
is the name (case sensitive!) of the font or its equivalent numerical ID
(e.g., Helvetica-Bold or 1). *fill* specifies the gray shade, color or
pattern of the text (see section `Specifying area fill attributes`_ above). Optionally, you may
append **=**\ *pen* to the *fill* value in order to draw the text
outline with the specified *pen*; if used you may optionally skip the
filling of the text by setting *fill* to **-**. If any of the attributes
is omitted their default or previous setting will be retained. See
Appendix `G. PostScript fonts used by GMT`_ for a list of all fonts recognized by GMT.

Stroke, Fill and Font Transparency
----------------------------------

The PostScript language has no built-in mechanism for transparency.
However, PostScript\ extensions make it possible to request
transparency, and tools that can render such extensions will produce
transparency effects. We specify transparency in percent: 0 is opaque
[Default] while 100 is fully transparent (i.e., nothing will show). As
noted in section `Layer PDF transparency: The -t option`_, we can control transparency on a
layer-by-layer basis using the **-t** option. However, we may also set
transparency as an attribute of stroke or fill (including for fonts)
settings. Here, transparency is requested by appending @\ *transparency*
to colors or pattern fills. The transparency *mode* can be changed by
using the GMT default parameter :ref:`PS_TRANSPARENCY <PS_TRANSPARENCY>`; the default is
Normal but you can choose among Color, ColorBurn, ColorDodge, Darken,
Difference, Exclusion, HardLight, Hue, Lighten, Luminosity, Multiply,
Normal, Overlay, Saturation, SoftLight, and Screen. For more
information, see for instance (search online for) the Adobe pdfmark
Reference Manual. Most printers and many PostScript viewers can
neither print nor show transparency. They will simply ignore your
attempt to create transparency and will plot any material as opaque.
Ghostscript and its derivatives such as GMT's
:doc:`ps2raster` support transparency (if
compiled with the correct build option). Note: If you use **Acrobat
Distiller** to create a PDF file you must first change some settings to
make transparency effective: change the parameter /AllowTransparency to
true in your \*.joboptions file.

Color palette tables
--------------------

Several programs, such as those which read 2-D gridded data sets and
create colored images or shaded reliefs, need to be told what colors to
use and over what *z*-range each color applies. This is the purpose of
the color palette table (CPT file). These files may also be used by
:doc:`psxy` and
:doc:`psxyz` to plot color-filled symbols. For
most applications, you will simply create a CPT file using the tool
:doc:`makecpt` which will take an existing
color table and resample it to fit your chosen data range, or use
:doc:`grd2cpt` to build a CPT file based on
the data distribution in one or more given grid files. However, in some
situations you will need to make a CPT file by hand or using text tools
like **awk** or **perl**.

Color palette tables (CPT) comes in two flavors: (1) Those designed to
work with categorical data (e.g., data where interpolation of values is
undefined) and (2) those designed for regular, continuously-varying
data. In both cases the *fill* information follows the format given in
Section `Specifying area fill attributes`_. The z-values in CPT files can
be scaled by using the **+u**\ \|\ **U**\ *unit* mechanism.  Append these
modifiers to your CPT filenames when used in GMT commands.  The **+u**\ *unit*
modifier will scale z *from unit to* meters, while **+U**\ *unit* does
the inverse (scale z *from meters to unit*).

Categorical CPT files
~~~~~~~~~~~~~~~~~~~~~

Categorical data are information on which normal numerical operations
are not defined. As an example, consider various land classifications
(desert, forest, glacier, etc.) and it is clear that even if we assigned
a numerical value to these categories (e.g., desert = 1, forest = 2,
etc) it would be meaningless to compute average values (what would 1.5
mean?). For such data a special format of the CPT files are provided.
Here, each category is assigned a unique key, a color or pattern, and an
optional label (usually the category name) marked by a leading
semi-colon. Keys must be monotonically increasing but do not need to be
consecutive. The format is

+-----------------+--------+--------------+
| key\ :sub:`1`   | *Fill* | [;\ *label*] |
+-----------------+--------+--------------+
| ...             |        |              |
+-----------------+--------+--------------+
| key\ :sub:`n`   | *Fill* | [;\ *label*] |
+-----------------+--------+--------------+

The *Fill* information follows the format given in Section `Specifying area fill attributes`_.
While not always applicable to categorical data, the background color
(for *key*-values < :math:`key_1`), foreground color (for
*key*-values > :math:`key_{n}`), and not-a-number (NaN)
color (for *key*-values = NaN) are all defined in the :doc:`gmt.conf` file, but can be
overridden by the statements

+-----+---------------------+
| B   | Fill\ :sub:`back`   |
+-----+---------------------+
| F   | Fill\ :sub:`fore`   |
+-----+---------------------+
| N   | Fill\ :sub:`nan`    |
+-----+---------------------+

Regular CPT files
~~~~~~~~~~~~~~~~~

Suitable for continuous data types and allowing for color
interpolations, the format of the regular CPT files is:

+---------------+-------------------+---------------+-------------------+----------+--------------+
| z\ :sub:`0`   | Color\ :sub:`min` | z\ :sub:`1`   | Color\ :sub:`max` | [**A**]  | [;\ *label*] |
+---------------+-------------------+---------------+-------------------+----------+--------------+
| ...                                                                                             |
+---------------+-------------------+---------------+-------------------+----------+--------------+
| z\ :sub:`n-2` | Color\ :sub:`min` | z\ :sub:`n-1` | Color\ :sub:`max` | [**A**]  | [;\ *label*] |
+---------------+-------------------+---------------+-------------------+----------+--------------+


Thus, for each "*z*-slice", defined as the interval between two
boundaries (e.g., :math:`z_0` to :math:`z_1`), the color can be
constant (by letting Color\ :math:`_{max}` = Color\ :math:`_{min}` or -)
or a continuous, linear function of *z*. If patterns are used then the
second (max) pattern must be set to -. The optional flag **A** is used
to indicate annotation of the color scale when plotted using
:doc:`psscale`. The optional flag **A** may
be **L**, **U**, or **B** to select annotation of the lower, upper, or
both limits of the particular *z*-slice, respectively. However,
the standard **-B** option can be used by
:doc:`psscale` to affect annotation and
ticking of color scales. Just as other GMT programs, the *stride* can
be omitted to determine the annotation and tick interval automatically
(e.g., **-Baf**). The optional semicolon followed by a text label will
make :doc:`psscale`, when used with the
**-L** option, place the supplied label instead of formatted *z*-values.

As for categorical tables, the background color (for *z*-values
< :math:`z_0`), foreground color (for *z*-values >
:math:`z_{n-1}`), and not-a-number (NaN) color (for *z*-values =
NaN) are all defined in the :doc:`gmt.conf` file, but can be overridden by the
statements

+-----+---------------------+
| B   | Fill\ :sub:`back`   |
+-----+---------------------+
| F   | Fill\ :sub:`fore`   |
+-----+---------------------+
| N   | Fill\ :sub:`nan`    |
+-----+---------------------+

which can be inserted into the beginning or end of the CPT file. If you
prefer the HSV system, set the :doc:`gmt.conf` parameter accordingly and replace red,
green, blue with hue, saturation, value. Color palette tables that
contain gray-shades only may replace the *r/g/b* triplets with a single
gray-shade in the 0--255 range. For CMYK, give *c/m/y/k* values in the
0--100 range.

A few programs (i.e., those that plot polygons such as
:doc:`grdview`, :doc:`psscale`,
:doc:`psxy` and
:doc:`psxyz`) can accept pattern fills instead
of gray-shades. You must specify the pattern as in Section `Specifying area fill attributes`_
(no leading **-G** of course), and only the first pattern (for low
*z*) is used (we cannot interpolate between patterns). Finally,
some programs let you skip features whose *z*-slice in the CPT
file has gray-shades set to -. As an example, consider

+-----+---------+-------------+-------+
| 30  | p200/16 | 80          | \-    |
+-----+---------+-------------+-------+
| 80  | \-      | 100         | \-    |
+-----+---------+-------------+-------+
| 100 | 200/0/0 | 200/255/255 | 0     |
+-----+---------+-------------+-------+
| 200 | yellow  | 300         | green |
+-----+---------+-------------+-------+

where slice 30 < z < 80 is painted with pattern # 16 at 200 dpi,
slice 80 < z < 100 is skipped, slice 100 < z < 200 is
painted in a range of dark red to yellow, whereas the slice
200 < z < 300 will linearly yield colors from yellow to green,
depending on the actual value of *z*.

Some programs like :doc:`grdimage` and
:doc:`grdview` apply artificial illumination
to achieve shaded relief maps. This is typically done by finding the
directional gradient in the direction of the artificial light source and
scaling the gradients to have approximately a normal distribution on the
interval [-1,+1]. These intensities are used to add "white" or "black"
to the color as defined by the *z*-values and the CPT file. An intensity
of zero leaves the color unchanged. Higher values will brighten the
color, lower values will darken it, all without changing the original
hue of the color (see Appendix [app:I] for more details). The
illumination is decoupled from the data grid file in that a separate
grid file holding intensities in the [-1,+1] range must be provided.
Such intensity files can be derived from the data grid using
:doc:`grdgradient` and modified with
:doc:`grdhisteq`, but could equally well be
a separate data set. E.g., some side-scan sonar systems collect both
bathymetry and backscatter intensities, and one may want to use the
latter information to specify the illumination of the colors defined by
the former. Similarly, one could portray magnetic anomalies superimposed
on topography by using the former for colors and the latter for shading.

The Drawing of Vectors
----------------------

GMT supports plotting vectors in various forms. A vector is one of
many symbols that may be plotted by :doc:`psxy`
and :doc:`psxyz`, is the main feature in
:doc:`grdvector`, and is indirectly used by
other programs. All vectors plotted by GMT consist of two separate
parts: The vector line (controlled by the chosen pen attributes) and the
optional vector head(s) (controlled by the chosen fill). We distinguish
between three types of vectors:

#. Cartesian vectors are plotted as straight lines. They can be
   specified by a start point and the direction and length (in map
   units) of the vector, or by its beginning and end point. They may
   also be specified giving the azimuth and length (in km) instead.

#. Circular vectors are (as the name implies) drawn as circular arcs and
   can be used to indicate opening angles. It accepts an origin, a
   radius, and the beginning and end angles.

#. Geo-vectors are drawn using great circle arcs. They are specified by
   a beginning point and the azimuth and length (in km) of the vector,
   or by its beginning and end point.

There are numerous attributes you can modify, including how the vector
should be justified relative to the given point (beginning, center, or
end), where heads (if any) should be placed, if the head should just be
the left or right half, if the vector attributes should shrink for
vectors whose length are less than a given cutoff length, and the size
and shape of the head. These attributes are detailed further in the
relevant manual pages.

Character escape sequences
--------------------------

For annotation labels or text strings plotted with
:doc:`pstext`, GMT provides several escape
sequences that allow the user to temporarily switch to the symbol font,
turn on sub- or superscript, etc., within words. These conditions are
toggled on/off by the escape sequence @\ **x**, where **x** can be one
of several types. The escape sequences recognized in GMT are listed in
Table :ref:`escape <tbl-escape>`. Only one level of sub- or superscript is supported.
Note that under Windows the percent symbol indicates a batch variable,
hence you must use two percent-signs for each one required in the escape
sequence for font switching.

.. _tbl-escape:

+-------------------+----------------------------------------------------------------+
+===================+================================================================+
| @~                | Turns symbol font on or off                                    |
+-------------------+----------------------------------------------------------------+
| @+                | Turns superscript on or off                                    |
+-------------------+----------------------------------------------------------------+
| @-                | Turns subscript on or off                                      |
+-------------------+----------------------------------------------------------------+
| @#                | Turns small caps on or off                                     |
+-------------------+----------------------------------------------------------------+
| @\_               | Turns underline on or off                                      |
+-------------------+----------------------------------------------------------------+
| @%\ *fontno*\ %   | Switches to another font; @%% resets to previous font          |
+-------------------+----------------------------------------------------------------+
| @:\ *size*:       | Switches to another font size; @:: resets to previous size     |
+-------------------+----------------------------------------------------------------+
| @;\ *color*;      | Switches to another font color; @;; resets to previous color   |
+-------------------+----------------------------------------------------------------+
| @!                | Creates one composite character of the next two characters     |
+-------------------+----------------------------------------------------------------+
| @@                | Prints the @ sign itself                                       |
+-------------------+----------------------------------------------------------------+

Shorthand notation for a few special European characters has also been added:


+----------+------------+----------+------------+
| *Code*   | *Effect*   | *Code*   | *Effect*   |
+==========+============+==========+============+
| @E       | Æ          | @e       | æ          |
+----------+------------+----------+------------+
| @O       | Ø          | @o       | ø          |
+----------+------------+----------+------------+
| @A       | Å          | @a       | å          |
+----------+------------+----------+------------+
| @C       | Ç          | @c       | ç          |
+----------+------------+----------+------------+
| @N       | Ñ          | @n       | ñ          |
+----------+------------+----------+------------+
| @U       | Ü          | @u       | ü          |
+----------+------------+----------+------------+
| @s       | ß          |          |            |
+----------+------------+----------+------------+

PostScript fonts used in GMT may be re-encoded to include several
accented characters used in many European languages. To access these,
you must specify the full octal code \\xxx allowed for
your choice of character encodings determined by the
:ref:`PS_CHAR_ENCODING <PS_CHAR_ENCODING>` setting described in the
:doc:`gmt.conf` man page. Only the special
characters belonging to a particular encoding will be available. Many
characters not directly available by using single octal codes may be
constructed with the composite character mechanism @!.

Some examples of escape sequences and embedded octal codes in
GMT strings using the Standard+ encoding:

| ``2@~p@~r@+2@+h@-0@- E\363tv\363s`` = 2\ :math:`\pi r^2h_0` Eötvös
| ``10@+-3 @Angstr@om`` = 10\ :math:`^{-3}` Ångstrøm
| ``Se@nor Gar@con`` = Señor Garçon
| ``M@!\305anoa stra@se`` = Manoa straße
| ``A@\#cceleration@\# (ms@+-2@+)`` =

The option in :doc:`pstext` to draw a
rectangle surrounding the text will not work for strings with escape
sequences. A chart of characters and their octal codes is given in
Appendix [app:F].

.. _grid-file-format:

Grid file format specifications
-------------------------------

GMT has the ability to read and write grids using
more than one grid file format (see Table :ref:`grdformats <tbl-grdformats>` for supported
format and their IDs). For reading, GMT will automatically determine
the format of grid files, while for writing you will normally have to
append *=ID* to the filename if you want GMT to use a different format
than the default.

By default, GMT will create new grid files using the **nf** format;
however, this behavior can be overridden by setting the
:ref:`IO_GRIDFILE_FORMAT <IO_GRIDFILE_FORMAT>` defaults parameter to any of the other
recognized values (or by appending *=ID*).

GMT can also read netCDF grid files produced by other software
packages, provided the grid files satisfy the COARDS and Hadley Centre
conventions for netCDF grids. Thus, products created under those
conventions (provided the grid is 2-, 3-, 4-, or 5-dimensional) can be
read directly by GMT and the netCDF grids written by GMT can be read
by other programs that conform to those conventions. Three such programs are
`ncview <http://meteora.ucsd.edu/~pierce/ncview_home_page.html>`_, `Panoply
<http://www.giss.nasa.gov/tools/panoply/>`_, and `ncBrowse
<http://www.epic.noaa.gov/java/ncBrowse/>`_ ; others can be found on the
`netCDF website <http://www.unidata.ucar.edu/software/netcdf/software.html>`_.

In addition, users with some C-programming experience may add their own
read/write functions and link them with the GMT library to extend the
number of predefined formats. Technical information on this topic can be
found in the source file ``gmt_customio.c``. Users who are considering this approach
should contact the GMT team.

.. _tbl-grdformats:

+----------+---------------------------------------------------------------+
| **ID**   | **Explanation**                                               |
+==========+===============================================================+
|          | *GMT 4 netCDF standard formats*                               |
+----------+---------------------------------------------------------------+
| nb       | GMT netCDF format (8-bit integer, COARDS, CF-1.5)             |
+----------+---------------------------------------------------------------+
| ns       | GMT netCDF format (16-bit integer, COARDS, CF-1.5)            |
+----------+---------------------------------------------------------------+
| ni       | GMT netCDF format (32-bit integer, COARDS, CF-1.5)            |
+----------+---------------------------------------------------------------+
| nf       | GMT netCDF format (32-bit float, COARDS, CF-1.5)              |
+----------+---------------------------------------------------------------+
| nd       | GMT netCDF format (64-bit float, COARDS, CF-1.5)              |
+----------+---------------------------------------------------------------+
|          | *GMT 3 netCDF legacy formats*                                 |
+----------+---------------------------------------------------------------+
| cb       | GMT netCDF format (8-bit integer, depreciated)                |
+----------+---------------------------------------------------------------+
| cs       | GMT netCDF format (16-bit integer, depreciated)               |
+----------+---------------------------------------------------------------+
| ci       | GMT netCDF format (32-bit integer, depreciated)               |
+----------+---------------------------------------------------------------+
| cf       | GMT netCDF format (32-bit float, depreciated)                 |
+----------+---------------------------------------------------------------+
| cd       | GMT netCDF format (64-bit float, depreciated)                 |
+----------+---------------------------------------------------------------+
|          | *GMT native binary formats*                                   |
+----------+---------------------------------------------------------------+
| bm       | GMT native, C-binary format (bit-mask)                        |
+----------+---------------------------------------------------------------+
| bb       | GMT native, C-binary format (8-bit integer)                   |
+----------+---------------------------------------------------------------+
| bs       | GMT native, C-binary format (16-bit integer)                  |
+----------+---------------------------------------------------------------+
| bi       | GMT native, C-binary format (32-bit integer)                  |
+----------+---------------------------------------------------------------+
| bf       | GMT native, C-binary format (32-bit float)                    |
+----------+---------------------------------------------------------------+
| bd       | GMT native, C-binary format (32-bit float)                    |
+----------+---------------------------------------------------------------+
|          | *Miscellaneous grid formats*                                  |
+----------+---------------------------------------------------------------+
| rb       | SUN raster file format (8-bit standard)                       |
+----------+---------------------------------------------------------------+
| rf       | GEODAS grid format GRD98 (NGDC)                               |
+----------+---------------------------------------------------------------+
| sf       | Golden Software Surfer format 6 (32-bit float)                |
+----------+---------------------------------------------------------------+
| sd       | Golden Software Surfer format 7 (64-bit float)                |
+----------+---------------------------------------------------------------+
| af       | Atlantic Geoscience Center AGC (32-bit float)                 |
+----------+---------------------------------------------------------------+
| ei       | ESRI Arc/Info ASCII Grid Interchange format (ASCII integer)   |
+----------+---------------------------------------------------------------+
| ef       | ESRI Arc/Info ASCII Grid Interchange format (ASCII float)     |
+----------+---------------------------------------------------------------+
| gd       | Import/export via GDAL [19]_                                  |
+----------+---------------------------------------------------------------+

Because some formats have limitations on the range of values they can
store it is sometimes necessary to provide more than simply the name of
the file and its ID on the command line. For instance, a native short
integer file may use a unique value to signify an empty node or NaN, and
the data may need translation and scaling prior to use. Therefore, all
GMT programs that read or write grid files will decode the given
filename as follows:

name[=\ *ID*\ [/*scale*/*offset*\ [/*nan*]]]

where everything in brackets is optional. If you are reading a grid then
no options are needed: just continue to pass the name of the grid file.
However, if you write another format you must append the =\ *ID* string,
where *ID* is the format code listed above. In addition, should you want
to (1) multiply the data by a scale factor, and (2) add a constant
offset you must append the /*scale*/*offset* modifier. Finally, if you
need to indicate that a certain data value should be interpreted as a
NaN (not-a-number) you must append the /*nan* suffix to the scaling
string (it cannot go by itself; note the nesting of the brackets!). The
/*scale* and /*offset* modifiers may be left empty to select default
values (scale = 1, offset = 0), or you may specify *a* for
auto-adjusting the scale and/or offset of packed integer grids
(=\ *ID*/*a* is a shorthand for =\ *ID*/*a*/*a*).

Some of the grid formats allow writing to standard output and reading
from standard input which means you can connect GMT programs that
operate on grid files with pipes, thereby speeding up execution and
eliminating the need for large, intermediate grid files. You specify
standard input/output by leaving out the filename entirely. That means
the "filename" will begin with "=\ *ID*". Note, that the netCDF format
does not allow piping.

Everything looks clearer after a few examples:

*  To write a native binary float grid file, specify the name as ``my_file.f4=bf`` .

*  To read a native short integer grid file, multiply the data by 10 and
   then add 32000, but first let values that equal 32767 be set to NaN,
   use the filename ``my_file.i2=bs/10/32000/32767``.

*  To read a Golden Software "surfer" format 6 grid file, just pass the
   file name, e.g., ``my_surferfile.grd``.

*  To read a 8-bit standard Sun raster file (with values in the 0--255
   range) and convert it to a 1 range, give the name as ``rasterfile=rb/7.84313725e-3/-1`` (i.e., 1/127.5).

*  To write a native binary short integer grid file to standard output
   after subtracting 32000 and dividing its values by 10, give filename
   as ``=bs/0.1/-3200``.

*  To write an 8-bit integer netCDF grid file with an auto-adjusted
   offset, give filename as ``=nb//a``.

Programs that both read and/or write more than one grid file may specify
different formats and/or scaling for the files involved. The only
restriction with the embedded grid specification mechanism is that no
grid files may actually use the "=" character as part of their name
(presumably, a small sacrifice).

One can also define special file suffixes to imply a specific file
format; this approach represents a more intuitive and user-friendly way
to specify the various file formats. The user may create a file called
``gmt.io`` in the current directory or home directory, or in the directory
``~/.gmt`` and define any number of custom formats. The following is an example of
a ``gmt.io`` file:

+---------------------------------------------------------------------------+
| # suffix format_id scale offset NaNxxxComments # GMT i/o shorthand file   |
| # It can have any number of comment lines like this one anywhere          |
| # suffix format\_id scale offset NaNComments                              |
+-------+-----+-----+---+-------+-------------------------------------------+
| grd   | nf  | \-  | \-| \-    | Default format                            |
+-------+-----+-----+---+-------+-------------------------------------------+
| b     | bf  | \-  | \-| \-    | Native binary floats                      |
+-------+-----+-----+---+-------+-------------------------------------------+
| i2    | bs  | \-  | \-| 32767 | 2-byte integers with NaN value            |
+-------+-----+-----+---+-------+-------------------------------------------+
| ras   | rb  | \-  | \-| \-    | Sun raster files                          |
+-------+-----+-----+---+-------+-------------------------------------------+
| byte  | bb  | \-  | \-| 255   | Native binary 1-byte grids                |
+-------+-----+-----+---+-------+-------------------------------------------+
| bit   | bm  | \-  | \-| \-    | Native binary 0 or 1 grids                |
+-------+-----+-----+---+-------+-------------------------------------------+
| mask  | bm  | \-  | \-| 0     | Native binary 1 or NaN masks              |
+-------+-----+-----+---+-------+-------------------------------------------+
| faa   | bs  | 0.1 | \-| 32767 | Native binary gravity in 0.1 mGal         |
+-------+-----+-----+---+-------+-------------------------------------------+
| ns    | ns  | a   | a | \-    | 16-bit integer netCDF grid with           |
|       |     |     |   |       | auto-scale and auto-offset                |
+-------+-----+-----+---+-------+-------------------------------------------+

These suffices can be anything that makes sense to the user. To activate
this mechanism, set parameter :ref:`IO_GRIDFILE_SHORTHAND <IO_GRIDFILE_SHORTHAND>` to TRUE in
your :doc:`gmt.conf` file. Then, using the filename ``stuff.i2`` is equivalent to saying ``stuff.i2=bs///32767``, and the
filename ``wet.mask`` means wet.mask=bm/1/0/0. For a file intended for masking, i.e.,
the nodes are either 1 or NaN, the bit or mask format file may be as
small as 1/32 the size of the corresponding grid float format file.

Modifiers for changing the grid coordinates
-------------------------------------------

A few GMT tools require that the two horizontal dimensions be
specified in meters. One example is
:doc:`grdfft` which must compute the 2-D
Fourier transform of a grid and evaluate wavenumbers in the proper units
(1/meter). There are two situations where the user may need to change
the coordinates of the grid passed to such programs:

-  You have a geographic grid (i.e., in longitude and latitude). Simply
   supply the **-fg** option and your grid coordinates will
   automatically be converted to meters via a "Flat Earth" approximation
   on the currently selected ellipsoid (Note: this is only possible in
   those few programs that require this capability. In general, **-fg**
   is used to specify table coordinates).

-  You have a Cartesian grid but the units are not meters (e.g., they
   may perhaps be in km or miles). In this case you may append the file
   modifier **+u**\ *unit*, where *unit* is one of non-arc units listed
   in Table :ref:`distunits <tbl-distunits>`. For example, reading in the grid (which has
   distance units of km) and converting them to meters is done by
   specifying the filename as *filename*\ **+u**\ k. On output, any derived grids will revert
   to their original units *unless* you specify another unit modifier to
   the output grid. This may be used, for instance, to save the original
   grid with distances in meters using some other unit.

For convenience, we also support the inverse translation, i.e.,
**+U**\ *unit*. This modifier can be used to convert your grid
coordinates *from* meters *to* the specified unit. Example 28 shows a
case where this is being used to change an UTM grid in meters to km.
These modifiers are only allowed when map projections are not selected
(or are Cartesian).

Modifiers for COARDS-compliant netCDF files
-------------------------------------------

When the netCDF grid file contains more than one 2-dimensional variable,
GMT programs will load the first such variable in the file and ignore
all others. Alternatively, the user can select the required variable by
adding the suffix "?\ *varname*" to the grid file name. For example, to
get information on the variable "slp" in file , use:

   ::

    gmt grdinfo "file.nc?slp"

Since COARDS-compliant netCDF files are the default, the additional
suffix "=nf" can be omitted.

In case the named grid is 3-dimensional, GMT will load the first
(bottom) layer. If another layer is required, either add "[*index*]"
or "(*level*)", where *index* is the index of the third (depth) variable
(starting at 0 for the first layer) and *level* is the numerical value
of the third (depth) variable associated with the requested layer. To
indicate the second layer of the 3-D variable "slp" use as file name: ``file.nc?slp[1]``.

When you supply the numerical value for the third variable using
"(*level*)", GMT will pick the layer closest to that value. No
interpolation is performed.

Note that the question mark, brackets and parentheses have special
meanings on Unix-based platforms. Therefore, you will need to either
*escape* these characters, by placing a backslash in front of them, or
place the whole file name plus modifiers between single quotes or double
quotes.

A similar approach is followed for loading 4-dimensional grids. Consider
a 4-dimensional grid with the following variables:

   ::

    lat(lat): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
    lon(lon): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
    depth(depth): 0, 10, 20, 30, 40, 50, 60, 70, 80, 90
    time(time): 0, 12, 24, 36, 48
    pressure(time,depth,lat,lon): (5000 values)

To get information on the 10x10 grid of pressure at
depth 10 and at time 24, one would use:

   ::

    gmt grdinfo "file.nc?pressure[2,1]"

or (only in case the coordinates increase linearly):

   ::

    gmt grdinfo "file.nc?pressure(24,10)"

Programs that generally deal with columns of one-dimensional data, like
or can use multi-dimensional netCDF files in a very similar way. If a
variable in a netCDF file is one-dimensional, there is nothing more
needed than name the variables on the command line. For example:

   ::

    gmt psxy "file.nc?lon/lat" ...
    gmt gmtconvert "file.nc?time/lat/lon"

If one or more of the selected variables are two-dimensional, and have
the same leading dimension as the other selected variables they will be
plotted in their entirety. For example, if a netCDF files contains 6
time steps recording temperature at 4 points, and the variable ``tmp`` is a 6 by
4 array, then the command ``gmtconvert "file.nc?time/temp"`` can result in:

   ::

    2012-06-25T00:00:00 20.1 20.2 20.1 20.3
    2012-06-25T12:00:00 24.2 23.2 24.5 23.5
    2012-06-26T00:00:00 16.1 16.2 16.1 16.3
    2012-06-26T12:00:00 22.1 23.0 23.9 23.5
    2012-06-27T00:00:00 17.5 16.9 17.2 16.8
    2012-06-27T12:00:00 27.2 27.2 27.5 27.5

If, for example, only the second temperature column is needed, use
``gmt gmtconvert "file.nc?time/temp"`` (indices start counting at 0).

The COARDS conventions set restrictions on the names that can be used
for the units of the variables and coordinates. For example, the units
of longitude and latitude are "degrees_east" and "degrees_north",
respectively. Here is an example of the header of a COARDS compliant
netCDF file (to be obtained using **ncdump**):

   ::

    netcdf M2_fes2004 {
    dimensions:
            lon = 2881 ;
            lat = 1441 ;
    variables:
            float lon(lon) ;
                    lon:long_name = "longitude" ;
                    lon:units = "degrees_east" ;
                    lon:actual_range = 0., 360. ;
            float lat(lat) ;
                    lat:long_name = "latitude" ;
                    lat:units = "degrees_north" ;
                    lat:actual_range = -90., 90. ;
            short amp(lat, lon) ;
                    amp:long_name = "amplitude" ;
                    amp:unit = "m" ;
                    amp:scale_factor = 0.0001 ;
                    amp:add_offset = 3. ;
                    amp:_FillValue = -32768s ;
            short pha(lat, lon) ;
                    pha:long_name = "phase" ;
                    pha:unit = "degrees" ;
                    pha:scale_factor = 0.01 ;
                    pha:_FillValue = -32768s ;

This file contains two grids, which can be plotted separately using the
names ``M2_fes2004.nc?amp`` and ``M2_fes2004.nc?pha``. The attributes ``long_name`` and ``unit`` for each variable
are combined in GMT to a single unit string. For example, after
reading the grid ``y_unit`` equals ``latitude [degrees_north]``. The
same method can be used in reverse to set the proper variable names and
units when writing a grid. However, when the coordinates are set
properly as geographical or time axes, GMT will take care of this. The
user is, however, still responsible for setting the variable name and
unit of the z-coordinate. The default is simply "z".

Modifiers to read and write grids and images via GDAL
-----------------------------------------------------

If the support has been configured during installation, then GMT can
read and write a variety of grid and image formats via GDAL. This
extends the capability of GMT to handle data sets from a variety of
sources.

Reading multi-band images
~~~~~~~~~~~~~~~~~~~~~~~~~

:doc:`grdimage` and :doc:`psimage` both lets the user select
individual bands in a multi-band image file and treats the result as an
image (that is the values, in the 0--255 range, are treated as colors,
not data). To select individual bands you use the **+b**\ *band-number*
mechanism that must be appended to the image filename. Here,
*band-number* can be the number of one individual band (the counting
starts at zero), or it could be a comma-separated list of bands. For example

   ::

    gmt psimage jpeg_image_with_three_bands.jpg+b0

will plot only the first band (i.e., the red band) of the jpeg image as
a gray-scale image, and

   ::

    gmt psimage jpeg_image_with_three_bands.jpg+b2,1,0

will plot the same image in color but where the RGB band order has been reversed.

Instead of treating them as images, all other GMT programs that
process grids can read individual bands from an image but will consider
the values to be regular data. For example, let ``multiband`` be the name of a
multi-band file with a near infrared component in band 4 and red in band
3. We will compute the NDVI (Normalized Difference Vegetation Index),
which is defined as NDVI = (NIR - R) / (NIR + R), as

   ::

    gmt grdmath multiband=gd+b3 multiband=gd+b2 SUB multiband=gd+b3 \
                multiband=gd+b2 ADD DIV = ndvi.nc

The resulting grid ``ndvi.nc`` can then be plotted as usual.

Reading more complex multi-band IMAGES or GRIDS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is also possible to access to sub-datasets in a multi-band grid. The
next example shows how we can extract the SST from the MODIS file ``A20030012003365.L3m_YR_NSST_9``
that is stored in the HDF "format". We need to run the GDAL program
**gdalinfo** on the file because we first
must extract the necessary metadata from the file:

   ::

    gdalinfo A20030012003365.L3m_YR_NSST_9
    Driver: HDF4/Hierarchical Data Format Release 4
    Files: A20030012003365.L3m_YR_NSST_9
    Size is 512, 512
    Coordinate System is `'
    Metadata:
     Product Name=A20030012003365.L3m_YR_NSST_9
     Sensor Name=MODISA
     Sensor=
     Title=MODISA Level-3 Standard Mapped Image
    ...
     Scaling=linear
     Scaling Equation=(Slope*l3m_data) + Intercept = Parameter value
     Slope=0.000717185
     Intercept=-2
     Scaled Data Minimum=-2
     Scaled Data Maximum=45
     Data Minimum=-1.999999
     Data Maximum=34.76
    Subdatasets:
     SUBDATASET_1_NAME=HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9":0
     SUBDATASET_1_DESC=[2160x4320] l3m_data (16-bit unsigned integer)
     SUBDATASET_2_NAME=HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9":1
     SUBDATASET_2_DESC=[2160x4320] l3m_qual (8-bit unsigned integer)

Now, to access this file with GMT we need to use the =gd mechanism and
append the name of the sub-dataset that we want to extract. Here, a
simple example using :doc:`grdinfo` would be

   ::

    grdinfo A20030012003365.L3m_YR_NSST_9=gd?HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9:0"

    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Title: Grid imported via GDAL
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Command:
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Remark:
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Gridline node registration used
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: Grid file format: gd = Import through GDAL (convert to float)
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: x_min: 0.5 x_max: 4319.5 x_inc: 1 name: x nx: 4320
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: y_min: 0.5 y_max: 2159.5 y_inc: 1 name: y ny: 2160
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: z_min: 0 z_max: 65535 name: z
    HDF4_SDS:UNKNOWN:A20030012003365.L3m_YR_NSST_9:0: scale_factor: 1 add_offset: 0

Be warned, however, that things are not yet completed because while the
data are scaled according to the equation printed above ("Scaling
Equation=(Slope\*l3m_data) + Intercept = Parameter value"), this
scaling is not applied by GDAL on reading so it cannot be done
automatically by GMT. One solution is to do the reading and scaling
via :doc:`grdmath` first, i.e.,

   ::

    gmt grdmath A20030012003365.L3m_YR_NSST_9=gd?HDF4_SDS:UNKNOWN:"A20030012003365.L3m_YR_NSST_9:0" \
                0.000717185 MUL -2 ADD = sst.nc

then plot the ``sst.nc`` directly.

Writing grids and images
~~~~~~~~~~~~~~~~~~~~~~~~

Saving images in the common raster formats is possible but, for the time
being, only from :doc:`grdimage` and even
that is restricted to raster type information. That is, vector data (for
instance, coast lines) or text will not be saved. To save an image with
:doc:`grdimage` use the
**-A**\ *outimg=driver* mechanism, where *driver* is the driver code
name used by GDAL (e.g. GTiff).

For all other programs that create grids, it is also possible to save
them using GDAL. To do it one need to use the =gd appended with the
necessary information regarding the driver and the data type to use.
Generically, =\ **gd**\ [/*scale*/*offset*\ [/*nan*][:<*driver*\ >[/\ *dataType*]]
where *driver* is the same as explained above and *dataType* is a 2 or 3
chars code from: u8\|u16\|i16\|u32\|i32\|float32, and where i\|u denotes
signed\|unsigned. If not provided the default type is float32. Both
driver names and data types are case insensitive. Note, that you have to
specify *nan* for integer types unless you whish that all NaN data values
are replaced by zero.

The NaN data value
------------------

For a variety of data processing and plotting tasks there is a need to
acknowledge that a data point is missing or unassigned. In the "old
days" such information was passed by letting a value like -9999.99 take
on the special meaning of "this is not really a value, it is missing".
The problem with this scheme is that -9999.99 (or any other floating
point value) may be a perfectly reasonable data value and in such a
scenario would be skipped. The solution adopted in GMT is to use the
IEEE concept Not-a-Number (NaN) for this purpose. Mathematically, a NaN
is what you get if you do an undefined mathematical operation like
0/0; in ASCII data files they appear as the textstring NaN. This
value is internally stored with a particular bit pattern defined by IEEE
so that special action can be taken when it is encountered by programs.
In particular, a standard library function called ``isnan`` is used to
test if a floating point is a NaN. GMT uses these tests extensively to
determine if a value is suitable for plotting or processing (if a NaN is
used in a calculation the result would become NaN as well). Data points
whose values equal NaN are not normally plotted (or plotted with the
special NaN color given in :doc:`gmt.conf`). Several tools such as
:doc:`xyz2grd`, :doc:`gmtmath`, and
:doc:`grdmath` can convert user data to NaN
and vice versa, thus facilitating arbitrary masking and clipping of data
sets. Note that a few computers do not have native IEEE hardware
support. At this point, this applies to some of the older Cray
super-computers. Users on such machines may have to adopt the old
'-9999.99' scheme to achieve the desired results.

Data records that contain NaN values for the *x* or *y* columns (or the
*z* column for cases when 3-D Cartesian data are expected) are usually
skipped during reading. However, the presence of these bad records can
be interpreted in two different ways, and this behavior is controlled by
the :ref:`IO_NAN_RECORDS <IO_NAN_RECORDS>` defaults parameter. The default setting (*gap*)
considers such records to indicate a gap in an otherwise continuous
series of points (e.g., a line), and programs can act upon this
information, e.g., not to draw a line across the gap or to break the
line into separate segments. The alternative setting (*bad*) makes no
such interpretation and simply reports back how many bad records were
skipped during reading; see Section `Data gap detection: The -g option`_ for details.

.. _Directory parameters:

Directory parameters
--------------------

GMT versions prior to GMT 5 relied on several environment variables
($GMT_SHAREDIR, $GMT_DATADIR, $GMT_USERDIR, and $GMT_TMPDIR), pointing
to folders with data files and program settings. Beginning with version
5, these locations are configurable with the :doc:`gmtset` utility.
The environment variables are still supported but are overridden by the
:ref:`directory parameters <DIR Parameters>` DIR_DATA, DIR_USER, and
DIR_TMP in :doc:`gmt.conf`.

Variable $GMT_SHAREDIR
    was sometimes required in previous GMT versions to locate the GMT
    share directory where all run-time support files such as coastlines,
    custom symbols, PostScript macros, color tables, and much more reside.
    If this parameter is not set (default), GMT will make a reasonable
    guess of the location of its share folder. Setting this variable is
    usually not required and recommended only under special circumstances.

Variable $GMT_DATADIR and parameter DIR_DATA
    may point to one or more directories where large and/or widely used
    data files can be placed. All GMT programs look in these directories
    when a file is specified on the command line and it is not present in
    the current directory. This allows maintainers to consolidate large
    data files and to simplify scripting that use these files since the
    absolute path need not be specified. Separate multiple directories
    with colons (:) -- under Windows use semi-colons (;). Any directory
    name that ends in a trailing slash (/) will be searched recursively
    (not under Windows).

Variable $GMT_USERDIR and parameter DIR_USER
    may point to a directory where the user places custom configuration
    files (e.g., an alternate ``coastline.conf`` file, preferred default
    settings in ``gmt.conf``, custom symbols and color palettes, math
    macros for :doc:`gmtmath` and :doc:`grdmath`, and shorthands for
    gridfile extensions via ``gmt_io``). Users may also place their own
    data files in this directory as GMT programs will search for files
    given on the command line in both DIR_DATA and DIR_USER.

Variable $GMT_TMPDIR and parameter DIR_TMP
    may indicate the location, where GMT will write its state parameters
    via the two files ``gmt.history`` and ``gmt.conf``. If DIR_TMP is not
    set, these files are written to the current directory. See Section
    `Isolation mode`_ for more information.

Note that files whose full path is given will never be searched for in
any of these directories.


GMT Coordinate Transformations
==============================

GMT programs read real-world coordinates and convert them to positions
on a plot. This is achieved by selecting one of several coordinate
transformations or projections. We distinguish between three sets of
such conversions:

-  Cartesian coordinate transformations

-  Polar coordinate transformations

-  Map coordinate transformations

The next chapter will be dedicated to GMT map projections in its
entirety. Meanwhile, the present chapter will summarize the properties
of the Cartesian and Polar coordinate transformations available in
GMT, list which parameters define them, and demonstrate how they are
used to create simple plot axes. We will mostly be using
:doc:`psbasemap` (and occasionally
:doc:`psxy`) to demonstrate the various
transformations. Our illustrations may differ from those you reproduce
with the same commands because of different settings in our ``gmt.conf`` file.)
Finally, note that while we will specify dimensions in inches (by
appending **i**), you may want to use cm (**c**), or points (**p**) as
unit instead (see the :doc:`gmt.conf` man page).

Cartesian transformations
-------------------------

GMT Cartesian coordinate transformations come in three flavors:

-  Linear coordinate transformation

-  Log\ :math:`_{10}` coordinate transformation

-  Power (exponential) coordinate transformation

These transformations convert input coordinates *(x,y)* to
locations *(x', y')* on a plot. There is no coupling between
*x* and *y* (i.e., *x' = f(x)* and *y' = f(y)*);
it is a **one-dimensional** projection. Hence, we may use separate
transformations for the *x*- and *y*-axes (and
*z*-axes for 3-D plots). Below, we will use the expression
*u' = f(u)*, where *u* is either *x* or *y* (or
*z* for 3-D plots). The coefficients in *f(u)* depend on the
desired plot size (or scale), the chosen *(x,y)* domain, and the
nature of *f* itself.

Two subsets of linear will be discussed separately; these are a polar
(cylindrical) projection and a linear projection applied to geographic
coordinates (with a 360 periodicity in the *x*-coordinate). We
will show examples of all of these projections using dummy data sets
created with :doc:`gmtmath`, a "Reverse
Polish Notation" (RPN) calculator that operates on or creates table data:

   ::

      gmt gmtmath -T0/100/1  T SQRT = sqrt.d
      gmt gmtmath -T0/100/10 T SQRT = sqrt.d10

Cartesian linear transformation (**-Jx** **-JX**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are in fact three different uses of the Cartesian linear
transformation, each associated with specific command line options. The
different manifestations result from specific properties of three kinds
of data:

#. Regular floating point coordinates

#. Geographic coordinates

#. Calendar time coordinates

.. figure:: /_images/GMT_arrows.*
   :width: 500 px
   :align: center

   Examples of Cartesian (left), circular (middle), and geo-vectors (right) for different
   attribute specifications. Note that both full and half arrow-heads can be specified, as well as no head at all.

Regular floating point coordinates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Selection of the Cartesian linear transformation with regular floating
point coordinates will result in a simple linear scaling
*u' = au + b* of the input coordinates. The projection is defined
by stating scale in inches/unit (**-Jx**) or axis length in inches (**-JX**).
If the *y*-scale or *y*-axis length is different from that of the
*x*-axis (which is most often the case), separate the two scales (or
lengths) by a slash, e.g., **-Jx**\ 0.1i/0.5i or **-JX**\ 8i/5i. Thus,
our :math:`y = \sqrt{x}` data sets will plot as shown in
Figure :ref:`Linear transformation of Cartesian coordinates <GMT_linear>`.

.. _GMT_linear:

.. figure:: /_images/GMT_linear.*
   :width: 400 px
   :align: center

   Linear transformation of Cartesian coordinates.


The complete commands given to produce this plot were

   ::

    gmt psxy -R0/100/0/10 -JX3i/1.5i -Bag -BWSne+gsnow -Wthick,blue,- -P -K sqrt.d > GMT_linear.ps
    gmt psxy -R -J -St0.1i -N -Gred -Wfaint -O sqrt.d10 >> GMT_linear.ps

Normally, the user's *x*-values will increase to the right and the
*y*-values will increase upwards. It should be noted that in many
situations it is desirable to have the direction of positive coordinates
be reversed. For example, when plotting depth on the *y*-axis it makes
more sense to have the positive direction downwards. All that is
required to reverse the sense of positive direction is to supply a
negative scale (or axis length). Finally, sometimes it is convenient to
specify the width (or height) of a map and let the other dimension be
computed based on the implied scale and the range of the other axis. To
do this, simply specify the length to be recomputed as 0.

Geographic coordinates
^^^^^^^^^^^^^^^^^^^^^^

.. _GMT_linear_d:

.. figure:: /_images/GMT_linear_d.*
   :width: 500 px
   :align: center

   Linear transformation of map coordinates.


While the Cartesian linear projection is primarily designed for regular
floating point *x*,\ *y* data, it is sometimes necessary to plot
geographical data in a linear projection. This poses a problem since
longitudes have a 360 periodicity. GMT therefore needs to be informed
that it has been given geographical coordinates even though a linear
transformation has been chosen. We do so by adding a **g** (for
geographical) or **d** (for degrees) directly after **-R** or by
appending a **g** or **d** to the end of the **-Jx** (or **-JX**)
option. As an example, we want to plot a crude world map centered on
125E. Our command will be

  ::

    gmt set MAP_GRID_CROSS_SIZE_PRIMARY 0.1i MAP_FRAME_TYPE FANCY FORMAT_GEO_MAP ddd:mm:ssF
    gmt pscoast -Rg-55/305/-90/90 -Jx0.014i -Bagf -BWSen -Dc -A1000 -Glightbrown -Wthinnest \
            -P -Slightblue > GMT_linear_d.ps

with the result reproduced in
Figure :ref:`Linear transformation of map coordinates <GMT_Linear_d>`.

Calendar time coordinates
^^^^^^^^^^^^^^^^^^^^^^^^^

.. _GMT_linear_cal:

.. figure:: /_images/GMT_linear_cal.*
   :width: 400 px
   :align: center

   Linear transformation of calendar coordinates.


Several particular issues arise when we seek to make linear plots using
calendar date/time as the input coordinates. As far as setting up the
coordinate transformation we must indicate whether our input data have
absolute time coordinates or relative time coordinates. For the former
we append **T** after the axis scale (or width), while for the latter we
append **t** at the end of the **-Jx** (or **-JX**) option. However,
other command line arguments (like the **-R** option) may already
specify whether the time coordinate is absolute or relative. An absolute
time entry must be given as [*date*\ ]\ **T**\ [*clock*\ ] (with *date*
given as *yyyy*\ [-*mm*\ [-*dd*]], *yyyy*\ [-*jjj*], or
*yyyy*\ [-**W**\ *ww*\ [-*d*]], and *clock* using the
*hh*\ [:*mm*\ [:*ss*\ [*.xxx*]]] 24-hour clock format) whereas the
relative time is simply given as the units of time since the epoch
followed by **t** (see :ref:`TIME_UNIT <TIME_UNIT>` and :ref:`TIME_EPOCH <TIME_EPOCH>` for
information on specifying the time unit and the epoch). As a simple
example, we will make a plot of a school week calendar
(Figure :ref:`Linear transformation of calendar coordinates <GMT_linear_cal>`).

When the coordinate ranges provided by the **-R** option and the
projection type given by **-JX** (including the optional **d**, **g**,
**t** or **T**) conflict, GMT will warn the users about it. In
general, the options provided with **-JX** will prevail.

   ::

    gmt set FORMAT_DATE_MAP o TIME_WEEK_START Sunday FORMAT_CLOCK_MAP=-hham \
            FORMAT_TIME_PRIMARY_MAP full
    gmt psbasemap -R2001-9-24T/2001-9-29T/T07:0/T15:0 -JX4i/-2i -Bxa1Kf1kg1d \
                  -Bya1Hg1h -BWsNe+glightyellow -P > GMT_linear_cal.ps

Cartesian logarithmic projection
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. _GMT_log:

.. figure:: /_images/GMT_log.*
   :width: 400 px
   :align: center

   Logarithmic transformation of x--coordinates.


The :math:`\log_{10}` transformation is simply
:math:`u' = a \log_{10}(u) + b` and is selected by appending an **l**
(lower case L) immediately following the scale (or axis length) value.
Hence, to produce a plot in which the *x*-axis is logarithmic (the
*y*-axis remains linear, i.e., a semi-log plot), try (Figure :ref:`Logarithmic
transformation <GMT_log>`)

   ::

    gmt psxy -R1/100/0/10 -Jx1.5il/0.15i -Bx2g3 -Bya2f1g2 -BWSne+gbisque \
             -Wthick,blue,- -P -K -h sqrt.d > GMT_log.ps
    gmt psxy -R -J -Ss0.1i -N -Gred -W -O -h sqrt.d10 >> GMT_log.ps

Note that if *x*- and *y*-scaling are different and a
:math:`\log_{10}-\log_{10}` plot is desired, the **l** must be
appended twice: Once after the *x*-scale (before the /) and once after
the *y*-scale.

Cartesian power projection
~~~~~~~~~~~~~~~~~~~~~~~~~~

.. _GMT_pow:

.. figure:: /_images/GMT_pow.*
   :width: 400 px
   :align: center

   Exponential or power transformation of x--coordinates.


This projection uses :math:`u' = a u^b + c` and allows us to explore
exponential relationships like :math:`x^p` versus :math:`y^q`.
While *p* and *q* can be any values, we will select *p
= 0.5* and *q = 1* which means we will plot *x* versus
:math:`\sqrt{x}`. We indicate this scaling by appending a **p** (lower
case P) followed by the desired exponent, in our case 0.5. Since
*q = 1* we do not need to specify **p**\ 1 since it is identical
to the linear transformation. Thus our command becomes (Figure :ref:`Power
transformation <GMT_pow>`)

   ::

    gmt psxy -R0/100/0/10 -Jx0.3ip0.5/0.15i -Bxa1p -Bya2f1 -BWSne+givory \
             -Wthick -P -K sqrt.d > GMT_pow.ps
    gmt psxy -R -J -Sc0.075i -Ggreen -W -O sqrt.d10 >> GMT_pow.ps

Linear projection with polar coordinates (**-Jp** **-JP**)
----------------------------------------------------------

.. _GMT_polar:

.. figure:: /_images/GMT_polar.*
   :width: 400 px
   :align: center

   Polar (Cylindrical) transformation of (:math:`\theta, r`) coordinates.


This transformation converts polar coordinates (angle :math:`\theta` and
radius *r*) to positions on a plot. Now :math:`x' = f(\theta,r)`
and :math:`y' = g(\theta,r)`, hence it is similar to a regular map
projection because *x* and *y* are coupled and *x*
(i.e., :math:`\theta`) has a 360 periodicity. With input and output
points both in the plane it is a **two-dimensional** projection. The
transformation comes in two flavors:

#. Normally, :math:`\theta` is understood to be directions
   counter-clockwise from the horizontal axis, but we may choose to
   specify an angular offset [whose default value is zero]. We will call
   this offset :math:`\theta_0`. Then,
   :math:`x' = f(\theta, r) = ar \cos (\theta-\theta_0) + b` and
   :math:`y' = g(\theta, r) = ar \sin (\theta-\theta_0) + c`.

#. Alternatively, :math:`\theta` can be interpreted to be azimuths
   clockwise from the vertical axis, yet we may again choose to specify
   the angular offset [whose default value is zero]. Then,
   :math:`x' = f(\theta, r) = ar \cos (90 - (\theta-\theta_0)) + b` and
   :math:`y' = g(\theta, r) = ar \sin (90 - (\theta-\theta_0)) + c`.

Consequently, the polar transformation is defined by providing

-  scale in inches/unit (**-Jp**) or full width of plot in inches (**-JP**)

-  Optionally, insert **a** after **p\ \| \ P** to indicate CW
   azimuths rather than CCW directions

-  Optionally, append /\ *origin* in degrees to indicate an angular offset [0]

-  Optionally, append **r** to reverse the radial direction (here,
   *south* and *north* must be elevations in 0--90 range).

-  Optionally, append **z** to annotate depths rather than radius.

As an example of this projection we will create a gridded data set in
polar coordinates :math:`z(\theta, r) = r^2 \cdot \cos{4\theta}` using
:doc:`grdmath`, a RPN calculator that
operates on or creates grid files.

   ::

    gmt grdmath -R0/360/2/4 -I6/0.1 X 4 MUL PI MUL 180 DIV COS Y 2 POW MUL = tt.nc
    gmt grdcontour tt.nc -JP3i -B30 -BNs+ghoneydew -P -C2 -S4 --FORMAT_GEO_MAP=+ddd > GMT_polar.ps

We used :doc:`grdcontour` to make a
contour map of this data. Because the data file only contains values
with :math:`2 \leq r \leq 4`, a donut shaped plot appears in
Figure :ref:`Polar transformation <GMT_polar>`.

GMT Map Projections
===================

GMT implements more than 30 different projections. They all project
the input coordinates longitude and latitude to positions on a map. In
general, *x' = f(x,y,z)* and *y' = g(x,y,z)*, where
*z* is implicitly given as the radial vector length to the
*(x,y)* point on the chosen ellipsoid. The functions *f* and
*g* can be quite nasty and we will refrain from presenting details
in this document. The interested read is referred to *Snyder*
[1987] [20]_. We will mostly be using the
:doc:`pscoast` command to demonstrate each of
the projections. GMT map projections are grouped into four categories
depending on the nature of the projection. The groups are

#. Conic map projections

#. Azimuthal map projections

#. Cylindrical map projections

#. Miscellaneous projections

Because *x* and *y* are coupled we can only specify one
plot-dimensional scale, typically a map *scale* (for lower-case map
projection code) or a map *width* (for upper-case map projection code).
However, in some cases it would be more practical to specify map
*height* instead of *width*, while in other situations it would be nice
to set either the *shortest* or *longest* map dimension. Users may
select these alternatives by appending a character code to their map
dimension. To specify map *height*, append **h** to the given dimension;
to select the minimum map dimension, append **-**, whereas you may
append **+** to select the maximum map dimension. Without the modifier
the map width is selected by default.

In GMT version 4.3.0 we noticed we ran out of the alphabet for
1-letter (and sometimes 2-letter) projection codes. To allow more
flexibility, and to make it easier to remember the codes, we implemented
the option to use the abbreviations used by the **Proj4** mapping
package. Since some of the GMT projections are not in **Proj4**, we
invented some of our own as well. For a full list of both the old 1- and
2-letter codes, as well as the **Proj4**-equivalents see the quick
reference cards in Section `GMT quick reference`_. For example, **-JM**\ 15c and
**-JMerc**\ /15c have the same meaning.

Conic projections
-----------------

Albers conic equal-area projection (**-Jb** **-JB**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This projection, developed by Albers in 1805, is predominantly used to
map regions of large east-west extent, in particular the United States.
It is a conic, equal-area projection, in which parallels are unequally
spaced arcs of concentric circles, more closely spaced at the north and
south edges of the map. Meridians, on the other hand, are equally spaced
radii about a common center, and cut the parallels at right angles.
Distortion in scale and shape vanishes along the two standard parallels.
Between them, the scale along parallels is too small; beyond them it is
too large. The opposite is true for the scale along meridians. To define
the projection in GMT you need to provide the following information:

-  Longitude and latitude of the projection center.

-  Two standard parallels.

-  Map scale in inch/degree or 1:xxxxx notation (**-Jb**), or map width (**-JB**).

Note that you must include the "1:" if you choose to specify the scale
that way. E.g., you can say 0.5 which means 0.5 inch/degree or 1:200000
which means 1 inch on the map equals 200,000 inches along the standard
parallels. The projection center defines the origin of the rectangular
map coordinates. As an example we will make a map of the region near
Taiwan. We choose the center of the projection to be at 125 E/20 N and
25 N and 45 N as our two standard parallels. We desire a map that is 5
inches wide. The complete command needed to generate the map below is
therefore given by:

   ::

    gmt set MAP_GRID_CROSS_SIZE_PRIMARY 0
    gmt pscoast -R110/140/20/35 -JB125/20/25/45/5i -Bag -Dl -Ggreen -Wthinnest \
                -A250 -P > GMT_albers.ps

.. figure:: /_images/GMT_albers.*
   :width: 500 px
   :align: center

   Albers equal-area conic map projection.


Equidistant conic projection (**-Jd** **-JD**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The equidistant conic projection was described by the Greek philosopher
Claudius Ptolemy about A.D. 150. It is neither conformal or equal-area,
but serves as a compromise between them. The scale is true along all
meridians and the standard parallels. To select this projection in
GMT you must provide the same information as for the other conic
projection, i.e.,

-  Longitude and latitude of the projection center.

-  Two standard parallels.

-  Map scale in inch/degree or 1:xxxxx notation (**-Jd**), or map width (**-JD**).

The equidistant conic projection is often used for atlases with maps of
small countries. As an example, we generate a map of Cuba:

   ::

    gmt set FORMAT_GEO_MAP ddd:mm:ssF MAP_GRID_CROSS_SIZE_PRIMARY 0.05i
    gmt pscoast -R-88/-70/18/24 -JD-79/21/19/23/4.5i -Bag -Di -N1/thick,red \
                -Glightgreen -Wthinnest -P > GMT_equidistant_conic.ps

.. figure:: /_images/GMT_equidistant_conic.*
   :width: 500 px
   :align: center

   Equidistant conic map projection.


Lambert conic conformal projection (**-Jl** **-JL**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This conic projection was designed by the Alsatian mathematician Johann
Heinrich Lambert (1772) and has been used extensively for mapping of
regions with predominantly east-west orientation, just like the Albers
projection. Unlike the Albers projection, Lambert's conformal projection
is not equal-area. The parallels are arcs of circles with a common
origin, and meridians are the equally spaced radii of these circles. As
with Albers projection, it is only the two standard parallels that are
distortion-free. To select this projection in GMT you must provide the
same information as for the Albers projection, i.e.,

-  Longitude and latitude of the projection center.

-  Two standard parallels.

-  Map scale in inch/degree or 1:xxxxx notation (**-Jl**), or map width (**-JL**).

The Lambert conformal projection has been used for basemaps for all the
48 contiguous States with the two fixed standard parallels 33N and 45N.
We will generate a map of the continental USA using these parameters.
Note that with all the projections you have the option of selecting a
rectangular border rather than one defined by meridians and parallels.
Here, we choose the regular WESN region, a "fancy" basemap frame, and
use degrees west for longitudes. The generating commands used were

   ::

    gmt set MAP_FRAME_TYPE FANCY FORMAT_GEO_MAP ddd:mm:ssF MAP_GRID_CROSS_SIZE_PRIMARY 0.05i
    gmt pscoast -R-130/-70/24/52 -Jl-100/35/33/45/1:50000000 -Bag -Dl -N1/thick,red \
                -N2/thinner -A500 -Gtan -Wthinnest,white -Sblue -P > GMT_lambert_conic.ps

.. figure:: /_images/GMT_lambert_conic.*
   :width: 500 px
   :align: center

   Lambert conformal conic map projection.


The choice for projection center does not affect the projection but it
indicates which meridian (here 100W) will be vertical on the map. The
standard parallels were originally selected by Adams to provide a
maximum scale error between latitudes 30.5N and 47.5N of 0.5--1%. Some
areas, like Florida, experience scale errors of up to 2.5%.

(American) polyconic projection (**-Jpoly** **-JPoly**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The polyconic projection, in Europe usually referred to as the American
polyconic projection, was introduced shortly before 1820 by the
Swiss-American cartographer Ferdinand Rodulph Hassler (1770--1843). As
head of the Survey of the Coast, he was looking for a projection that
would give the least distortion for mapping the coast of the United
States. The projection acquired its name from the construction of each
parallel, which is achieved by projecting the parallel onto the cone
while it is rolled around the globe, along the central meridian, tangent
to that parallel. As a consequence, the projection involves many cones
rather than a single one used in regular conic projections.

The polyconic projection is neither equal-area, nor conformal. It is
true to scale without distortion along the central meridian. Each
parallel is true to scale as well, but the meridians are not as they get
further away from the central meridian. As a consequence, no parallel is
standard because conformity is lost with the lengthening of the meridians.

Below we reproduce the illustration by *Snyder* [1987], with a gridline
every 10 and annotations only every 30 in longitude:

   ::

    gmt pscoast -R-180/-20/0/90 -JPoly/4i -Bx30g10 -By10g10 -Dc -A1000 -Glightgray \
                -Wthinnest -P > GMT_polyconic.ps

.. figure:: /_images/GMT_polyconic.*
   :width: 500 px
   :align: center

   (American) polyconic projection.


Azimuthal projections
---------------------

Lambert Azimuthal Equal-Area (**-Ja** **-JA**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This projection was developed by Lambert in 1772 and is typically used
for mapping large regions like continents and hemispheres. It is an
azimuthal, equal-area projection, but is not perspective. Distortion is
zero at the center of the projection, and increases radially away from
this point. To define this projection in GMT you must provide the
following information:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (<= 180, default is 90).

-  Scale as 1:xxxxx or as radius/latitude where radius is the projected
   distance on the map from projection center to an oblique latitude
   (**-Ja**), or map width (**-JA**).

Two different types of maps can be made with this projection depending
on how the region is specified. We will give examples of both types.

Rectangular map
^^^^^^^^^^^^^^^

In this mode we define our region by specifying the longitude/latitude
of the lower left and upper right corners instead of the usual *west,
east, south, north* boundaries. The reason for specifying our area this
way is that for this and many other projections, lines of equal
longitude and latitude are not straight lines and are thus poor choices
for map boundaries. Instead we require that the map boundaries be
rectangular by defining the corners of a rectangular map boundary. Using
0E/40S (lower left) and 60E/10S (upper right) as our corners we try

   ::

    gmt set FORMAT_GEO_MAP ddd:mm:ssF MAP_GRID_CROSS_SIZE_PRIMARY 0
    gmt pscoast -R0/-40/60/-10r -JA30/-30/4.5i -Bag -Dl -A500 -Gp300/10 \
                -Wthinnest -P > GMT_lambert_az_rect.ps

.. figure:: /_images/GMT_lambert_az_rect.*
   :width: 500 px
   :align: center

   Rectangular map using the Lambert azimuthal equal-area projection.


Note that an "r" is appended to the **-R** option to inform GMT that
the region has been selected using the rectangle technique, otherwise it
would try to decode the values as *west, east, south, north* and report
an error since *'east'* < *'west'*.

Hemisphere map
^^^^^^^^^^^^^^

Here, you must specify the world as your region (**-Rg** or
**-Rd**). E.g., to obtain a hemisphere view that shows the Americas, try

   ::

    gmt pscoast -Rg -JA280/30/3.5i -Bg -Dc -A1000 -Gnavy -P > GMT_lambert_az_hemi.ps

.. figure:: /_images/GMT_lambert_az_hemi.*
   :width: 400 px
   :align: center

   Hemisphere map using the Lambert azimuthal equal-area projection.


To geologists, the Lambert azimuthal equal-area projection (with origin
at 0/0) is known as the *equal-area* (Schmidt) stereonet and used for
plotting fold axes, fault planes, and the like. An *equal-angle* (Wulff)
stereonet can be obtained by using the stereographic projection
(discussed later). The stereonets produced by these two projections appear below.

.. _GMT_stereonets:

.. figure:: /_images/GMT_stereonets.*
   :width: 500 px
   :align: center

   Equal-Area (Schmidt) and Equal-Angle (Wulff) stereo nets.


Stereographic Equal-Angle projection (**-Js** **-JS**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is a conformal, azimuthal projection that dates back to the Greeks.
Its main use is for mapping the polar regions. In the polar aspect all
meridians are straight lines and parallels are arcs of circles. While
this is the most common use it is possible to select any point as the
center of projection. The requirements are

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (< 180, default is 90).

-  Scale as 1:xxxxx (true scale at pole), slat/1:xxxxx (true scale at
   standard parallel slat), or radius/latitude where radius is distance
   on map in inches from projection center to a particular [possibly
   oblique] latitude (**-Js**), or simply map width (**-JS**).

A default map scale factor of 0.9996 will be applied by default
(although you may change this with :ref:`PROJ_SCALE_FACTOR <PROJ_SCALE_FACTOR>`). However,
the setting is ignored when a standard parallel has been specified since
the scale is then implicitly given. We will look at two different types
of maps.

Polar Stereographic Map
^^^^^^^^^^^^^^^^^^^^^^^

In our first example we will let the projection center be at the north
pole. This means we have a polar stereographic projection and the map
boundaries will coincide with lines of constant longitude and latitude.
An example is given by

   ::

    gmt pscoast -R-30/30/60/72 -Js0/90/4.5i/60 -B10g -Dl -A250 -Groyalblue \
                -Sseashell -P > GMT_stereographic_polar.ps

.. figure:: /_images/GMT_stereographic_polar.*
   :width: 500 px
   :align: center

   Polar stereographic conformal projection.


Rectangular stereographic map
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As with Lambert's azimuthal equal-area projection we have the option to
use rectangular boundaries rather than the wedge-shape typically
associated with polar projections. This choice is defined by selecting
two points as corners in the rectangle and appending an "r" to the
**-R** option. This command produces a map as presented in
Figure :ref:`Polar stereographic <GMT_stereographic_rect>`:

   ::

    gmt set MAP_ANNOT_OBLIQUE 30
    gmt pscoast -R-25/59/70/72r -JS10/90/11c -B20g -Dl -A250 -Gdarkbrown -Wthinnest \
                -Slightgray -P > GMT_stereographic_rect.ps

.. _GMT_stereographic_rect:

.. figure:: /_images/GMT_stereographic_rect.*
   :width: 500 px
   :align: center

   Polar stereographic conformal projection with rectangular borders.


General stereographic map
^^^^^^^^^^^^^^^^^^^^^^^^^

In terms of usage this projection is identical to the Lambert azimuthal
equal-area projection. Thus, one can make both rectangular and
hemispheric maps. Our example shows Australia using a projection pole at
130E/30S. The command used was

   ::

    gmt set MAP_ANNOT_OBLIQUE 0
    gmt pscoast -R100/-42/160/-8r -JS130/-30/4i -Bag -Dl -A500 -Ggreen -Slightblue \
                -Wthinnest -P > GMT_stereographic_general.ps

.. figure:: /_images/GMT_stereographic_general.*
   :width: 500 px
   :align: center

   General stereographic conformal projection with rectangular borders.


By choosing 0/0 as the pole, we obtain the conformal stereonet presented
next to its equal-area cousin in the Section `Lambert Azimuthal Equal-Area (-Ja -JA)`_ on the Lambert
azimuthal equal-area projection (Figure :ref:`Stereonets <GMT_stereonets>`).

Perspective projection (**-Jg** **-JG**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The perspective projection imitates in 2 dimensions the 3-dimensional
view of the earth from space. The implementation in GMT is very
flexible, and thus requires many input variables. Those are listed and
explained below, with the values used in
Figure :ref:`Perspective projection <GMT_perspective>` between brackets.

-  Longitude and latitude of the projection center (4E/52N).

-  Altitude of the viewer above sea level in kilometers (230 km). If
   this value is less than 10, it is assumed to be the distance of the
   viewer from the center of the earth in earth radii. If an "r" is
   appended, it is the distance from the center of the earth in
   kilometers.

-  Azimuth in degrees (90, due east). This is the direction in which you
   are looking, measured clockwise from north.

-  Tilt in degrees (60). This is the viewing angle relative to zenith.
   So a tilt of 0 is looking straight down, 60 is looking from 30 above
   the horizon.

-  Twist in degrees (180). This is the boresight rotation (clockwise) of
   the image. The twist of 180 in the example mimics the fact that the
   Space Shuttle flies upside down.

-  Width and height of the viewpoint in degrees (60). This number
   depends on whether you are looking with the naked eye (in which case
   you view is about 60 wide), or with binoculars, for example.

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in inches from projection center to a particular [possibly
   oblique] latitude (**-Jg**), or map width (**-JG**) (5 inches).

The imagined view of northwest Europe from a Space Shuttle at 230 km
looking due east is thus accomplished by the following
:doc:`pscoast` command:

   ::

    gmt pscoast -Rg -JG4/52/230/90/60/180/60/60/5i -Bx2g2 -By1g1 -Ia -Di -Glightbrown \
                -Wthinnest -P -Slightblue --MAP_ANNOT_MIN_SPACING=0.25i > GMT_perspective.ps

.. _GMT_perspective:

.. figure:: /_images/GMT_perspective.*
   :width: 500 px
   :align: center

   View from the Space Shuttle in Perspective projection.


Orthographic projection (**-Jg** **-JG**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The orthographic azimuthal projection is a perspective projection from
infinite distance. It is therefore often used to give the appearance of
a globe viewed from outer space. As with Lambert's equal-area and the
stereographic projection, only one hemisphere can be viewed at any time.
The projection is neither equal-area nor conformal, and much distortion
is introduced near the edge of the hemisphere. The directions from the
center of projection are true. The projection was known to the Egyptians
and Greeks more than 2,000 years ago. Because it is mainly used for
pictorial views at a small scale, only the spherical form is necessary.

To specify the orthographic projection the same options **-Jg** or
**-JG** as the perspective projection are used, but with fewer variables to supply:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (<= 90, default is 90).

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in inches from projection center to a particular [possibly
   oblique] latitude (**-Jg**), or map width (**-JG**).

Our example of a perspective view centered on 75W/40N can therefore be
generated by the following :doc:`pscoast` command:

   ::

    gmt pscoast -Rg -JG-75/41/4.5i -Bg -Dc -A5000 -Gpink -Sthistle -P > GMT_orthographic.ps

.. figure:: /_images/GMT_orthographic.*
   :width: 400 px
   :align: center

   Hemisphere map using the Orthographic projection.


Azimuthal Equidistant projection (**-Je** **-JE**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The most noticeable feature of this azimuthal projection is the fact
that distances measured from the center are true. Therefore, a circle
about the projection center defines the locus of points that are equally
far away from the plot origin. Furthermore, directions from the center
are also true. The projection, in the polar aspect, is at least several
centuries old. It is a useful projection for a global view of locations
at various or identical distance from a given point (the map center).

To specify the azimuthal equidistant projection you must supply:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (<= 180, default is 180).

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in inches from projection center to a particular [possibly
   oblique] latitude (**-Je**), or map width (**-JE**).

Our example of a global view centered on 100W/40N can therefore be
generated by the following :doc:`pscoast`
command. Note that the antipodal point is 180 away from the center, but
in this projection this point plots as the entire map perimeter:

   ::

    gmt pscoast -Rg -JE-100/40/4.5i -Bg -Dc -A10000 -Glightgray -Wthinnest -P \
                > GMT_az_equidistant.ps

.. figure:: /_images/GMT_az_equidistant.*
   :width: 400 px
   :align: center

   World map using the equidistant azimuthal projection.


Gnomonic projection (**-Jf** **-JF**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Gnomonic azimuthal projection is a perspective projection from the
center onto a plane tangent to the surface. Its origin goes back to the
old Greeks who used it for star maps almost 2500 years ago. The
projection is neither equal-area nor conformal, and much distortion is
introduced near the edge of the hemisphere; in fact, less than a
hemisphere may be shown around a given center. The directions from the
center of projection are true. Great circles project onto straight
lines. Because it is mainly used for pictorial views at a small scale,
only the spherical form is necessary.

To specify the Gnomonic projection you must supply:

-  Longitude and latitude of the projection center.

-  Optionally, the horizon, i.e., the number of degrees from the center
   to the edge (< 90, default is 60).

-  Scale as 1:xxxxx or as radius/latitude where radius is distance on
   map in inches from projection center to a particular [possibly
   oblique] latitude (**-Jf**), or map width (**-JF**).

Using a horizon of 60, our example of this projection centered on
120W/35N can therefore be generated by the following :doc:`pscoast` command:

   ::

    gmt pscoast -Rg -JF-120/35/60/4.5i -B30g15 -Dc -A10000 -Gtan -Scyan -Wthinnest \
                -P > GMT_gnomonic.ps

.. figure:: /_images/GMT_gnomonic.*
   :width: 500 px
   :align: center

   Gnomonic azimuthal projection.


Cylindrical projections
-----------------------

Cylindrical projections are easily recognized for its shape: maps are
rectangular and meridians and parallels are straight lines crossing at
right angles. But that is where similarities between the cylindrical
projections supported by GMT (Mercator, transverse Mercator, universal
transverse Mercator, oblique Mercator, Cassini, cylindrical equidistant,
cylindrical equal-area, Miller, and cylindrical stereographic
projections) stops. Each have a different way of spacing the meridians
and parallels to obtain certain desirable cartographic properties.

Mercator projection (**-Jm** **-JM**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Probably the most famous of the various map projections, the Mercator
projection takes its name from the Flemish cartographer Gheert Cremer,
better known as Gerardus Mercator, who presented it in 1569. The
projection is a cylindrical and conformal, with no distortion along the
equator. A major navigational feature of the projection is that a line
of constant azimuth is straight. Such a line is called a rhumb line or
*loxodrome*. Thus, to sail from one point to another one only had to
connect the points with a straight line, determine the azimuth of the
line, and keep this constant course for the entire voyage [21]_. The
Mercator projection has been used extensively for world maps in which
the distortion towards the polar regions grows rather large, thus
incorrectly giving the impression that, for example, Greenland is larger
than South America. In reality, the latter is about eight times the size
of Greenland. Also, the Former Soviet Union looks much bigger than
Africa or South America. One may wonder whether this illusion has had
any influence on U.S. foreign policy.

In the regular Mercator projection, the cylinder touches the globe along
the equator. Other orientations like vertical and oblique give rise to
the Transverse and Oblique Mercator projections, respectively. We will
discuss these generalizations following the regular Mercator projection.

The regular Mercator projection requires a minimum of parameters. To use
it in GMT programs you supply this information (the first two items
are optional and have defaults):

-  Central meridian [Middle of your map].

-  Standard parallel for true scale [Equator]. When supplied, central
   meridian must be supplied as well.

-  Scale along the equator in inch/degree or 1:xxxxx (**-Jm**), or map
   width (**-JM**).

Our example presents a world map at a scale of 0.012 inch pr degree
which will give a map 4.32 inch wide. It was created with the command:

   ::

    gmt set MAP_FRAME_TYPE fancy
    gmt pscoast -R0/360/-70/70 -Jm1.2e-2i -Bxa60f15 -Bya30f15 -Dc -A5000 -Gred \
                -P > GMT_mercator.ps

.. figure:: /_images/GMT_mercator.*
   :width: 500 px
   :align: center

   Simple Mercator map.


While this example is centered on the Dateline, one can easily choose
another configuration with the **-R** option. A map centered on
Greenwich would specify the region with **-R**-180/180/-70/70.

Transverse Mercator projection (**-Jt** **-JT**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The transverse Mercator was invented by Lambert in 1772. In this
projection the cylinder touches a meridian along which there is no
distortion. The distortion increases away from the central meridian and
goes to infinity at 90 from center. The central meridian, each meridian
90 away from the center, and equator are straight lines; other parallels
and meridians are complex curves. The projection is defined by
specifying:

-  The central meridian.

-  Optionally, the latitude of origin (default is the equator).

-  Scale along the equator in inch/degree or 1:xxxxx (**-Jt**), or map
   width (**-JT**).

The optional latitude of origin defaults to Equator if not specified.
Although defaulting to 1, you can change the map scale factor via the
:ref:`PROJ_SCALE_FACTOR <PROJ_SCALE_FACTOR>` parameter. Our example shows a transverse
Mercator map of south-east Europe and the Middle East with 35E as the
central meridian:

   ::

    gmt pscoast -R20/30/50/45r -Jt35/0.18i -Bag -Dl -A250 -Glightbrown -Wthinnest \
                -P -Sseashell > GMT_transverse_merc.ps

.. figure:: /_images/GMT_transverse_merc.*
   :width: 500 px
   :align: center

   Rectangular Transverse Mercator map.


The transverse Mercator can also be used to generate a global map - the
equivalent of the 360 Mercator map. Using the command

   ::

    gmt pscoast -R0/360/-80/80 -JT330/-45/3.5i -Ba30g -BWSne -Dc -A2000 \
                -Slightblue -G0 -P > GMT_TM.ps

we made the map illustrated in Figure :ref:`Global transverse Mercator
<GMT_TM>`. Note that
when a world map is given (indicated by **-R**\ *0/360/s/n*), the
arguments are interpreted to mean oblique degrees, i.e., the 360 range
is understood to mean the extent of the plot along the central meridian,
while the "south" and "north" values represent how far from the central
longitude we want the plot to extend. These values correspond to
latitudes in the regular Mercator projection and must therefore be less
than 90.

.. _GMT_TM:

.. figure:: /_images/GMT_TM.*
   :width: 450 px
   :align: center

   A global transverse Mercator map.


Universal Transverse Mercator (UTM) projection (**-Ju** **-JU**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A particular subset of the transverse Mercator is the Universal
Transverse Mercator (UTM) which was adopted by the US Army for
large-scale military maps. Here, the globe is divided into 60 zones
between 84S and 84N, most of which are 6 wide. Each of these UTM zones
have their unique central meridian. Furthermore, each zone is divided
into latitude bands but these are not needed to specify the projection
for most cases. See Figure :ref:`Universal Transverse Mercator
<GMT_utm_zones>` for all zone designations.

.. _GMT_utm_zones:

.. figure:: /_images/GMT_utm_zones.*
   :width: 700 px
   :align: center

   Universal Transverse Mercator zone layout.


GMT implements both the transverse Mercator and the UTM projection.
When selecting UTM you must specify:

-  UTM zone (A, B, 1--60, Y, Z). Use negative values for numerical zones
   in the southern hemisphere or append the latitude modifiers C--H, J--N,
   P--X) to specify an exact UTM grid zone.

-  Scale along the equator in inch/degree or 1:xxxxx (**-Ju**), or map
   width (**-JU**).

In order to minimize the distortion in any given zone, a scale factor of
0.9996 has been factored into the formulae. (although a standard, you
can change this with :ref:`PROJ_SCALE_FACTOR <PROJ_SCALE_FACTOR>`). This makes the UTM
projection a *secant* projection and not a *tangent* projection like the
transverse Mercator above. The scale only varies by 1 part in 1,000 from
true scale at equator. The ellipsoidal projection expressions are
accurate for map areas that extend less than 10 away from the central
meridian. For larger regions we use the conformal latitude in the
general spherical formulae instead.

Oblique Mercator projection (**-Jo** **-JO**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Oblique configurations of the cylinder give rise to the oblique Mercator
projection. It is particularly useful when mapping regions of large
lateral extent in an oblique direction. Both parallels and meridians are
complex curves. The projection was developed in the early 1900s by
several workers. Several parameters must be provided to define the
projection. GMT offers three different definitions:

#. Option **-Joa** or **-JOa**:

   -  Longitude and latitude of projection center.

   -  Azimuth of the oblique equator.

   -  Scale in inch/degree or 1:xxxxx along oblique equator (**-Joa**),
      or map width (**-JOa**).

#. Option **-Job** or **-JOb**:

   -  Longitude and latitude of projection center.

   -  Longitude and latitude of second point on oblique equator.

   -  Scale in inch/degree or 1:xxxxx along oblique equator (**-Job**),
      or map width (**-JOb**).

#. Option **-Joc** or **-JOc**:

   -  Longitude and latitude of projection center.

   -  Longitude and latitude of projection pole.

   -  Scale in inch/degree or 1:xxxxx along oblique equator (**-Joc**),
      or map width (**-JOc**).

Our example was produced by the command

   ::

    gmt pscoast -R270/20/305/25r -JOc280/25.5/22/69/4.8i -Bag -Di -A250 -Gburlywood \
                -Wthinnest -P -Tf301.5/23/0.4i/2 -Sazure --FONT_TITLE=8p \
                --MAP_TITLE_OFFSET=0.05i > GMT_obl_merc.ps

.. figure:: /_images/GMT_obl_merc.*
   :width: 500 px
   :align: center

   Oblique Mercator map using **-Joc**. We make it clear which direction is North by
   adding a star rose with the **-T** option.


It uses definition 3 for an oblique view of some Caribbean islands. Note
that we define our region using the rectangular system described
earlier. If we do not append an "r" to the **-R** string then the
information provided with the **-R** option is assumed to be oblique
degrees about the projection center rather than the usual geographic
coordinates. This interpretation is chosen since in general the
parallels and meridians are not very suitable as map boundaries.

Cassini cylindrical projection (**-Jc** **-JC**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This cylindrical projection was developed in 1745 by César-François
Cassini de Thury for the survey of France. It is occasionally called
Cassini-Soldner since the latter provided the more accurate mathematical
analysis that led to the development of the ellipsoidal formulae. The
projection is neither conformal nor equal-area, and behaves as a
compromise between the two end-members. The distortion is zero along the
central meridian. It is best suited for mapping regions of north-south
extent. The central meridian, each meridian 90 away, and equator are
straight lines; all other meridians and parallels are complex curves.
The requirements to define this projection are:

-  Longitude and latitude of central point.

-  Scale in inch/degree or as 1:xxxxx (**-Jc**), or map width (**-JC**).

A detailed map of the island of Sardinia centered on the 845'E meridian
using the Cassini projection can be obtained by running the command:

   ::

    gmt pscoast -R7:30/38:30/10:30/41:30r -JC8.75/40/2.5i -Bafg -Lf9.5/38.8/40/60 -Gspringgreen \
                -Dh -Sazure -Wthinnest -Ia/thinner -P --FONT_LABEL=12p > GMT_cassini.ps

.. figure:: /_images/GMT_cassini.*
   :width: 400 px
   :align: center

   Cassini map over Sardinia.


As with the previous projections, the user can choose between a
rectangular boundary (used here) or a geographical (WESN) boundary.

Cylindrical equidistant projection (**-Jq** **-JQ**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This simple cylindrical projection is really a linear scaling of
longitudes and latitudes. The most common form is the Plate Carrée
projection, where the scaling of longitudes and latitudes is the same.
All meridians and parallels are straight lines. The projection can be
defined by:

-  The central meridian [Middle of your map].

-  Standard parallel [Equator].

-  Scale in inch/degree or as 1:xxxxx (**-Jq**), or map width (**-JQ**).

The first two of these are optional and have defaults. When the standard
parallel is defined, the central meridian must be supplied as well.

A world map centered on the dateline using this projection can be
obtained by running the command:

   ::

    gmt pscoast -Rg -JQ4.5i -B60f30g30 -Dc -A5000 -Gtan4 -Slightcyan -P > GMT_equi_cyl.ps

.. figure:: /_images/GMT_equi_cyl.*
   :width: 500 px
   :align: center

   World map using the Plate Carr\'{e}e projection.


Different relative scalings of longitudes and latitudes can be obtained
by selecting a standard parallel different from the equator. Some
selections for standard parallels have practical properties as shown in
Table :ref:`JQ <tbl-JQ>`.

.. _tbl-JQ:

+-----------------------------------------------------+--------+
+=====================================================+========+
| Grafarend and Niermann, minimum linear distortion   | 61.7   |
+-----------------------------------------------------+--------+
| Ronald Miller Equirectangular                       | 50.5   |
+-----------------------------------------------------+--------+
| Ronald Miller, minimum continental distortion       | 43.5   |
+-----------------------------------------------------+--------+
| Grafarend and Niermann                              | 42     |
+-----------------------------------------------------+--------+
| Ronald Miller, minimum overall distortion           | 37.5   |
+-----------------------------------------------------+--------+
| Plate Carrée, Simple Cylindrical, Plain/Plane       | 0      |
+-----------------------------------------------------+--------+

Cylindrical equal-area projections (**-Jy** **-JY**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This cylindrical projection is actually several projections, depending
on what latitude is selected as the standard parallel. However, they are
all equal area and hence non-conformal. All meridians and parallels are
straight lines. The requirements to define this projection are:

-  The central meridian.

-  The standard parallel.

-  Scale in inch/degree or as 1:xxxxx (**-Jy**), or map width (**-JY**)

While you may choose any value for the standard parallel and obtain your
own personal projection, there are seven choices of standard parallels
that result in known (or named) projections. These are listed in Table :ref:`JY <tbl-JY>`.

.. _tbl-JY:

+-------------------+---------------------+
+===================+=====================+
| Balthasart        | 50                  |
+-------------------+---------------------+
| Gall-Peters       | 45                  |
+-------------------+---------------------+
| Hobo-Dyer         | 3730' (= 37.5)      |
+-------------------+---------------------+
| Trystan Edwards   | 3724' (= 37.4)      |
+-------------------+---------------------+
| Caster            | 3704' (= 37.0666)   |
+-------------------+---------------------+
| Behrman           | 30                  |
+-------------------+---------------------+
| Lambert           | 0                   |
+-------------------+---------------------+

For instance, a world map centered on the 35E meridian using the Behrman
projection (Figure :ref:`Behrman cylindrical projection <GMT_general_cyl>`)
can be obtained by running the command:

   ::

    gmt pscoast -R-145/215/-90/90 -JY35/30/4.5i -B45g45 -Dc -A10000 -Sdodgerblue \
                -Wthinnest -P > GMT_general_cyl.ps

.. _GMT_general_cyl:

.. figure:: /_images/GMT_general_cyl.*
   :width: 600 px
   :align: center

   World map using the Behrman cylindrical equal-area projection.


As one can see there is considerable distortion at high latitudes since
the poles map into lines.

Miller Cylindrical projection (**-Jj** **-JJ**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This cylindrical projection, presented by Osborn Maitland Miller of the
American Geographic Society in 1942, is neither equal nor conformal. All
meridians and parallels are straight lines. The projection was designed
to be a compromise between Mercator and other cylindrical projections.
Specifically, Miller spaced the parallels by using Mercator's formula
with 0.8 times the actual latitude, thus avoiding the singular poles;
the result was then divided by 0.8. There is only a spherical form for
this projection. Specify the projection by:

-  Optionally, the central meridian (default is the middle of your map).

-  Scale in inch/degree or as 1:xxxxx (**-Jj**), or map width (**-JJ**).

For instance, a world map centered on the 90E meridian at a map scale of
1:400,000,000 (Figure :ref:`Miller projection <GMT_miller>`) can be obtained as
follows:

   ::

    gmt pscoast -R-90/270/-80/90 -Jj1:400000000 -Bx45g45 -By30g30 -Dc -A10000 \
                -Gkhaki -Wthinnest -P -Sazure > GMT_miller.ps

.. _GMT_miller:

.. figure:: /_images/GMT_miller.*
   :width: 500 px
   :align: center

   World map using the Miller cylindrical projection.


Cylindrical stereographic projections (**-Jcyl_stere** **-JCyl_stere**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The cylindrical stereographic projections are certainly not as notable
as other cylindrical projections, but are still used because of their
relative simplicity and their ability to overcome some of the downsides
of other cylindrical projections, like extreme distortions of the higher
latitudes. The stereographic projections are perspective projections,
projecting the sphere onto a cylinder in the direction of the antipodal
point on the equator. The cylinder crosses the sphere at two standard
parallels, equidistant from the equator. The projections are defined by:

-  The central meridian (uses the middle of the map when omitted).

-  The standard parallel (default is the Equator). When used, central
   meridian needs to be given as well.

-  Scale in inch/degree or as 1:xxxxx (**-Jcyl_stere**), or map width
   (**-JCyl_stere**)

Some of the selections of the standard parallel are named for the
cartographer or publication that popularized the projection
(Table :ref:`JCylstere <tbl-JCylstere>`).

.. _tbl-JCylstere:

+---------------------------------------------------------+-------------+
+=========================================================+=============+
| Miller's modified Gall                                  | 66.159467   |
+---------------------------------------------------------+-------------+
| Kamenetskiy's First                                     | 55          |
+---------------------------------------------------------+-------------+
| Gall's stereographic                                    | 45          |
+---------------------------------------------------------+-------------+
| Bolshoi Sovietskii Atlas Mira or Kamenetskiy's Second   | 30          |
+---------------------------------------------------------+-------------+
| Braun's cylindrical                                     | 0           |
+---------------------------------------------------------+-------------+

A map of the world, centered on the Greenwich meridian, using the Gall's
stereographic projection (standard parallel is 45,
Figure :ref:`Gall's stereographic projection <GMT_gall_stereo>`),
is obtained as follows:

   ::

    gmt set FORMAT_GEO_MAP dddA
    gmt pscoast -R-180/180/-60/80 -JCyl_stere/0/45/4.5i -Bxa60f30g30 -Bya30g30 -Dc -A5000 \
                -Wblack -Gseashell4 -Santiquewhite1 -P > GMT_gall_stereo.ps

.. _GMT_gall_stereo:

.. figure:: /_images/GMT_gall_stereo.*
   :width: 500 px
   :align: center

   World map using Gall's stereographic projection.


Miscellaneous projections
-------------------------

GMT supports 8 common projections for global presentation of data or
models. These are the Hammer, Mollweide, Winkel Tripel, Robinson, Eckert
IV and VI, Sinusoidal, and Van der Grinten projections. Due to the small
scale used for global maps these projections all use the spherical
approximation rather than more elaborate elliptical formulae.

In all cases, the specification of the central meridian can be skipped.
The default is the middle of the longitude range of the plot, specified
by the (R) option.

Hammer projection (**-Jh** **-JH**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The equal-area Hammer projection, first presented by the German
mathematician Ernst von Hammer in 1892, is also known as Hammer-Aitoff
(the Aitoff projection looks similar, but is not equal-area). The border
is an ellipse, equator and central meridian are straight lines, while
other parallels and meridians are complex curves. The projection is
defined by selecting:

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Jh**), or map width (**-JH**).

A view of the Pacific ocean using the Dateline as central meridian is accomplished thus

   ::

    gmt pscoast -Rg -JH4.5i -Bg -Dc -A10000 -Gblack -Scornsilk -P > GMT_hammer.ps

.. figure:: /_images/GMT_hammer.*
   :width: 500 px
   :align: center

   World map using the Hammer projection.


Mollweide projection (**-Jw** **-JW**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This pseudo-cylindrical, equal-area projection was developed by the
German mathematician and astronomer Karl Brandan Mollweide in 1805.
Parallels are unequally spaced straight lines with the meridians being
equally spaced elliptical arcs. The scale is only true along latitudes
4044' north and south. The projection is used mainly for global maps
showing data distributions. It is occasionally referenced under the name
homalographic projection. Like the Hammer projection, outlined above, we
need to specify only two parameters to completely define the mapping of
longitudes and latitudes into rectangular *x*/*y* coordinates:

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Jw**), or map width (**-JW**).

An example centered on Greenwich can be generated thus:

   ::

    gmt pscoast -Rd -JW4.5i -Bg -Dc -A10000 -Gtomato1 -Sskyblue -P > GMT_mollweide.ps

.. figure:: /_images/GMT_mollweide.*
   :width: 500 px
   :align: center

   World map using the Mollweide projection.


Winkel Tripel projection (**-Jr** **-JR**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In 1921, the German mathematician Oswald Winkel a projection that was to
strike a compromise between the properties of three elements (area,
angle and distance). The German word "tripel" refers to this junction of
where each of these elements are least distorted when plotting global
maps. The projection was popularized when Bartholomew and Son started to
use it in its world-renowned "The Times Atlas of the World" in the mid
20th century. In 1998, the National Geographic Society made the Winkel
Tripel as its map projection of choice for global maps.

Naturally, this projection is neither conformal, nor equal-area. Central
meridian and equator are straight lines; other parallels and meridians
are curved. The projection is obtained by averaging the coordinates of
the Equidistant Cylindrical and Aitoff (not Hammer-Aitoff) projections.
The poles map into straight lines 0.4 times the length of equator. To
use it you must enter

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Jr**), or map width (**-JR**).

Centered on Greenwich, the example in Figure :ref:`Winkel Tripel projection
<GMT_winkel>` was created by this command:

   ::

    gmt pscoast -Rd -JR4.5i -Bg -Dc -A10000 -Gburlywood4 -Swheat1 -P > GMT_winkel.ps

.. _GMT_winkel:

.. figure:: /_images/GMT_winkel.*
   :width: 500 px
   :align: center

   World map using the Winkel Tripel projection.


Robinson projection (**-Jn** **-JN**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Robinson projection, presented by the American geographer and
cartographer Arthur H. Robinson in 1963, is a modified cylindrical
projection that is neither conformal nor equal-area. Central meridian
and all parallels are straight lines; other meridians are curved. It
uses lookup tables rather than analytic expressions to make the world
map "look" right [22]_. The scale is true along latitudes 38. The
projection was originally developed for use by Rand McNally and is
currently used by the National Geographic Society. To use it you must
enter

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Jn**), or map width
   (**-JN**).

Again centered on Greenwich, the example below was created by this command:

   ::

    gmt pscoast -Rd -JN4.5i -Bg -Dc -A10000 -Ggoldenrod -Ssnow2 -P > GMT_robinson.ps

.. figure:: /_images/GMT_robinson.*
   :width: 500 px
   :align: center

   World map using the Robinson projection.


Eckert IV and VI projection (**-Jk** **-JK**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Eckert IV and VI projections, presented by the German cartographer
Max Eckert-Greiffendorff in 1906, are pseudocylindrical equal-area
projections. Central meridian and all parallels are straight lines;
other meridians are equally spaced elliptical arcs (IV) or sinusoids
(VI). The scale is true along latitudes 4030' (IV) and 4916' (VI). Their
main use is in thematic world maps. To select Eckert IV you must use
**-JKf** (**f** for "four") while Eckert VI is selected with **-JKs**
(**s** for "six"). If no modifier is given it defaults to Eckert VI. In
addition, you must enter

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Jk**), or map width
   (**-JK**).

Centered on the Dateline, the Eckert IV example below was created by
this command:

   ::

    gmt pscoast -Rg -JKf4.5i -Bg -Dc -A10000 -Wthinnest -Givory -Sbisque3 -P > GMT_eckert4.ps

.. figure:: /_images/GMT_eckert4.*
   :width: 500 px
   :align: center

   World map using the Eckert IV projection.


The same script, with **s** instead of **f**, yields the Eckert VI map:

.. figure:: /_images/GMT_eckert6.*
   :width: 500 px
   :align: center

   World map using the Eckert VI projection.


Sinusoidal projection (**-Ji** **-JI**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The sinusoidal projection is one of the oldest known projections, is
equal-area, and has been used since the mid-16th century. It has also
been called the "Equal-area Mercator" projection. The central meridian
is a straight line; all other meridians are sinusoidal curves. Parallels
are all equally spaced straight lines, with scale being true along all
parallels (and central meridian). To use it, you need to select:

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Ji**), or map width
   (**-JI**).

A simple world map using the sinusoidal projection is therefore obtained by

   ::

     gmt pscoast -Rd -JI4.5i -Bxg30 -Byg15 -Dc -A10000 -Ggray -P > GMT_sinusoidal.ps

.. figure:: /_images/GMT_sinusoidal.*
   :width: 500 px
   :align: center

   World map using the Sinusoidal projection.


To reduce distortion of shape the interrupted sinusoidal projection was
introduced in 1927. Here, three symmetrical segments are used to cover
the entire world. Traditionally, the interruptions are at 160W, 20W, and
60E. To make the interrupted map we must call
:doc:`pscoast` for each segment and superpose
the results. To produce an interrupted world map (with the traditional
boundaries just mentioned) that is 5.04 inches wide we use the scale
5.04/360 = 0.014 and offset the subsequent plots horizontally by their
widths (140\ :math:`\cdot`\ 0.014 and 80\ :math:`\cdot`\ 0.014):

   ::

     gmt pscoast -R200/340/-90/90 -Ji0.014i -Bxg30 -Byg15 -A10000 -Dc \
                 -Gblack -K -P > GMT_sinus_int.ps
     gmt pscoast -R-20/60/-90/90 -Ji0.014i -Bxg30 -Byg15 -Dc -A10000 \
                 -Gblack -X1.96i -O -K >> GMT_sinus_int.ps
     gmt pscoast -R60/200/-90/90 -Ji0.014i -Bxg30 -Byg15 -Dc -A10000 \
                 -Gblack -X1.12i -O >> GMT_sinus_int.ps

.. figure:: /_images/GMT_sinus_int.*
   :width: 500 px
   :align: center

   World map using the Interrupted Sinusoidal projection.


The usefulness of the interrupted sinusoidal projection is basically
limited to display of global, discontinuous data distributions like
hydrocarbon and mineral resources, etc.

Van der Grinten projection (**-Jv** **-JV**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Van der Grinten projection, presented by Alphons J. van der Grinten
in 1904, is neither equal-area nor conformal. Central meridian and
Equator are straight lines; other meridians are arcs of circles. The
scale is true along the Equator only. Its main use is to show the entire
world enclosed in a circle. To use it you must enter

-  The central meridian [Middle of your map].

-  Scale along equator in inch/degree or 1:xxxxx (**-Jv**), or map width (**-JV**).

Centered on the Dateline, the example below was created by this command:

    ::

      gmt pscoast -Rg -JV4i -Bxg30 -Byg15 -Dc -Glightgray -A10000 \
                  -Wthinnest -P > GMT_grinten.ps

.. figure:: /_images/GMT_grinten.*
   :width: 400 px
   :align: center

   World map using the Van der Grinten projection.


.. include:: examples_chapter.rst_


A. GMT Supplemental Packages
============================

These packages are for the most part written and supported by us, but
there are some exceptions. They provide extensions of GMT that are
needed for particular rather than general applications. The software is
provided in a separate, supplemental archive (GMT_suppl.tar.gz (or .bz2)).
Questions or bug reports for this software
should be addressed to the person(s) listed in the ``README`` file associated with
the particular program. It is not guaranteed that these programs are
fully ANSI-C, Y2K, or POSIX compliant, or that they necessarily will
install smoothly on all platforms, but most do. Note that the data sets
some of these programs work on are not distributed with these packages;
they must be obtained separately. The contents of the supplemental
archive may change without notice; at this writing it contains these directories:

gshhg: GSHHG data extractor
---------------------------

This package contains :doc:`gshhg <supplements/gshhg/gshhg>` which you
can use to extract shoreline polygons from the Global Self-consistent
Hierarchical High-resolution Shorelines (GSHHG) available separately
from or the (GSHHG is the polygon data base from which the
GMT coastlines derive). The package is maintained by Paul Wessel.

img: gridded altimetry extractor
--------------------------------

This package consists of the program
:doc:`img2grd <supplements/img/img2grd>` to extract subsets of the
global gravity and predicted topography solutions derived from satellite
altimetry [28]_. The package is maintained by Walter Smith and Paul Wessel.

meca: seismology and geodesy symbols
------------------------------------

This package contains the programs
:doc:`pscoupe <supplements/meca/pscoupe>`,
:doc:`psmeca <supplements/meca/psmeca>`,
:doc:`pspolar <supplements/meca/pspolar>`, and
:doc:`psvelo <supplements/meca/psvelo>` which are used by seismologists
and geodesists for plotting focal mechanisms (including cross-sections
and polarities), error ellipses, velocity arrows, rotational wedges, and
more. The package was developed by Kurt Feigl and Genevieve
Patau but is now maintained by the GMT team.

mgd77: MGD77 extractor and plotting tools
-----------------------------------------

This package currently holds the programs
:doc:`mgd77convert <supplements/mgd77/mgd77convert>`,
:doc:`mgd77info <supplements/mgd77/mgd77info>`,
:doc:`mgd77list <supplements/mgd77/mgd77list>`,
:doc:`mgd77magref <supplements/mgd77/mgd77magref>`,
:doc:`mgd77manage <supplements/mgd77/mgd77manage>`,
:doc:`mgd77path <supplements/mgd77/mgd77path>`,
:doc:`mgd77sniffer <supplements/mgd77/mgd77sniffer>`, and
:doc:`mgd77track <supplements/mgd77/mgd77track>` which can be used to
extract information or data values from or plot marine geophysical data
files in the ASCII MGD77 or netCDF MGD77+ formats [29]_). This package
has replaced the old **mgg** package. The package is maintained by Paul Wessel.

misc: Miscellaneous tools
-------------------------

At the moment, this package contains the program
:doc:`dimfilter <supplements/misc/dimfilter>`, which is an extension of
:doc:`grdfilter` in that it allows for spatial directional filtering.
The package is maintained by Paul Wessel.

potential: Geopotential tools
-----------------------------

At the moment, this package contains the programs
:doc:`gravfft <supplements/potential/gravfft>`, which performs gravity,
isostasy, and admittance calculation for grids,
:doc:`grdredpol <supplements/potential/grdredpol>`, which compute the
continuous reduction to the pole, AKA differential RTP for magnetic
data, :doc:`grdseamount <supplements/potential/grdseamount>`, which computes
synthetic bathymetry over various seamount shapes, and
:doc:`gmtgravmag3d <supplements/potential/gmtgravmag3d>` and
:doc:`grdgravmag3d <supplements/potential/grdgravmag3d>`,
which computes the gravity or
magnetic anomaly of a body by the method of Okabe [30]_. The package is
maintained by Joaquim Luis and Paul Wessel.

segyprogs: plotting SEGY seismic data
-------------------------------------

This package contains programs to plot SEGY seismic data files using the
GMT mapping transformations and postscript library.
:doc:`pssegy <supplements/segy/pssegy>` generates a 2-D plot (x:location
and y:time/depth) while :doc:`pssegyz <supplements/segy/pssegyz>`
generates a 3-D plot (x and y: location coordinates, z: time/depth).
Locations may be read from predefined or arbitrary portions of each
trace header. Finally, :doc:`segy2grd <supplements/segy/segy2grd>` can
convert SEGY data to a GMT grid file. The package is maintained by Tim Henstock [31]_.

spotter: backtracking and hotspotting
-------------------------------------

This package contains the plate tectonic programs
:doc:`backtracker <supplements/spotter/backtracker>`, which you can use to
move geologic markers forward or backward in time,
:doc:`grdpmodeler <supplements/spotter/grdpmodeler>` which evaluates
predictions of a plate motion model on a grid,
:doc:`grdrotater <supplements/spotter/grdrotater>` which rotates entire
grids using a finite rotation,
:doc:`hotspotter <supplements/spotter/hotspotter>` which generates CVA
grids based on seamount locations and a set of absolute plate motion
stage poles (:doc:`grdspotter <supplements/spotter/grdspotter>` does the
same using a bathymetry grid instead of seamount locations),
:doc:`originator <supplements/spotter/originator>`, which associates
seamounts with the most likely hotspot origins, and
:doc:`rotconverter <supplements/spotter/rotconverter>` which does various
operations involving finite rotations on a sphere. The package is
maintained by Paul Wessel.

x2sys: track crossover error estimation
---------------------------------------

This package contains the tools
:doc:`x2sys_datalist <supplements/x2sys/x2sys_datalist>`, which allows
you to extract data from almost any binary or ASCII data file, and
:doc:`x2sys_cross <supplements/x2sys/x2sys_cross>` which determines
crossover locations and errors generated by one or several geospatial
tracks. Newly added are the tools
:doc:`x2sys_init <supplements/x2sys/x2sys_init>`,
:doc:`x2sys_binlist <supplements/x2sys/x2sys_binlist>`,
:doc:`x2sys_get <supplements/x2sys/x2sys_get>`,
:doc:`x2sys_list <supplements/x2sys/x2sys_list>`,
:doc:`x2sys_put <supplements/x2sys/x2sys_put>`,
:doc:`x2sys_report <supplements/x2sys/x2sys_report>`,
:doc:`x2sys_solve <supplements/x2sys/x2sys_solve>` and
:doc:`x2sys_merge <supplements/x2sys/x2sys_merge>` which extends the
track-management system employed by the mgg supplement to generic track
data of any format. This package represents a new generation of tools
and replaces the old **x_system** package. The package is maintained by
Paul Wessel.

.. _App-file-formats:

B. GMT File Formats
===================

Table data
----------

These files have *N* records which have *M* fields each. All programs
that handle tables can read multicolumn files. GMT can read both
ASCII, native binary, and netCDF table data.

ASCII tables
~~~~~~~~~~~~

Optional file header records
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The first data record may be preceded by one or more header records. Any
records that begins with '#' is considered a header or comment line and
are always processed correctly. If your data file has leading header
records that do *not* start with '#' then you must make sure to use the
**-h** option and set the parameter :ref:`IO_N_HEADER_RECS <IO_N_HEADER_RECS>` in the :doc:`gmt.conf` file
(GMT default is one header record if **-h** is given; you may also use
**-h**\ *nrecs* directly). Fields within a record must be separated by
spaces, tabs, or commas. Each field can be an integer or floating-point
number or a geographic coordinate string using the
[+\ \|\ -]dd[:mm[:ss]][W:\ \|\ S\ \|\ N\ \|\ E\ \|\ w\ \|\ s\ \|\ n\ \|\ e]
format. Thus, 12:30:44.5W, 17.5S, 1:00:05, and 200:45E are all valid
input strings. On output, fields will be separated by the character
given by the parameter :ref:`IO_COL_SEPARATOR <IO_COL_SEPARATOR>`, which by default is a TAB.

Optional segment header records
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When dealing with time- or (*x,y*)-series it is usually convenient to
have each profile in separate files. However, this may sometimes prove
impractical due to large numbers of profiles. An example is files of
digitized lineations where the number of individual features may range
into the thousands. One file per feature would in this case be
unreasonable and furthermore clog up the directory. GMT provides a
mechanism for keeping more than one profile in a file. Such files are
called *multiple segment files* and are identical to the ones just
outlined except that they have segment headers interspersed with data
records that signal the start of a new segment. The segment headers may
be of any format, but all must have the same character in the first
column. The unique character is by default '\ >\ ', but you can
override that by modifying the :ref:`IO_SEGMENT_MARKER <IO_SEGMENT_MARKER>` default setting.
Programs can examine the segment headers to see if they contain **-D**
for a distance value, **-W** and **-G** options for specifying pen and
fill attributes for individual segments, **-Z** to change color via a
CPT file, **-L** for label specifications, or **-T** for general-purpose
text descriptions. These settings (and occasionally others) will
override the corresponding command line options. GMT also provides for
two special values for :ref:`IO_SEGMENT_MARKER <IO_SEGMENT_MARKER>` that can make
interoperability with other software packages easier. Choose the marker
**B** to have blank lines recognized as segment breaks, or use **N** to
have data records whose fields equal NaN mean segment breaks (e.g., as
used by Matlab or Octave). When these markers are used then no other
segment header will be considered. Note that :ref:`IO_SEGMENT_MARKER <IO_SEGMENT_MARKER>` can
be set differently for input and output.  Finally, if a segment represents
a closed polygon that is a hole inside another polygon you indicate this
with **-Ph**.  This setting will be read and processed if converting a
file to the OGR format.

Binary tables
~~~~~~~~~~~~~

GMT programs also support native binary tables to speed up
input-output for i/o-intensive tasks like gridding and preprocessing.
This is discussed in more detail in section `Binary table i/o: The -b option`_.

NetCDF tables
~~~~~~~~~~~~~

More and more programs are now producing binary data in the netCDF
format, and so GMT programs started to support tabular netCDF data
(files containing one or more 1-dimensional arrays) starting with
GMT version 4.3.0. Because of the meta data contained in those files,
reading them is much less complex than reading native binary tables, and
even than ASCII tables. GMT programs will read as many 1-dimensional
columns as are needed by the program, starting with the first
1-dimensional it can find in the file. To specifically specify which
variables are to be read, append the suffix
**?**\ *var1*\ **/**\ *var2*\ **/**\ *...* to the netCDF file name or
add the option **-bic**\ *var1*\ **/**\ *var2*\ **/**\ *...*, where
*var1*, *var2*, etc.are the names of the variables to be processed. The
latter option is particularly practical when more than one file is read:
the **-bic** option will apply to all files. Currently, GMT only
reads, but does not write, netCDF tabular data.

Grid files
----------

GMT allows numerous grid formats to be read. In addition to the default
netCDF format it can use binary floating points, short integers, bytes, and
bits, as well as 8-bit Sun raster files (colormap ignored).  Additional
formats may be used by supplying read/write functions and linking these with
the GMT libraries. The source file ``gmt_customio.c`` has the information
that programmers will need to augment GMT to read custom grid files. See
Section `Grid file format specifications`_ for more information.

NetCDF files
~~~~~~~~~~~~

By default, GMT stores 2-D grids as COARDS-compliant netCDF files.
COARDS (which stands for Cooperative Ocean/Atmosphere Research Data
Service) is a convention used by many agencies distributing gridded data
for ocean and atmosphere research. Sticking to this convention allows
GMT to read gridded data provided by other institutes and other
programs. Conversely, other general domain programs will be able to read
grids created by GMT. COARDS is a subset of a more extensive
convention for netCDF data called CF-1.5 (Climate and Forecast, version
1.5). Hence, GMT grids are also automatically CF-1.5-compliant.
However, since CF-1.5 has more general application than COARDS, not all
CF-1.5 compliant netCDF files can be read by GMT.

The netCDF grid file in GMT has several attributes (See Table
:ref:`netcdf-format <tbl-netcdf-format>`) to describe the content. The routine
that deals with netCDF grid files is sufficiently flexible so that grid files
slightly deviating from the standards used by GMT can also be read.

.. _tbl-netcdf-format:

+----------------------+--------------------------------------------------------------------+
| **Atributte**        | **Description**                                                    |
+======================+====================================================================+
|                      | *Global attributes*                                                |
+----------------------+--------------------------------------------------------------------+
| Conventions          | COARDS, CF-1.5 (optional)                                          |
+----------------------+--------------------------------------------------------------------+
| title                | Title (optional)                                                   |
+----------------------+--------------------------------------------------------------------+
| source               | How file was created (optional)                                    |
+----------------------+--------------------------------------------------------------------+
| node_offset          | 0 for gridline node registration (default),                        |
|                      | 1 for pixel registration                                           |
+----------------------+--------------------------------------------------------------------+
|                      | *x- and y-variable attributes*                                     |
+----------------------+--------------------------------------------------------------------+
| long_name            | Coordinate name (e.g., "Longitude" and "Latitude")                 |
+----------------------+--------------------------------------------------------------------+
| units                | Unit of the coordinate (e.g., "degrees_east" and "degrees_north")  |
+----------------------+--------------------------------------------------------------------+
| actual range         | Minimum and maximum *x* and *y* of region; if absent the           |
| (or valid range)     | first and last *x*- and *y*-values are queried                     |
+----------------------+--------------------------------------------------------------------+
|                      | *z-variable attributes*                                            |
+----------------------+--------------------------------------------------------------------+
| long_name            | Name of the variable (default: "z")                                |
+----------------------+--------------------------------------------------------------------+
| units                | Unit of the variable                                               |
+----------------------+--------------------------------------------------------------------+
| scale_factor         | Factor to multiply *z* with (default: 1)                           |
+----------------------+--------------------------------------------------------------------+
| add_offset           | Offset to add to scaled *z* (default: 0)                           |
+----------------------+--------------------------------------------------------------------+
| actual_range         | Minimum and maximum *z* (in unpacked units, optional) and *z*      |
+----------------------+--------------------------------------------------------------------+
| \_FillValue          | Value associated with missing or invalid data points; if absent an |
| (or missing_value)   | appropriate default value is assumed, depending on data type.      |
+----------------------+--------------------------------------------------------------------+

By default, the first 2-dimensional variable in a netCDF file will be read as
the *z* variable and the coordinate axes *x* and *y* will be determined from
the dimensions of the *z* variable. GMT will recognize whether the *y*
(latitude) variable increases or decreases. Both forms of data storage are
handled appropriately.

For more information on the use of COARDS-compliant netCDF files, and on how
to load multi-dimensional grids, read Section `Modifiers for COARDS-compliant netCDF files`_.

Chunking and compression with netCDF
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GMT supports reading and writing of netCDF-4 files since release 5.0.  For
performance reasons with ever-increasing grid sizes, the default output format
of GMT is netCDF-4 with chunking enabled for grids with more than 16384 cells.
Chunking means that the data is not stored sequentially in rows along latitude
but rather split up into tiles.  Figure :ref:`netcdf_chunking` illustrates
the layout in a chunked netCDF file.  To access a subset of the data (e.g.,
the four blue tiles in the lower left), netCDF only reads those tiles
("chunks") instead of extracting data from long rows.

.. _netcdf_chunking:

.. figure:: /_images/GMT_chunking.*
   :align: center

   Grid split into 3 by 3 chunks

Gridded datasets in the earth sciences usually exhibit a strong spatial
dependence (e.g. topography, potential fields, illustrated by blue and white
cells in Figure :ref:`netcdf_chunking`) and deflation can greatly reduce the
file size and hence the file access time (deflating/inflating is faster than
hard disk I/O).  It is therefore convenient to deflate grids with spatial
dependence (levels 1--3 give the best speed/size-tradeoff).

You may control the size of the chunks of data and compression with the
configuration parameters :ref:`IO_NC4_CHUNK_SIZE <IO_NC4_CHUNK_SIZE>`
and :ref:`IO_NC 4_DEFLATION_LEVEL <IO_NC4_DEFLATION_LEVEL>` as specified in
:doc:`gmt.conf` and you can check the netCDF format with :doc:`grdinfo`.

Classic netCDF files were the de facto standard until netCDF 4.0 was released
in 2008.  Most programs supporting netCDF by now are using the netCDF-4
library and are thus capable of reading netCDF files generated with GMT 5,
this includes official GMT releases since revision 4.5.8.  In rare occasions,
when you have to load netCDF files with old software, you may be forced to
export your grids in the old classic format.  This can be achieved by setting
:ref:`IO_NC4_CHUNK_SIZE <IO_NC4_CHUNK_SIZE>` to **c**\ lassic.

Further reading:

- `Unidata NetCDF Workshop: NetCDF Formats and Performance <http://www.unidata.ucar.edu/software/netcdf/workshops/most-recent/performance/index.html>`_
- `Unidata NetCDF Workshop: What is Chunking? <http://www.unidata.ucar.edu/software/netcdf/workshops/most-recent/nc4chunking/WhatIsChunking.html>`_
- `HDF NetCDF-4 Performance Report <http://www.hdfgroup.org/pubs/papers/2008-06_netcdf4_perf_report.pdf>`_

Gridline and Pixel node registration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Scanline format means that the data are stored in rows (*y* = constant)
going from the "top" (:math:`y = y_{max}` (north)) to the "bottom"
(:math:`y = y_{min}` (south)). Data within each row are ordered from
"left" (:math:`x = x_{min}` (west)) to "right" (:math:`x = x_{max}`
(east)). The *registration* signals how the nodes are laid out. The grid
is always defined as the intersections of all
*x* ( :math:`x = x_{min}, x_{min} + x_{inc}, x_{min} + 2 \cdot x_{inc}, \ldots, x_{max}` )
and *y* ( :math:`y = y_{min}, y_{min} + y_{inc}, y_{min} + 2 \cdot y_{inc}, \ldots, y_{max}` )
lines. The two scenarios differ as to which area each data point
represents. The default node registration in GMT is gridline node
registration. Most programs can handle both types, and for some programs
like :doc:`grdimage` a pixel registered file
makes more sense. Utility programs like
:doc:`grdsample` and
:doc:`grdproject` will allow you to
convert from one format to the other;
:doc:`grdedit` can make changes to the grid
header and convert a pixel- to a gridline-registered grid, or *vice
versa*. The grid registration is determined by the common GMT **-r**
option (see Section `Grid registration: The -r option`_).

Boundary Conditions for operations on grids
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GMT has the option to specify boundary conditions in some programs
that operate on grids (e.g.,
:doc:`grdsample`, :doc:`grdgradient`,
:doc:`grdtrack`, :doc:`nearneighbor`, and
:doc:`grdview`, to name a few. The desired
condition can be set with the common GMT option **-n**; see Section
`Grid interpolation parameters: The -n option`_. The boundary conditions come into play when
interpolating or computing derivatives near the limits of the region
covered by the grid. The *default* boundary conditions used are those
which are "natural" for the boundary of a minimum curvature
interpolating surface. If the user knows that the data are periodic in
*x* (and/or *y*), or that the data cover a sphere with *x*,\ *y*
representing *longitude*,\ *latitude*, then there are better choices for
the boundary conditions. Periodic conditions on *x* (and/or *y*) are
chosen by specifying *x* (and/or *y*) as the boundary condition flags;
global spherical cases are specified using the *g* (geographical) flag.
Behavior of these conditions is as follows:

Periodic
    conditions on *x* indicate that the data are periodic in the
    distance (:math:`x_{max} - x_{min}`) and thus repeat values after
    every :math:`N = (x_{max} - x_{min})/x_{inc}`. Note that this
    implies that in a grid-registered file the values in the first and
    last columns are equal, since these are located at
    :math:`x = x_{min}` and :math:`x = x_{max}`, and there are
    *N + 1* columns in the file. This is not the case in a
    pixel-registered file, where there are only *N* and the first
    and last columns are located at :math:`x_{min} + x_{inc}/2` and
    :math:`x_{max} - x_{inc}/2`. If *y* is periodic all the same
    holds for *y*.

Geographical
    conditions indicate the following:

    #. If :math:`(x_{max} - x_{min}) \geq 360` and also 180 modulo
       :math:`x_{inc} = 0` then a periodic condition is used on
       *x* with a period of 360; else a default condition is used
       on the *x* boundaries.

    #. If condition 1 is true and also :math:`y_{max} = 90` then a
       "north pole condition" is used at :math:`y_{max}`, else a default
       condition is used there.

    #. If condition 1 is true and also :math:`y_{min} = -90` then a
       "south pole condition" is used at :math:`y_{min}`, else a default
       condition is used there.

    "Pole conditions" use a 180 phase-shift of the data, requiring 180
    modulo :math:`x_{inc} = 0`.

Default
    boundary conditions are

    .. math:: \nabla^2 f = \frac{\partial}{\partial n} \nabla^2 f = 0

    on the boundary, where :math:`f(x, y)` is represented by the values
    in the grid file, and :math:`\partial/\partial n` is the derivative
    in the direction normal to a boundary, and

    .. math:: \nabla^2 = \left(\frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2}\right)

    is the two-dimensional Laplacian operator.

Native binary grid files
~~~~~~~~~~~~~~~~~~~~~~~~

The old style native grid file format that was common in earlier version
of GMT is still supported, although the use of netCDF files is
strongly recommended. The file starts with a header of 892 bytes
containing a number of attributes defining the content. The
:doc:`grdedit` utility program will allow you
to edit parts of the header of an existing grid file. The attributes
listed in Table :ref:`grdheader <tbl-grdheader>` are contained within the header record
in the order given (except the *z*-array which is not part of the
header structure, but makes up the rest of the file). As this header was
designed long before 64-bit architectures became available, the jump
from the first three integers to the subsequent doubles in the structure
does not occur on a 16-byte alignment. While GMT handles the reading
of these structures correctly, enterprising programmers must take care
to read this header correctly (see our code for details).

.. _tbl-grdheader:

+---------------------------------+--------------------------------------------------------+
| **Parameter**                   | **Description**                                        |
+=================================+========================================================+
| **int** *nx*                    | Number of nodes in the *x*-dimension                   |
+---------------------------------+--------------------------------------------------------+
| **int** *ny*                    | Number of nodes in the *y*-dimension                   |
+---------------------------------+--------------------------------------------------------+
| **int** *registration*          | 0 for grid line registration, 1 for pixel registration |
+---------------------------------+--------------------------------------------------------+
| **double** *x_min*              | Minimum *x*-value of region                            |
+---------------------------------+--------------------------------------------------------+
| **double** *x_max*              | Maximum *x*-value of region                            |
+---------------------------------+--------------------------------------------------------+
| **double** *y_min*              | Minimum *y*-value of region                            |
+---------------------------------+--------------------------------------------------------+
| **double** *y_max*              | Maximum *y*-value of region                            |
+---------------------------------+--------------------------------------------------------+
| **double** *z_min*              | Minimum *z*-value in data set                          |
+---------------------------------+--------------------------------------------------------+
| **double** *z_max*              | Maximum *z*-value in data set                          |
+---------------------------------+--------------------------------------------------------+
| **double** *x_inc*              | Node spacing in *x*-dimension                          |
+---------------------------------+--------------------------------------------------------+
| **double** *y_inc*              | Node spacing in *y*-dimension                          |
+---------------------------------+--------------------------------------------------------+
| **double** *z_scale_factor*     | Factor to multiply *z*-values after read               |
+---------------------------------+--------------------------------------------------------+
| **double** *z_add_offset*       | Offset to add to scaled *z*-values                     |
+---------------------------------+--------------------------------------------------------+
| **char** *x_units*\ [80]        | Units of the *x*-dimension                             |
+---------------------------------+--------------------------------------------------------+
| **char** *y_units*\ [80]        | Units of the *y*-dimension                             |
+---------------------------------+--------------------------------------------------------+
| **char** *z_units*\ [80]        | Units of the *z*-dimension                             |
+---------------------------------+--------------------------------------------------------+
| **char** *title*\ [80]          | Descriptive title of the data set                      |
+---------------------------------+--------------------------------------------------------+
| **char** *command*\ [320]       | Command line that produced the grid file               |
+---------------------------------+--------------------------------------------------------+
| **char** *remark*\ [160]        | Any additional comments                                |
+---------------------------------+--------------------------------------------------------+
| **TYPE** *z*\ [nx\*ny]          | 1-D array with *z*-values in scanline format           |
+---------------------------------+--------------------------------------------------------+

Sun raster files
----------------

The Sun raster file format consists of a header followed by a series of
unsigned 1-byte integers that represents the bit-pattern. Bits are
scanline oriented, and each row must contain an even number of bytes.
The predefined 1-bit patterns in GMT have dimensions of 64 by 64, but
other sizes will be accepted when using the **-Gp|P** option. The Sun
header structure is outline in Table :ref:`sunheader <tbl-sunheader>`.

.. _tbl-sunheader:

+---------------------------+-------------------------------------+
| **Parameter**             | **Description**                     |
+===========================+=====================================+
| **int** *ras_magic*       | Magic number                        |
+---------------------------+-------------------------------------+
| **int** *ras_width*       | Width (pixels) of image             |
+---------------------------+-------------------------------------+
| **int** *ras_height*      | Height (pixels) of image            |
+---------------------------+-------------------------------------+
| **int** *ras_depth*       | Depth (1, 8, 24, 32 bits) of pixel  |
+---------------------------+-------------------------------------+
| **int** *ras_length*      | Length (bytes) of image             |
+---------------------------+-------------------------------------+
| **int** *ras_type*        | Type of file; see RT\_ below        |
+---------------------------+-------------------------------------+
| **int** *ras_maptype*     | Type of colormap; see RMT\_ below   |
+---------------------------+-------------------------------------+
| **int** *ras_maplength*   | Length (bytes) of following map     |
+---------------------------+-------------------------------------+

After the header, the color map (if *ras_maptype* is not RMT_NONE)
follows for *ras_maplength* bytes, followed by an image of
*ras_length* bytes. Some related definitions are given in
Table :ref:`sundef <tbl-sundef>`.

.. _tbl-sundef:

+---------------------+-------------------------------------------+
| **Macro name**      | **Description**                           |
+=====================+===========================================+
| RAS_MAGIC           | 0x59a66a95                                |
+---------------------+-------------------------------------------+
| RT_STANDARD         | 1 (Raw pixrect image in 68000 byte order) |
+---------------------+-------------------------------------------+
| RT_BYTE_ENCODED     | 2 (Run-length compression of bytes)       |
+---------------------+-------------------------------------------+
| RT_FORMAT_RGB       | 3 ([X]RGB instead of [X]BGR)              |
+---------------------+-------------------------------------------+
| RMT_NONE            | 0 (ras_maplength is expected to be 0)     |
+---------------------+-------------------------------------------+
| RMT_EQUAL_RGB       | 1 (red[ras_maplength/3],green[],blue[])   |
+---------------------+-------------------------------------------+

Numerous public-domain programs exist, such as **xv** and
**convert** (in the ImageMagick package), that will translate between
various raster file formats such as tiff, gif, jpeg, and Sun raster.
Raster patterns may be created with GMT plotting tools by generating
PostScript plots that can be rasterized by ghostscript and
translated into the right raster format.

.. _include-gmt-graphics:

C. Including GMT Graphics into your Documents
=============================================


Now that you made some nice graphics with GMT, it is time to add them
to a document, an article, a report, your dissertation, a poster, a web
page, or a presentation. Of course, you could try the old-fashioned
scissors and glue stick. More likely, you want to incorporate your
graphics electronically into the document. Depending on the application,
the GMT PostScript file will need to be converted to Encapsulated
PostScript (EPS), Portable Document Format (PDF), or some raster
format (e.g., JPEG, PNG, or TIFF) in order to incorporate them into the
document.

-  When creating a document intended for printing (article,
   dissertation, or poster) it is best to preserve the scalable vector
   characteristics of the PostScript file. Many applications can
   directly incorporate PostScript in the form of EPS files. Modern
   programs will often allow the inclusion of PDF files. Either way, the
   sharpness of lines and fonts will be preserved and can be scaled up
   or down as required.

-  When the aim is to display the graphics on a computer screen or
   present it using a projector, it is wise to convert the
   PostScript into a raster format. Although applications like
   PowerPoint can do this for you, you can best take the
   conversion into your own hands for the best results.

A large number of questions to the GMT-Help mailing list are related to
these rendering issues, showing that something as seemingly
straightforward as incorporating a PostScript file into a document is
a far from trivial exercise. This Appendix will show how to include
GMT graphics into documents and how to achieve the best quality results.

Making GMT Encapsulated PostScript Files
------------------------------------------

GMT produces freeform PostScript files. Note that a freeform
PostScript file may contain special operators (such as
``Setpagedevice``) that is specific to printers (e.g., selection of
paper tray). Some previewers (among them, Sun's pageview) may not
understand these valid instructions and may fail to image the file.
Also, embedding freeform PostScript with such instructions in it into
a larger document can cause printing to fail. While you could choose
another viewer (we recommend **ghostview**) to view single plots
prepared by GMT, it is generally wiser to convert PostScript to EPS
output when you are creating a plot intended for inclusion into a larger
document. Some programs (and some publishers as well) do not allow the
use of instructions like ``Setpagedevice`` as part of embedded graphics.

An EPS file that is to be placed into another document needs to have
correct bounding box parameters. These are found in the
PostScript Document Comment %%BoundingBox. Applications that generate
EPS files should set these parameters correctly. Because GMT\ makes
the PostScript files on the fly, often with several overlays, it is
not possible to do so accurately. Therefore, if you need and EPS version
with a "tight" BoundingBox you need to post-process your
PostScript file. There are several ways in which this can be
accomplished.

-  Programs such as Adobe Illustrator, Aldus Freehand, and
   Corel Draw will allow you to edit the BoundingBox graphically.

-  A command-line alternative is to use freely-available program
   **epstool** from the makers of Aladdin ghostscript. Running

      ::

       epstool -c -b myplot.ps

   should give a tight BoundingBox; **epstool** assumes the plot is
   page size and not a huge poster.

-  Another option is to use **ps2epsi** which also comes with the
   ghostscript package. Running

      ::

       ps2epsi myplot.ps myplot.eps

   should also do the trick. The downside is that this program adds an
   "image" of the plot in the preamble of the EPS file, thus increasing
   the file size significantly. This image is a rough rendering of your
   PostScript graphics that some programs will show on screen while
   you are editing your document. This image is basically a placeholder
   for the PostScript graphics that will actually be printed.

-  The preferred option is to use the GMT utility
   :doc:`ps2raster`. Its **-A** option will
   figure out the tightest BoundingBox, again using ghostscript in
   the background. For example, running

      ::

       ps2raster -A -Te myplot.ps

   will convert the PostScript file ``myplot.ps`` into an encapsulated
   PostScript file ``myplot.eps`` which is exactly cropped to the tightest possible
   BoundingBox.

If you do not want to modify your illustration but just include it in a
text document: many word processors (such as Microsoft Word  or Apple Pages) will let you include a
PostScript file that you may place but not edit. Newer versions of
those programs also allow you to include PDF versions of your graphics.
Except for Pages, you will not be able to view the figure
on-screen, but it will print correctly.

Converting GMT PostScript to PDF or raster images
---------------------------------------------------

Since Adobe's PDF (Portable Document Format) seems to have become the
*de facto* standard for vector graphics, you are often well off
converting GMT produced PostScript files to PDF. Being both vector
formats (i.e., they basically describe all objects, text and graphics as
lines and curves), such conversion sounds awfully straightforward and
not worth a full section in this document. But experience has shown
differently, since most converters cut corners by using the same tool
(Aladdin's ghostscript) with basic default options that are not
devised to produce the best quality PDF files.

For some applications it is practical or even essential that you convert
your PostScript file into a raster format, such as GIF (Graphics
Interchange Format), TIFF (Tagged Image File Format), PNG (Portable
Network Graphics), or JPEG (Joint Photographic Experts Group). A web
page is better served with a raster image that will immediately show on
a web browser, than with a PostScript file that needs to be downloaded
to view, despite the better printing quality of the PostScript image.
A less obvious reason to convert your image to a raster format is to
by-pass PowerPoint's rendering engine in case you want to embed
the image into a presentation.

The are a number of programs that will convert PostScript files to PDF
or raster formats, like Aladdin's **pstopdf**, pbmplus' **pstoimg**,
or ImageMagick's **convert**, most of which run ghostscript
behind the scenes. The same is true for viewers like **ghostview** and
Apple's Preview*. So a lot of the times when people report that
their PostScript plot does not look right but prints fine, it is the
way ghostscript is used with its most basic settings that is to blame.

When converting or viewing PostScript goes awry
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are some notorious pitfalls with ghostscript (and other
rendering programs for that matter).

Rendering.
    When you are converting to a raster format, make sure you use a high
    enough resolution so that the pixels do not show when it is enlarged
    onto a screen or using a projector. The right choice of resolution
    depends on the application, but do not feel limited to the default
    72 dpi (dots-per-inch) that is offered by most converters.

Image compression.
    There are *lossy* and *non-lossy* compressions. A compression
    algorithm is called "lossy" when information is lost in the
    conversion: there is no way back to get the full original. The
    effect can be seen when there are sharp color transitions in your
    image: the edges will get blurry in order to allow a more efficient
    compression. JPEG uses a lossy compression, PNG is non-lossy, and
    TIFF generally does not use compression at all. We therefore
    recommend you convert to PNG if you need to rasterize your plot, and
    leave JPEG to photographs.

Embedded image compression.
    When your GMT plot includes objects produced by
    :doc:`grdimage`, :doc:`psimage` or
    :doc:`pslegend`, they are seen as
    "images". The default options of ghostscript will use a
    *lossy* compression (similar to JPEG) on those images when
    converting them to PDF objects. This can be avoided, however, by
    inhibiting the compression altogether, or using the non-lossy
    *flate* compression, similar to the one used in the old
    **compress** program. This compression is fully reversible, so
    that your image does not suffer any loss.

Auto-rotation.
    The ghostscript engine has the annoying habit to automatically
    rotate an image produced with portrait orientation (using the **-P**
    option) so that the height is always larger than the width. So if
    you have an image that was printed in portrait mode but happens to
    have a width larger than height (for example a global map), it would
    suddenly get rotated. Again, this function needs to be switched off.
    Apple's Preview uses the ghostscript engine and suffers
    from the same annoying habit. Oddly enough, ghostscript does
    not force landscape plots to be "horizontal".

Anti-aliasing.
    This is not something to worry about when converting to PDF, but
    certainly when producing raster images (discussed below).
    *Anti-aliasing* in this context means that the rendering tries to
    avoid *aliasing*, for example, sampling only the blacks in a
    black-and-white hachure. It does so by first oversampling the image
    and then using "gray-shades" when a target pixel is only partially
    white or black.

    Clearly, this can lead to some unwanted results. First, all edges
    and lines get blurry and second, the assumption of a white
    background causes the gray shades to stand out when transferring the
    image to background with a different color (like the popular
    sleep-inducing blue in PowerPoint presentations). A more
    surprising effect of anti-aliasing is that the seams between tiles
    that make up the land mask when using
    :doc:`pscoast` will become visible. The
    anti-aliasing somehow decides to blur the edges of all polygons,
    even when they are seamlessly connected to other polygons.

    It is therefore wise to overrule the default anti-aliasing option
    and over-sample the image yourself by choosing a higher resolution.

Including fonts.
    When you are producing print-ready copy to publishers, they will
    often (and justifiably) ask that you include all fonts in your PDF
    document. Again, ghostscript (and all converters relying on
    that engine) will not do so by default.

Using :doc:`ps2raster`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The remedy to all the problems mentioned in the previous section is
readily available to you in the form of the GMT utility
:doc:`ps2raster`. It is designed to provide
the best quality PDF and raster files using ghostscript as a
rendering engine. The program :doc:`ps2raster` avoids anti-aliasing and
lossy compression techniques that are default to ghostscript and
includes the fonts into the resulting PDF file to ensure portability. By
default the fonts are rendered at 720 dots-per-inch in a PDF file and
images are sampled to 300 dpi, but that can be changed with the **-E**
option. Simply run

   ::

    gmt ps2raster -A -P -Tf *.ps

to convert all PostScript files to PDF while cropping it to the
smallest possible BoundingBox. Or use the **-Tg** option to convert your
files to PNG.

The **-P** option of :doc:`ps2raster` may
also come in handy. When you have *not* supplied the **-P** option in
your first GMT plot command, your plot will be in Landscape mode. That
means that the plot will be rotated 90 degrees (anti-clockwise) to fit
on a Portrait mode page when coming out of the printer. The **-P**
option of :doc:`ps2raster` will undo that
rotation, so that you do not have to do so within your document. This
will only affect Landscape plots; Portrait plots will not be rotated.

Examples
--------

GMT graphics in LaTeX
~~~~~~~~~~~~~~~~~~~~~

Nearly all illustrations in this GMT documentation were GMT-produced
PostScript files. They were converted to PDF files using
:doc:`ps2raster` and then included into a
LaTeX document that was processed with **pdflatex** to create the PDF
document you are reading.

To add the graphics into the LaTeX document we use the
``\includegraphics`` command supplied by the package. In the preamble of
your LaTeX document you will need to include the line

   ::

     \usepackage{graphicx}

The inclusion of the graphics will probably be inside a floating figure
environment; something like this

   ::

     \begin{figure}
        \includegraphics{myplot}
        \caption{This is my first plot in \LaTeX.}
        \label{fig:myplot}
     \end{figure}

Note that the ``\includegraphics`` command does not require you to add
the suffix ``.pdf`` to the file name. If you run **pdflatex**, it will
look automatically for ``myplot.pdf``. If you run **latex**, it will use ``myplot.eps`` instead.

You can scale your plot using the options ``width=``, ``height=``, or
``scale=``. In addition, if your original graphics was produced in
Landscape mode (i.e., you did *not* use GMT's **-P** option: not
while plotting, nor in :doc:`ps2raster`),
you will need to rotate the plot as well. For example,

   ::

     \includegraphics[angle=-90,width=0.8\textwidth]{myplot}

will rotate the image 90 clockwise and scale it such that its width
(after rotation) will be 80% of the width of the text column.

GMT graphics in **PowerPoint**
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. _Rendering:

.. figure:: /_images/rendering.png
   :height: 540 px
   :width: 720 px
   :align: center
   :scale: 50 %

   Examples of rendered images in a PowerPoint presentation


.. _PowerPoint_dialogue:

.. figure:: /_images/formatpicture.png
   :height: 516 px
   :width: 545 px
   :align: center
   :scale: 50 %

   PowerPoint's Format Picture dialogue to set scale and rotation.

In Figure :ref:`Rendered images <Rendering>` we have attempted to include
Figure :ref:`Example 20 <Fig_example_20>` into a PowerPoint presentation.
First the PostScript file was converted to PDF (using
:doc:`ps2raster`), then loaded into
PowerPoint and the white background color was made transparent
using the formatting toolbar (shown on the left side of
Figure :ref:`Rendered images <Rendering>`). Clearly, when we let PowerPoint
do the rendering, we do not get the best result:

*  The anti-aliasing causes the tiles that make up the land to stand
   out. This is because the anti-aliasing algorithm blurs all edges,
   even when the tiles join seamlessly.

*  The background color was assumed to be white, hence the text is
   "smoothed" using gray shades. Instead, shades of blue which would be
   appropriate for the background we are using.

On the central column of Figure :ref:`Rendered images <Rendering>` we have
included PNG
versions of a portion of the same example. This shows the workings of
anti-aliasing and different resolutions. All samples were obtained with
**convert**. The one on the top uses all default settings, resulting
in an anti-aliased image at 72 dpi resolution (very much like the PDF
included directly into PowerPoint).

Just switching anti-aliasing off (middle) is clearly not an option
either. It is true that we got rid of the gray blurring and the seams
between the tiles, but without anti-aliasing the image becomes very
blocky. The solution is to render the image at a higher resolution
(e.g., 300 dpi) without anti-aliasing and then shrink the image to the
appropriate size (bottom of the central column in
Figure :ref:`Rendered images <Rendering>`). The scaling, rotation as well as
the selection
of the transparent color can be accomplished through the "Formatting"
tool bar and the "Format Picture" dialogue box of PowerPoint
(Figure :ref:`PowerPoint dialogue box <PowerPoint_dialogue>`), which can be
found by double clicking the
included image (or selecting and right-clicking or control-clicking on a
one-button mouse).

Concluding remarks
------------------

These examples do not constitute endorsements of the products mentioned
above; they only represent our limited experience with adding
PostScript to various types of documents. For other solutions and
further help, please post messages to.

E. Predefined Bit and Hachure Patterns in GMT
=============================================


GMT provides 90 different bit and hachure patterns that can be
selected with the **-Gp** or **-GP** option in most plotting programs.
The left side of each image was created using **-Gp**, the right side
shows the inverted version using **-GP**. These patterns are reproduced
below at 300 dpi using the default black and white shades.

.. figure:: /_images/GMT_App_E.*
   :width: 500 px
   :align: center


F. Chart of Octal Codes for Characters
======================================

The characters and their octal codes in the Standard and ISOLatin1
encoded fonts are shown in
Figure :ref:`Octal codes for Standard and ISO <Octal_codes_stand_iso>`. Light red areas signify
codes reserved for control characters. In order to use all the extended
characters (shown in the light green boxes) you need to set
:ref:`PS_CHAR_ENCODING <PS_CHAR_ENCODING>` to Standard+ or ISOLatin1+ in your :doc:`gmt.conf` file [32]_.

.. _Octal_codes_stand_iso:

.. figure:: /_images/GMT_App_F_stand+_iso+.*
   :width: 500 px
   :align: center

   Octal codes and corresponding symbols for StandardEncoding (left) and ISOLatin1Encoding (right) fonts.

The chart for the Symbol character set (GMT font number 12) and Pifont
ZapfDingbats character set (font number 34) are presented in
Figure :ref:`Octal codes for Symbol and ZapfDingbats <Octal_codes_symbol_zap>` below. The octal code
is obtained by appending the column value to the \\??
value, e.g., :math:`\partial` is \\266 in the Symbol
font. The euro currency symbol is \\240 in the Symbol
font and will print if your printer supports it (older printer's
firmware will not know about the euro).

.. _Octal_codes_symbol_zap:

.. figure:: /_images/GMT_App_F_symbol_dingbats.*
   :width: 500 px
   :align: center

   Octal codes and corresponding symbols for Symbol (left) and ZapfDingbats (right) fonts.


G. PostScript Fonts Used by GMT
=================================

GMT uses the standard 35 fonts that come with most
PostScript laserwriters. If your printer does not support some of
these fonts, it will automatically substitute the default font (which is
usually Courier). The following is a list of the GMT fonts:

.. figure:: /_images/GMT_App_G.*
   :width: 500 px
   :align: center

   The standard 35 PostScript fonts recognized by GMT.


For the special fonts Symbol (12) and ZapfDingbats (34), see the octal
charts in Appendix [app:F]. When specifying fonts in GMT, you can
either give the entire font name *or* just the font number listed in
this table. To change the fonts used in plotting basemap frames, see the
man page for :doc:`gmt.conf`. For direct
plotting of text-strings, see the man page for :doc:`pstext`.

Using non-default fonts with GMT
--------------------------------

To add additional fonts that you may have purchased or that are
available freely in the internet or at your institution, see the
instructions in the ``CUSTOM_font_info.d`` under the ``share/pslib`` directory and continue reading. GMT does
not read or process any font files and thus does not know anything about
installed fonts and their metrics. In order to use extra fonts in
GMT you need to specify the PostScript name of the relevant fonts in
the file ``CUSTOM_font_info.d``. You can either edit the existing file distributed with
GMT to make the changes global or you can create a new file in the
current working directory, e.g.,

   ::

    LinBiolinumO      0.700    0
    LinLibertineOB    0.700    0

The format is a space delimited list of the PostScript font name, the
font height-point size-ratio, and a boolean variable that tells GMT to
re-encode the font (if set to zero). The latter has to be set to zero as
additional fonts will most likely not come in standard
PostScript encoding. GMT determines how tall typical annotations
might be from the font size ratio so that the vertical position of
labels and titles can be adjusted to a more uniform typesetting. Now,
you can set the GMT font parameters to your non-standard fonts:

   ::

    gmt set FONT              LinBiolinumO \
            FONT_TITLE        28p,LinLibertineOB \
            PS_CHAR_ENCODING  ISO-8859-1 \
            MAP_DEGREE_SYMBOL degree

After setting the encoding and the degree symbol, the configuration part
for GMT is finished and you can proceed to create GMT-maps as usual.
An example script is discussed in Example [sec:non-default-fonts-example].

Embedding fonts in PostScript and PDF
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you have Type 1 fonts in PFA (Printer Font ASCII) format you can
embed them directly by copying them at the very top of your
PostScript-file, before even the %!PS header comment. PFB (Printer
Font Binary), TrueType or OpenType fonts cannot be embedded in
PostScript directly and therefore have to be converted to PFA first.

However, you most likely will have to tell Ghostscript where to
find your custom fonts in order to convert your GMT-PostScript-plot
to PDF or an image with :doc:`ps2raster`.
When you have used the correct PostScript-names of the fonts in ``CUSTOM_font_info.d`` you
only need to point the ``GS_FONTPATH`` environment variable to the
directory where the font files can be found and invoke
:doc:`ps2raster` in the usual way. Likewise
you can specify Ghostscript's ``-sFONTPATH`` option on the
command line with ``C -sFONTPATH=/path/to/fontdir``. Ghostscript,
which is invoked by :doc:`ps2raster`, does
not depend on file names. It will automatically find the relevant font
files by their PostScript-names and embed and subset them in
PDF-files. This is quite convenient as the document can be displayed and
printed even on other computers when the font is not available locally.
There is no need to convert your fonts as Ghostscript can handle
all Type 1, TrueType and OpenType fonts. Note also, that you do not need
to edit Ghostscript's Fontmap.

If you do not want or cannot embed the fonts you can convert them to
outlines (shapes with fills) with Ghostscript in the following
way:

   ::

     gs -q -dNOCACHE -dSAFER -dNOPAUSE -dBATCH -dNOPLATFONTS \
        -sDEVICE=pswrite -sFONTPATH="/path/to/fontdir" \
        -sOutputFile=mapWithOutlinedFonts.ps map.ps

Note, that this only works with the *pswrite* device. If you need
outlined fonts in PDF, create the PDF from the converted
PostScript-file. Also, :doc:`ps2raster`
cannot correctly crop Ghostscript converted PostScript-files
anymore. Use Heiko Oberdiek's instead or crop with
:doc:`ps2raster` **-A** **-Te** before (See
Example [sec:non-default-fonts-example]).

Character encoding
~~~~~~~~~~~~~~~~~~

Since PostScript itself does not support Unicode fonts,
Ghostscript will re-encode the fonts on the fly. You have to make
sure to set the correct :ref:`PS_CHAR_ENCODING <PS_CHAR_ENCODING>`
with :doc:`gmtset` and save your
script file with the same character encoding. Alternatively, you can
substitute all non ASCII characters with their corresponding octal
codes, e.g., \\265 instead of μ. Note, that PostScript fonts support
only a small range of glyphs and you may have to switch the
:ref:`PS_CHAR_ENCODING <PS_CHAR_ENCODING>` within your script.

H. Color Space: The Final Frontier
==================================


In this Appendix, we are going to try to explain the relationship
between the RGB, CMYK, and HSV color systems so as to (hopefully) make
them more intuitive. GMT allows users to specify colors in CPT files
in either of these three systems. Interpolation between colors is
performed in either RGB or HSV, depending on the specification in the
CPT file. Below, we will explain why this all matters.

RGB color system
----------------

Remember your (parents') first color television set? Likely it had three
little bright colored squares on it: red, green, and blue. And that is
exactly what each color on the tube is made of: varying levels of red,
green and blue light. Switch all of them off, *r=g=b=0*, then you
have black. All of them at maximum, *r=g=b=255*, creates white.
Your computer screen works the same way.

A mix of levels of red, green, and blue creates basically any color
imaginable. In GMT each color can be represented by the triplet
*r7g7b*. For example, 127/255/0 (half red, full
green, and no blue) creates a color called chartreuse. The color sliders
in the graphics program GIMP are an excellent way to experiment
with colors, since they show you in advance how moving one of the color
sliders will change the color. As Figure :ref:`Chartreuse in GIMP <GIMP>`\ *a*
shows: increase
the red and you will get a more yellow color, while lowering the blue
level will turn it into brown.

.. _GIMP:

.. figure:: /_images/gimp-sliders+panel.png
   :height: 230 px
   :width: 775 px
   :align: center
   :scale: 75 %

   Chartreuse in GIMP. *(a)* Sliders indicate how the color is altered
   when changing the H, S, V, R, G, or B levels. *(b)* For a constant hue (here 90)
   value increases to the right and saturation increases up, so the pure
   color is on the top right.


Is chocolate your favorite color, but you do not know the RGB equivalent
values? Then look them up in Figure :ref:`RGB chart <RGBchart>` or type
``man gmtcolors`` for a full list. It's 210/105/30. But GMT makes it easy
on you: you can specify pen, fill, and palette colors by any of the more
than 500 unique colors found in that file.

Are you very web-savvy and work best with hexadecimal color codes as
they are used in HTML? Even that is allowed in GMT. Just start with a
hash mark (``#``) and follow with the 2 hexadecimal characters for red,
green, and blue. For example, you can use ``#79ff00`` for chartreuse,
``#D2691E`` for chocolate.

.. _RGBchart:

.. figure:: /_images/GMT_RGBchart_a4.*
   :width: 700 px
   :align: center

   The 555 unique color names that can be used in GMT. Lower, upper, or mixed cases, as well as
   the british spelling of grey are allowed. A4, Letter, and Tabloid sized versions of this RGB chart can be
   found in the GMT documentation directory.


HSV color system
----------------

If you have played around with RGB color sliders, you will have noticed
that it is not intuitive to make a chosen color lighter or darker, more
saturated or more gray. It would involve changing three sliders. To make
it easier to manipulate colors in terms of lightness and saturation,
another coordinate system was invented: HSV (hue, saturation, value).
Those terms can be made clear best by looking at the color sliders in
Figure :ref:`Chartreuse in GIMP <GIMP>`\ *a*. Hue (running from 0 to 360) gives you the full
spectrum of saturated colors. Saturation (from 0 to 1, or 100%) tells
you how ‘full' your color is: reduce it to zero and you only have gray
scales. Value (from 0 to 1, or 100%) will bring you from black to a
fully saturated color. Note that "value" is not the same as "intensity",
or "lightness", used in other color geometries. "Brilliance" may be the
best alternative word to describe "value". Apple calls it as
"brightness", and hence refers to HSB for this color space.

Want more chartreuse or chocolate? You can specify them in GMT as
90-1-1 and 25-0.86-0.82, respectively.

The color cube
--------------

We are going to try to give you a geometric picture of color mixing in
RGB and HSV by means of a tour of the RGB cube depicted in
Figure :ref:`Example 11 <Fig_example_11>`. The geometric picture is most
helpful, we think, since HSV are not orthogonal coordinates and not
found from RGB by a simple algebraic transformation. So here goes: Look
at the cube face with black, red, magenta, and blue corners. This is the
*g* = 0 face. Orient the cube so that you are looking at this face
with black in the lower left corner. Now imagine a right-handed
cartesian (*rgb*) coordinate system with
origin at the black point; you are looking at the *g = 0* plane
with *r* increasing to your right, *g* increasing away from
you, and *b* increasing up. Keep this sense of (*rgb*) as you look at the cube.

Now tip the cube such that the black corner faces down and the white
corner up. When looking from the top, you can see the hue, contoured in
gray solid lines, running around in 360 counter-clockwise. It starts
with shades of red (0), then goes through green (120) and blue (240),
back to red.

On the three faces that are now on the lower side (with the white print)
one of (*rgb*) is equal to 0. These three
faces meet at the black corner, where *r = g = b = 0*. On these
three faces the colors are fully saturated: *s = 1*. The dashed
white lines indicate different levels of *v*, ranging from 0 to 1
with contours every 0.1.

On the upper three faces (with the black print), one of
(*rgb*) is equal to the maximum value. These
three faces meet at the white corner, where *r = g = b = 255*. On
these three faces value is at its maximum: *v = 1* (or 100%). The
dashed black lines indicate varying levels of saturation: *s*
ranges from 0 to 1 with contours every 0.1.

Now turn the cube around on its vertical axis (running from the black to
the white corner). Along the six edges that zigzag around the "equator",
both saturation and value are maximum, so *s = v = 1*. Twirling
the cube around and tracing the zigzag, you will visit six of the eight
corners of the cube, with changing hue (*h*): red (0), yellow
(60), green (120), cyan (180), blue (240), and magenta (300). Three of
these are the RGB colors; the other three are the CMY colors which are
the complement of RGB and are used in many color hardcopy devices (see
below). The only cube corners you did not visit on this path are the
black and white corners. They lie on the vertical axis where hue is
undefined and *r = g = b*. Any point on this axis is a shade of gray.

Let us call the points where *s = v = 1* (points along the RYGCBM
path described above) the "pure" colors. If we start at a pure color and
we want to whiten it, we can keep *h* constant and *v = 1*
while decreasing *s*; this will move us along one of the cube
faces toward the white point. If we start at a pure color and we want to
blacken it, we can keep *h* constant and *s = 1* while
decreasing *v*; this will move us along one of the cube faces
toward the black point. Any point in (*rgb*)
space which can be thought of as a mixture of pure color + white, or
pure color + black, is on a face of the cube.

The points in the interior of the cube are a little harder to describe.
The definition for *h* above works at all points in (non-gray)
(*rgb*) space, but so far we have only
looked at (*s*, *v*) on the cube faces, not inside it. At
interior points, none of (*rgb*) is equal to
either 0 or 255. Choose such a point, not on the gray axis. Now draw a
line through your point so that the line intersects the gray axis and
also intersects the RYGCBM path of edges somewhere. It is always
possible to construct this line, and all points on this line have the
same hue. This construction shows that any point in RGB space can be
thought of as a mixture of a pure color plus a shade of gray. If we move
along this line away from the gray axis toward the pure color, we are
"purifying" the color by "removing gray"; this move increases the
color's saturation. When we get to the point where we cannot remove any
more gray, at least one of (*rgb*) will have
become zero and the color is now fully saturated; *s = 1*.
Conversely, any point on the gray axis is completely undersaturated, so
that *s = 0* there. Now we see that the black point is special,
*s* is both 0 and 1 at the same time. In other words, at the black
point saturation in undefined (and so is hue). The convention is to use
*h = s = v = 0* at this point.

It remains to define value. To do so, try this: Take your point in RGB
space and construct a line through it so that this line goes through the
black point; produce this line from black past your point until it hits
a face on which *v = 1*. All points on this line have the same
hue. Note that this line and the line we made in the previous paragraph
are both contained in the plane whose hue is constant. These two lines
meet at some arbitrary angle which varies depending on which point you
chose. Thus HSV is not an orthogonal coordinate system. If the line you
made in the previous paragraph happened to touch the gray axis at the
black point, then these two lines are the same line, which is why the
black point is special. Now, the line we made in this paragraph
illustrates the following: If your chosen point is not already at the
end of the line, where *v = 1*, then it is possible to move along
the line in that direction so as to increase
(*rgb*) while keeping the same hue. The
effect this has on a color monitor is to make the color more
"brilliant", your hue will become "stronger"; if you are already on a
plane where at least one of (*rgb*) = 255,
then you cannot get a stronger version of the same hue. Thus, *v*
measures brilliance or strength. Note that it is not quite true to say
that *v* measures distance away from the black point, because
*v* is not equal to :math:`\sqrt{r^2 + g^2 + b^2}/255`.

Another representation of the HSV space is the color cone illustrated in
Figure :ref:`hsv_cone`.

.. _hsv_cone:

.. figure:: /_images/hsv-cone.png
   :height: 508 px
   :width: 750 px
   :align: center
   :scale: 50 %

   The HSV color space

Color interpolation
-------------------

From studying the RGB cube, we hope you will have understood that there
are different routes to follow between two colors, depending whether you
are in the RGB or HSV system. Suppose you would make an interpolation
between blue and red. In the RGB system you would follow a path
diagonally across a face of the cube, from 0/0/255 (blue) via 127/0/127
(purple) to 255/0/0 (red). In the HSV system, you would trace two edges,
from 240-1-1 (blue) via 300-1-1 (magenta) to 360-1-1 (red). That is even
assuming software would be smart enough to go the shorter route. More
likely, red will be recorded as 0-1-1, so hue will be interpolated the
other way around, reducing hue from 240 to 0, via cyan, green, and yellow.

Depending on the design of your color palette, you may want to have it
either way. By default, GMT interpolates in RGB space, even when the
original color palette is in the HSV system. However, when you add the
line ``#COLOR_MODEL=+HSV`` (with the leading ‘+' sign) in the header of
the color palette file, GMT will not only read the color
representation as HSV values, but also interpolate colors in the HSV
system. That means that H, S, and V values are interpolated linearly
between two colors, instead of their respective R, G, and B values.

The top row in Figure :ref:`Interpolating colors <color_interpolate>`
illustrates two examples: a blue-white-red scale (the palette in
Appendix [app:M]) interpolated in RGB and the palette interpolated in
HSV. The bottom row of the Figure demonstrates how things can go
terribly wrong when you do the interpolation in the other system.

.. _color_interpolate:

.. figure:: /_images/GMT_color_interpolate.*
   :width: 500 px
   :align: center

   When interpolating colors, the color system matters. The polar palette on the left needs to
   be interpolated in RGB, otherwise hue will change between blue (240) and white (0). The rainbow
   palette should be interpolated in HSV, since only hue should change between magenta (300) and red (0).
   Diamonds indicate which colors are defined in the palettes; they are fixed, the rest is interpolated.


Artificial illumination
-----------------------

GMT uses the HSV system to achieve artificial illumination of colored
images (e.g., **-I** option in :doc:`grdimage`) by changing the saturation
*s* and value *v* coordinates of the color. When the intensity is zero
(flat illumination), the data are colored according to the CPT file. If
the intensity is non-zero, the color is either lightened or darkened
depending on the illumination. The color is first converted to HSV (if
necessary) and then darkened by moving (*sv*) toward
(:ref:`COLOR_HSV_MIN_S <COLOR_HSV_MIN_S>`, :ref:`COLOR_HSV_MIN_V <COLOR_HSV_MIN_V>`)
if the intensity is negative, or lightened by sliding (*sv*) toward
(:ref:`COLOR_HSV_MAX_S <COLOR_HSV_MAX_S>`, :ref:`COLOR_HSV_MAX_V <COLOR_HSV_MAX_V>`)
if the illumination is positive. The extremes of the *s* and *v* are defined in the
:doc:`gmt.conf` file and are usually chosen so the corresponding points are nearly black
(*s = 1*, *v = 0*) and white (*s = 0*, *v = 1*).
The reason this works is that the HSV system allows movements in color
space which correspond more closely to what we mean by "tint" and
"shade"; an instruction like "add white" is easy in HSV and not so
obvious in RGB.

Thinking in RGB or HSV
----------------------

The RGB system is understandable because it is cartesian, and we all
learned cartesian coordinates in school. But it doesn't help us create a
tint or shade of a color; we cannot say, "We want orange, and a lighter
shade of orange, or a less vivid orange". With HSV we can do this, by
saying, "Orange must be between red and yellow, so its hue is about
*h = 30*; a less vivid orange has a lesser *s*, a darker
orange has a lesser *v*". On the other hand, the HSV system is a
peculiar geometric construction, more like a cone
(Figure :ref:`hsv_cone`). It is not an orthogonal coordinate system, and
it is not found by a matrix transformation of RGB; these make it
difficult in some cases too. Note that a move toward black or a move
toward white will change both *s* and *v*, in the general
case of an interior point in the cube. The HSV system also doesn't
behave well for very dark colors, where the gray point is near black and
the two lines we constructed above are almost parallel. If you are
trying to create nice colors for drawing chocolates, for example, you
may be better off guessing in RGB coordinates.

CMYK color system
-----------------

Finally, you can imagine that printers work in a different way: they mix
different paints to make a color. The more paint, the darker the color,
which is the reverse of adding more light. Also, mixing more colored
paints does not give you true black, so that means that you really need
four colors to do it right. Open up your color printer and you'll
probably find four cartridges: cyan, magenta, yellow (often these are
combined into one), and black. They form the CMYK system of colors, each
value running from 0 to 1 (or 100%). In GMT CMYK color coding can be
achieved using *c/m/y/k* quadruplets.

Obviously, there is no unique way to go from the 3-dimensional RGB
system to the 4-dimensional CMYK system. So, again, there is a lot of
hand waving applied in the transformation. Strikingly, CMYK actually
covers a smaller color space than RGB. We will not try to explain you
the details behind it, just know that there is a transformation needed
to go from the colors on your screen to the colors on your printer. It
might explain why what you see is not necessarily what you get. If you
are really concerned about how your color plots will show up in your PhD
thesis, for example, it might be worth trying to save and print all your
color plots using the CMYK system. Letting GMT do the conversion to
CMYK may avoid some nasty surprises when it comes down to printing. To
specify the color space of your PostScript file, set
:ref:`PS_COLOR_MODEL <PS_COLOR_MODEL>` in the :doc:`gmt.conf` file to RGB, HSV, or CMYK.

I. Filtering of Data in GMT
===========================

The GMT programs :doc:`filter1d` (for
tables of data indexed to one independent variable) and
:doc:`grdfilter` (for data given as
2-dimensional grids) allow filtering of data by a moving-window process.
(To filter a grid by Fourier transform use
:doc:`grdfft`.) Both programs use an argument
**-F**\ <\ *type*\ ><\ *width*> to specify the
type of process and the window's width (in 1-D) or diameter (in 2-D).
(In :doc:`filter1d` the width is a length of
the time or space ordinate axis, while in
:doc:`grdfilter` it is the diameter of a
circular area whose distance unit is related to the grid mesh via the
**-D** option). If the process is a median, mode, or extreme value
estimator then the window output cannot be written as a convolution and
the filtering operation is not a linear operator. If the process is a
weighted average, as in the boxcar, cosine, and gaussian filter types,
then linear operator theory applies to the filtering process. These
three filters can be described as convolutions with an impulse response
function, and their transfer functions can be used to describe how they
alter components in the input as a function of wavelength.

Impulse responses are shown here for the boxcar, cosine, and gaussian
filters. Only the relative amplitudes of the filter weights shown; the
values in the center of the window have been fixed equal to 1 for ease
of plotting. In this way the same graph can serve to illustrate both the
1-D and 2-D impulse responses; in the 2-D case this plot is a
diametrical cross-section through the filter weights
(Figure :ref:`Impulse responses <Impulse_responses>`).

.. _Impulse_responses:

.. figure:: /_images/GMT_App_J_1.*
   :width: 500 px
   :align: center

   Impulse responses for GMT filters.


Although the impulse responses look the same in 1-D and 2-D, this is not
true of the transfer functions; in 1-D the transfer function is the
Fourier transform of the impulse response, while in 2-D it is the Hankel
transform of the impulse response. These are shown in Figures
:ref:`Transfer functions for 1D <GMT_1D_filters>` and
:ref:`2D <GMT_2D_filters>`,
respectively. Note that in 1-D the
boxcar transfer function has its first zero crossing at *f = 1*,
while in 2-D it is around :math:`f \sim 1.2`. The 1-D cosine transfer
function has its first zero crossing at *f = 2*; so a cosine
filter needs to be twice as wide as a boxcar filter in order to zero the
same lowest frequency. As a general rule, the cosine and gaussian
filters are "better" in the sense that they do not have the "side lobes"
(large-amplitude oscillations in the transfer function) that the boxcar
filter has. However, they are correspondingly "worse" in the sense that
they require more work (doubling the width to achieve the same cut-off wavelength).

.. _GMT_1D_filters:

.. figure:: /_images/GMT_App_J_2.*
   :width: 500 px
   :align: center

   Transfer functions for 1-D GMT filters.


One of the nice things about the gaussian filter is that its transfer
functions are the same in 1-D and 2-D. Another nice property is that it
has no negative side lobes. There are many definitions of the gaussian
filter in the literature (see page 7 of Bracewell [33]_). We define
:math:`\sigma` equal to 1/6 of the filter width, and the impulse
response proportional to :math:`\exp[-0.5(t/\sigma)^2)`. With this
definition, the transfer function is :math:`\exp[-2(\pi\sigma f)^2]` and
the wavelength at which the transfer function equals 0.5 is about 5.34
:math:`\sigma`, or about 0.89 of the filter width.

.. _GMT_2D_filters:

.. figure:: /_images/GMT_App_J_3.*
   :width: 500 px
   :align: center

   Transfer functions for 2-D (radial) GMT filters.


J. The GMT High-Resolution Coastline Data
=========================================


Starting with version 3.0, GMT use a completely new coastline database
and the :doc:`pscoast` utility was been
completely rewritten to handle the new file format. Many users have
asked us why it has taken so long for GMT to use a high-resolution
coastline database; after all, such data have been available in the
public domain for years. To answer such questions we will take you along
the road that starts with these public domain data sets and ends up with
the database used by GMT.

Selecting the right data
------------------------

There are two well-known public-domain data sets that could be used for
this purpose. Once is known as the World Data Bank II or CIA Data Bank
(WDB) and contains coastlines, lakes, political boundaries, and rivers.
The other, the World Vector Shoreline (WVS) only contains shorelines
between saltwater and land (i.e., no lakes). It turns out that the WVS
data is far superior to the WDB data as far as data quality goes, but as
noted it lacks lakes, not to mention rivers and borders. We decided to
use the WVS whenever possible and supplement it with WDB data. We got
these data over the Internet; they are also available on CD-ROM from the
National Geophysical Data Center in Boulder, Colorado [34]_.

Format required by GMT
----------------------

In order to paint continents or oceans it is necessary that the
coastline data be organized in polygons that may be filled. Simple line
segments can be used to draw the coastline, but for painting polygons
are required. Both the WVS and WDB data consists of unsorted line
segments: there is no information included that tells you which segments
belong to the same polygon (e.g., Australia should be one large
polygon). In addition, polygons enclosing land must be differentiated
from polygons enclosing lakes since they will need different paint.
Finally, we want :doc:`pscoast` to be
flexible enough that it can paint the land *or* the oceans *or* both. If
just land (or oceans) is selected we do not want to paint those areas
that are not land (or oceans) since previous plot programs may have
drawn in those areas. Thus, we will need to combine polygons into new
polygons that lend themselves to fill land (or oceans) only (Note that
older versions of :doc:`pscoast` always
painted lakes and wiped out whatever was plotted beneath).

The long and winding road
-------------------------

The WVS and WDB together represent more than 100 Mb of binary data and
something like 20 million data points. Hence, it becomes obvious that
any manipulation of these data must be automated. For instance, the
reasonable requirement that no coastline should cross another coastline
becomes a complicated processing step.

*  To begin, we first made sure that all data were "clean", i.e., that
   there were no outliers and bad points. We had to write several
   programs to ensure data consistency and remove "spikes" and bad
   points from the raw data. Also, crossing segments were automatically
   "trimmed" provided only a few points had to be deleted. A few hundred
   more complicated cases had to be examined semi-manually.

*  Programs were written to examine all the loose segments and determine
   which segments should be joined to produce polygons. Because not all
   segments joined exactly (there were non-zero gaps between some
   segments) we had to find all possible combinations and choose the
   simplest combinations. The WVS segments joined to produce more than
   200,000 polygons, the largest being the Africa-Eurasia polygon which
   has 1.4 million points. The WDB data resulted in a smaller data base
   (~25% of WVS).

*  We now needed to combine the WVS and WDB data bases. The main problem
   here is that we have duplicates of polygons: most of the features in
   WVS are also in WDB. However, because the resolution of the data
   differ it is nontrivial to figure out which polygons in WDB to
   include and which ones to ignore. We used two techniques to address
   this problem. First, we looked for crossovers between all possible
   pairs of polygons. Because of the crossover processing in step 1
   above we know that there are no remaining crossovers within WVS and
   WDB; thus any crossovers would be between WVS and WDB polygons.
   Crossovers could mean two things: (1) A slightly misplaced WDB
   polygon crosses a more accurate WVS polygon, both representing the
   same geographic feature, or (2) a misplaced WDB polygon (e.g., a
   small coastal lake) crosses the accurate WVS shoreline. We
   distinguished between these cases by comparing the area and centroid
   of the two polygons. In almost all cases it was obvious when we had
   duplicates; a few cases had to be checked manually. Second, on many
   occasions the WDB duplicate polygon did not cross its WVS counterpart
   but was either entirely inside or outside the WVS polygon. In those
   cases we relied on the area-centroid tests.

*  While the largest polygons were easy to identify by visual
   inspection, the majority remain unidentified. Since it is important
   to know whether a polygon is a continent or a small pond inside an
   island inside a lake we wrote programs that would determine the
   hierarchical level of each polygon. Here, level = 1 represents
   ocean/land boundaries, 2 is land/lakes borders, 3 is
   lakes/islands-in-lakes, and 4 is
   islands-in-lakes/ponds-in-islands-in-lakes. Level 4 was the highest
   level encountered in the data. To automatically determine the
   hierarchical levels we wrote programs that would compare all possible
   pairs of polygons and find how many polygons a given polygon was
   inside. Because of the size and number of the polygons such programs
   would typically run for 3 days on a Sparc-2 workstation.

*  Once we know what type a polygon is we can enforce a common
   "orientation" for all polygons. We arranged them so that when you
   move along a polygon from beginning to end, your left hand is
   pointing toward "land". At this step we also computed the area of all
   polygons since we would like the option to plot only features that
   are bigger than a minimum area to be specified by the user.

*  Obviously, if you need to make a map of Denmark then you do not want
   to read the entire 1.4 million points making up the Africa-Eurasia
   polygon. Furthermore, most plotting devices will not let you paint
   and fill a polygon of that size due to memory restrictions. Hence, we
   need to partition the polygons so that smaller subsets can be
   accessed rapidly. Likewise, if you want to plot a world map on a
   letter-size paper there is no need to plot 10 million data points as
   most of them will plot several times on the same pixel and the
   operation would take a very long time to complete. We chose to make 5
   versions on the database, corresponding to different resolutions. The
   decimation was carried out using the Douglas-Peucker (DP)
   line-reduction algorithm [35]_. We chose the cutoffs so that each
   subset was approximately 20% the size of the next higher resolution.
   The five resolutions are called **f**\ ull, **h**\ igh,
   **i**\ ntermediate, **l**\ ow, and **c**\ rude; they are accessed in
   :doc:`pscoast`, :doc:`gmtselect`, and
   :doc:`grdlandmask` with the **-D**
   option [36]_. For each of these 5 data sets (**f**, **h**, **i**,
   **l**, **c**) we specified an equidistant grid (1, 2, 5, 10, 20) and
   split all polygons into line-segments that each fit inside one of the
   many boxes defined by these grid lines. Thus, to paint the entire
   continent of Australia we instead paint many smaller polygons made up
   of these line segments and gridlines. Some book-keeping has to be
   done since we need to know which parent polygon these smaller pieces
   came from in order to prescribe the correct paint or ignore if the
   feature is smaller than the cutoff specified by the user. The
   resulting segment coordinates were then scaled to fit in short
   integer format to preserve precision and written in netCDF format for
   ultimate portability across hardware platforms [37]_.

*  While we are now back to a file of line-segments we are in a much
   better position to create smaller polygons for painting. Two problems
   must be overcome to correctly paint an area:

   -  We must be able to join line segments and grid cell borders into
      meaningful polygons; how we do this will depend on whether we want
      to paint the land or the oceans.

   -  We want to nest the polygons so that no paint falls on areas that
      are "wet" (or "dry"); e.g., if a grid cell completely on land
      contains a lake with a small island, we do not want to paint the
      lake and then draw the island, but paint the annulus or "donut"
      that is represented by the land and lake, and then plot the
      island.

   GMT uses a polygon-assembly routine that carries out these tasks on the fly.

The Five Resolutions
--------------------

We will demonstrate the power of the new database by starting with a
regional hemisphere map centered near Papua New Guinea and zoom in on a
specified point. The map regions will be specified in projected km from
the projection center, e.g., we may want the map to go from km to km in
the longitudinal and the latitudinal direction.
Also, as we zoom in on the projection center we want to draw the outline
of the next map region on the plot. To do that we use the **-D** option
in :doc:`psbasemap`.

The crude resolution (**-Dc**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We begin with an azimuthal equidistant map of the hemisphere centered on
130\ |degree|\ 21'E, 0\ |degree|\ 12'S, which is slightly west of New Guinea, near the Strait of
Dampier. The edges of the map are all 9000 km true distance from the
projection center. At this scale (and for global maps) the crude
resolution data will usually be adequate to capture the main geographic
features. To avoid cluttering the map with insignificant detail we only
plot features (i.e., polygons) that exceed 500 km^2 in area.
Smaller features would only occupy a few pixels on the plot and make the
map look "dirty". We also add national borders to the plot. The crude
database is heavily decimated and simplified by the DP-routine: The
total file size of the coastlines, rivers, and borders database is only
283 kbytes. The plot is produced by the script:

  ::

    gmt set MAP_GRID_CROSS_SIZE_PRIMARY 0 MAP_ANNOT_OBLIQUE 22 MAP_ANNOT_MIN_SPACING 0.3i
    gmt pscoast -Rk-9000/9000/-9000/9000 -JE130.35/-0.2/3.5i -P -Dc -A500 \
                -Gburlywood -Sazure -Wthinnest -N1/thinnest,- -B20g20 -BWSne -K > GMT_App_K_1.ps
    gmt psbasemap -R -J -O -Dk2000+c130.35/-0.2+pthicker >> GMT_App_K_1.ps

.. figure:: /_images/GMT_App_K_1.*
   :width: 500 px
   :align: center

   Map using the crude resolution coastline data.


Here, we use the :ref:`MAP_ANNOT_OBLIQUE <MAP_ANNOT_OBLIQUE>` bit flags to achieve horizontal
annotations and set :ref:`MAP_ANNOT_MIN_SPACING <MAP_ANNOT_MIN_SPACING>` to suppress some
longitudinal annotations near the S pole that otherwise would overprint.
The square box indicates the outline of the next map.

The low resolution (**-Dl**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We have now reduced the map area by zooming in on the map center. Now,
the edges of the map are all 2000 km true distance from the projection
center. At this scale we choose the low resolution data that faithfully
reproduce the dominant geographic features in the region. We cut back on
minor features less than 100 km^2 in area. We still add
national borders to the plot. The low database is less decimated and
simplified by the DP-routine: The total file size of the coastlines,
rivers, and borders combined grows to 907 kbytes; it is the default
resolution in GMT. The plot is generated by the script:

  ::

    gmt pscoast -Rk-2000/2000/-2000/2000 -JE130.35/-0.2/3.5i -P -Dl -A100 -Gburlywood \
                -Sazure -Wthinnest -N1/thinnest,- -B10g5 -BWSne -K > GMT_App_K_2.ps
    gmt psbasemap -R -J -O -Dk500+c130.35/-0.2+pthicker >> GMT_App_K_2.ps

.. figure:: /_images/GMT_App_K_2.*
   :width: 500 px
   :align: center

   Map using the low resolution coastline data.


The intermediate resolution (**-Di**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We continue to zoom in on the map center. In this map, the edges of the
map are all 500 km true distance from the projection center. We abandon
the low resolution data set as it would look too jagged at this scale
and instead employ the intermediate resolution data that faithfully
reproduce the dominant geographic features in the region. This time, we
ignore features less than 20 km^2 in area. Although the script
still asks for national borders none exist within our region. The
intermediate database is moderately decimated and simplified by the
DP-routine: The combined file size of the coastlines, rivers, and
borders now exceeds 3.35 Mbytes. The plot is generated by the script:

  ::

    gmt pscoast -Rk-500/500/-500/500 -JE130.35/-0.2/3.5i -P -Di -A20 -Gburlywood \
                -Sazure -Wthinnest -N1/thinnest,- -B2g1 -BWSne -K > GMT_App_K_3.ps
    echo 133 2 | gmt psxy -R -J -O -K -Sc1.4i -Gwhite >> GMT_App_K_3.ps
    gmt psbasemap -R -J -O -K --FONT_TITLE=12p --MAP_TICK_LENGTH_PRIMARY=0.05i \
                  -Tm133/2/1i::+45/10/5 --FONT_ANNOT_SECONDARY=8p >> GMT_App_K_3.ps
    gmt psbasemap -R -J -O -Dk100+c130.35/-0.2+pthicker >> GMT_App_K_3.ps

.. figure:: /_images/GMT_App_K_3.*
   :width: 500 px
   :align: center

   Map using the intermediate resolution coastline data. We have added a compass
   rose just because we have the power to do so.


The high resolution (**-Dh**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The relentless zooming continues! Now, the edges of the map are all 100
km true distance from the projection center. We step up to the high
resolution data set as it is needed to accurately portray the detailed
geographic features within the region. Because of the small scale we
only ignore features less than 1 km^2 in area. The high
resolution database has undergone minor decimation and simplification by
the DP-routine: The combined file size of the coastlines, rivers, and
borders now swells to 12.3 Mbytes. The map and the final outline box are
generated by these commands:

  ::

    gmt pscoast -Rk-100/100/-100/100 -JE130.35/-0.2/3.5i -P -Dh -A1 -Gburlywood \
                -Sazure -Wthinnest -N1/thinnest,- -B30mg10m -BWSne -K > GMT_App_K_4.ps
    gmt psbasemap -R -J -O -Dk20+c130.35/-0.2+pthicker >> GMT_App_K_4.ps

.. figure:: /_images/GMT_App_K_4.*
   :width: 500 px
   :align: center

   Map using the high resolution coastline data.


The full resolution (**-Df**)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We now arrive at our final plot, which shows a detailed view of the
western side of the small island of Waigeo. The map area is
approximately 40 by 40 km. We call upon the full resolution data set to
portray the richness of geographic detail within this region; no
features are ignored. The full resolution has undergone no decimation
and it shows: The combined file size of the coastlines, rivers, and
borders totals a (once considered hefty) 55.9 Mbytes. Our final map is
reproduced by the single command:

  ::

    gmt pscoast -Rk-20/20/-20/20 -JE130.35/-0.2/3.5i -P -Df -Gburlywood \
                -Sazure -Wthinnest -N1/thinnest,- -B10mg2m -BWSne > GMT_App_K_5.ps

.. figure:: /_images/GMT_App_K_5.*
   :width: 500 px
   :align: center

   Map using the full resolution coastline data.


We hope you will study these examples to enable you to make efficient
and wise use of this vast data set.

K. GMT on non-\ UNIX Platforms
================================


Introduction
------------

While GMT was ported to non-\ UNIX systems such as Windows, it is
also true that one of the strengths of GMT lies its symbiotic
relationship with UNIX. We therefore recommend that GMT be installed
in a POSIX-compliant UNIX environment such as traditional
UNIX-systems, Linux, or Mac OS X. If abandoning your
non-\ UNIX operating system is not an option, consider one of these
solutions:

WINDOWS:
    Choose among these three possibilities:

    #. Install GMT under MinGW/MSYS (A collection of GNU utilities).

    #. Install GMT under Cygwin (A GNU port to Windows).

    #. Install GMT in Windows using Microsoft C/C++ or other
       compilers. Unlike the first two, this option will not provide you
       with any UNIX tools so you will be limited to what you can do
       with DOS batch files.

Cygwin and GMT
--------------

Because GMT works best in conjugation with UNIX tools we suggest you
install GMT using the Cygwin product from Cygnus (now assimilated by
Redhat, Inc.). This free version works on any Windows version and it
comes with both the Bourne Again shell **bash** and the **tcsh**.
You also have access to most standard GNU development tools such as
compilers and text processing tools (**awk**, **grep**, **sed**,
etc.). Note that executables prepared for Windows will also run under Cygwin.

Follow the instructions on the Cygwin page on how to install the
package; note you must explicitly add all the development tool packages
(e.g., **gcc** etc) as the basic installation does not include them by
default. Once you are up and running under Cygwin, you may install
GMT  the same way you do under any other UNIX platform by either
running the automated install via **install_gmt** or manually running
configure first, then type make all. If you install via the web form,
make sure you save the parameter file without DOS CR/LF endings. Use
**dos2unix** to get rid of those if need be.

Finally, from Cygwin's User Guide: By default, no Cygwin program can
allocate more than 384 MB of memory (program and data). You should not
need to change this default in most circumstances. However, if you need
to use more real or virtual memory in your machine you may add an entry
in either the **HKEY_LOCAL_MACHINE** (to change the limit for all
users) or **HKEY_CURRENT_USER** (for just the current user) section of
the registry. Add the DWORD value **heap_chunk_in_mb** and set it to
the desired memory limit in decimal Mb. It is preferred to do this in
Cygwin using the **regtool** program included in the Cygwin package.
(For more information about **regtool** or the other Cygwin utilities,
see the Section called Cygwin Utilities in Chapter 3 of the Cygwin's
User Guide or use the help option of each utility.) You should always be
careful when using **regtool** since damaging your system registry can
result in an unusable system. This example sets the local machine memory
limit to 1024 Mb:

   ::

    regtool -i set /HKLM/Software/Cygnus\ Solutions/Cygwin/heap_chunk_in_mb 1024
    regtool -v list /HKLM/Software/Cygnus\ Solutions/Cygwin

For more installation details see the general README file.

MINGW|MSYS and GMT
------------------

Though one can install GMT natively using CMake, the simplest way of installing
under MINGW|MSYS is to just install the Windows binaries and use them from
the msys bash shell. As simple as that. Furthermore, GMT programs launch
faster here than on Cygwin so this is the recommended way of running
GMT on Windows.

L. Of Colors and Color Legends
==============================


Built-in color palette tables
-----------------------------

Figures :ref:`CPT files a <CPT_files_a>` and
:ref:`b <CPT_files_b>` show the 36 built-in
color palettes, stored in so-called CPT tables [38]_. The programs
:doc:`makecpt` and
:doc:`grd2cpt` are used to access these
master CPT tables and translate/scale them to fit the user's range of
*z*-values. The top half of the color bars in the Figure shows the
original color scale, which can be either discrete or continuous, though
some (like **globe**) are a mix of the two. The bottom half the color
bar are built by using :doc:`makecpt`
**-T**-1/1/0.25, thus splitting the color scale into 8 discrete colors.

.. _CPT_files_a:

.. figure:: /_images/GMT_App_M_1a.*
   :width: 500 px
   :align: center

   The first 18 of the standard 36 CPT files supported by GMT

.. _CPT_files_b:

.. figure:: /_images/GMT_App_M_1b.*
   :width: 500 px
   :align: center

   The second 18 of the standard 36 CPT files supported by GMT


Labeled and non-equidistant color legends
-----------------------------------------

[app:colorbars] The use of color legends has already been introduced in
Chapter [ch:7] (examples 2, 16, and 17). Things become a bit more
complicated when you want to label the legend with names for certain
intervals (like geological time periods in the example below). To
accomplish that, one should add a semi-colon and the label name at the
end of a line in the CPT table and add the **-L** option to the
:doc:`psscale` command that draws the color
legend. This option also makes all intervals in the legend of equal
length, even it the numerical values are not equally spaced.

Normally, the name labels are plotted at the lower end of the intervals.
But by adding a *gap* amount (even when zero) to the **-L** option, they
are centered. The example below also shows how to annotate ranges using
**-Li** (in which case no name labels should appear in the CPT file),
and how to switch the color bar around (by using a negative length).

.. figure:: /_images/GMT_App_M_2.*
   :width: 600 px
   :align: center

For additional color tables, visit
`cpt-city <http://soliton.vm.bytemark.co.uk/pub/cpt-city/>`_.


.. _App-custom_symbols:

M. Custom Plot Symbols
======================

Background
----------

The GMT tools :doc:`psxy` and :doc:`psxyz` are capable of using custom
symbols as alternatives to the built-in, standard geometrical shapes
like circles, triangles, and many others. One the command line, custom
symbols are selected via the **-Sk**\ *symbolname*\ [*size*] symbol
selection, where *symbolname* refers to a special symbol definition file
called ``symbolname.def`` that must be available via the standard GMT user paths. Several
custom symbols comes pre-configured with GMT\ (see
Figure :ref:`Custom symbols <Custom_symbols>`)

.. _Custom_symbols:

.. figure:: /_images/GMT_App_N_1.*
   :width: 500 px
   :align: center

   Custom plot symbols supported by GMT. Note that we only show
   the symbol outline and not any fill. These are all single-parameter symbols.
   Be aware that some symbols may have a hardwired fill or no-fill component,
   while others duplicate what is already available as standard built-in symbols.


You may find it convenient to examine some of these and use them as a
starting point for your own design; they can be found in GMT's
share/custom directory.

The macro language
------------------

To make your own custom plot symbol, you will need to design your own
\*.def files. This section defines the language used to build custom
symbols. You can place these definition files in your current directory
or your .gmt user directory. When designing the symbol, you are doing so
in a relative coordinate system centered on (0,0). This point will be
mapped to the actual location specified by your data coordinates.
Furthermore, your symbol should be constructed within the domain
:math:`{-\frac{1}{2},+\frac{1}{2},-\frac{1}{2},+\frac{1}{2}}`, resulting
in a 1 by 1 relative canvas area. This 1 x 1 square will be scaled to your
actual symbol size when plotted.

Comment lines
~~~~~~~~~~~~~

Your definition file may have any number of comment lines, defined to
begin with the character #. These are skipped by GMT but provides a
mechanism for you to clarify what your symbol does.

Symbol variables
~~~~~~~~~~~~~~~~

Simple symbols, such as circles and triangles, only take a single
parameter: the symbol size, which is either given on the command line
(via **-Sk**) or as part of the input data. However, more complicated
symbols, such as the ellipse or vector symbols, may require more
parameters. If your custom symbol requires more than the single size
parameter you must include the line

    **N**: *n_extra_parameters* [*types*]

before any other macro commands. It is an optional statement in that
*n_extra_parameters* will default to 0 unless explicitly set. By
default the extra parameters are considered to be quantities that should
be passed directly to the symbol machinery. However, you can use the
*types* argument to specify different types of parameters. The available
types are

  **a** Geographic angle, to be converted to map angle given the current
  map projection.

  **l** Length, i.e., an additional length scale (in cm, inch, or point as
  per :ref:`PROJ_LENGTH_UNIT <PROJ_LENGTH_UNIT>`) in addition to the given symbol size.

  **o** Other, i.e., a numerical quantity to be passed to the custom symbol as is.

  **s** String, i.e., a single column of text to be placed by the **l** command.
  Use octal \\040 to include spaces while still remaining a single word.

To use the extra parameters in your macro you address them as $1, $2, etc.

Macro commands
~~~~~~~~~~~~~~

The custom symbol language contains commands to rotate the relative
coordinate system, draw free-form polygons and lines, change the current
fill and/or pen, place text, and include basic geometric symbols as part of the
overall design (e.g., circles, triangles, etc.). The available commands
are listed in Table :ref:`custsymb <tbl-custsymb>`.

.. _tbl-custsymb:

+---------------+------------+----------------------------------------+--------------------------------------------+
| **Name**      | **Code**   | **Purpose**                            | **Arguments**                              |
+===============+============+========================================+============================================+
| rotate        | **R**      | Rotate the coordinate system           | :math:`\alpha`\[**a**]                     |
+---------------+------------+----------------------------------------+--------------------------------------------+
| moveto        | **M**      | Set a new anchor point                 | :math:`x_0, y_0`                           |
+---------------+------------+----------------------------------------+--------------------------------------------+
| drawto        | **D**      | Draw line from previous point          | :math:`x, y`                               |
+---------------+------------+----------------------------------------+--------------------------------------------+
| arc           | **A**      | Append circular arc to existing path   | :math:`x_c, y_c, r, \alpha_1, \alpha_2`    |
+---------------+------------+----------------------------------------+--------------------------------------------+
| stroke        | **S**      | Stroke existing path only              |                                            |
+---------------+------------+----------------------------------------+--------------------------------------------+
| texture       | **T**      | Change current pen and fill            |                                            |
+---------------+------------+----------------------------------------+--------------------------------------------+
| star          | **a**      | Plot a star                            | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| circle        | **c**      | Plot a circle                          | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| diamond       | **d**      | Plot a diamond                         | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| ellipse       | **e**      | Plot a ellipse                         | :math:`x, y, \alpha`,\ *major*,\ *minor*   |
+---------------+------------+----------------------------------------+--------------------------------------------+
| octagon       | **g**      | Plot an octagon                        | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| hexagon       | **h**      | Plot a hexagon                         | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| invtriangle   | **i**      | Plot an inverted triangle              | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| letter        | **l**      | Plot a letter                          | :math:`x, y`,\ *size*, *string*            |
+---------------+------------+----------------------------------------+--------------------------------------------+
| marc          | **m**      | Plot a math arc                        | :math:`x, y, r, \alpha_1, \alpha_2`        |
+---------------+------------+----------------------------------------+--------------------------------------------+
| pentagon      | **n**      | Plot a pentagon                        | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| plus          | **+**      | Plot a plus sign                       | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| rect          | **r**      | Plot a rectangle                       | :math:`x, y`, *width*, *height*            |
+---------------+------------+----------------------------------------+--------------------------------------------+
| square        | **s**      | Plot a square                          | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| triangle      | **t**      | Plot a triangle                        | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| wedge         | **w**      | Plot a wedge                           | :math:`x, y, r, \alpha_1, \alpha_2`        |
+---------------+------------+----------------------------------------+--------------------------------------------+
| cross         | **x**      | Plot a cross                           | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| x-dash        | **-**      | Plot a x-dash                          | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+
| y-dash        | **y**      | Plot a y-dash                          | :math:`x, y`,\ *size*                      |
+---------------+------------+----------------------------------------+--------------------------------------------+

Note for **R**\: if an **a** is appended then :math:`\alpha` is considered
to be a map azimuth; otherwise it is a Cartesian angle.
For **M**, **T**, and all the lower-case symbol codes you may optionally
append specific pens (with **-W**\ *pen*) and fills (with
**-G**\ *pen*). These settings will override the pens and fills you may
have specified on the command line. Passing **-G**- or **-W**- means no
fill or outline, respectively.

Text substitution
~~~~~~~~~~~~~~~~~

Normally, the **l** macro code will place a hard-wired text string.  However,
you can also obtain the entire string from your input file via a single symbol
variable that must be declared with type  **s** (string).  The string read
from your input file must be a single word, so if you need spaces you must
use the octal \\040 code.  Similarly, to place the dollar sign $ you must
use octal \\044 so as to not confuse the parser with a symbol variable.
The string itself, if obtained from the symbol definition file,
may contain special codes that will be expanded given the current record.  You
can embed %X or %Y to add the current longitude (or x) and latitude (or y) in
your label string. You may also use $n to embed a numerical symbol variable as text.
It will be formatted according to :ref:`FORMAT_FLOAT_MAP <FORMAT_FLOAT_MAP>`,
unless you append the modifiers **+X** (longitude via :ref:`FORMAT_GEO_MAP <FORMAT_GEO_MAP>`),
**+Y** (latitude via :ref:`FORMAT_GEO_MAP <FORMAT_GEO_MAP>`), or **+T** (calendar time via
:ref:`FORMAT_DATE_MAP <FORMAT_DATE_MAP>` and :ref:`FORMAT_CLOCK_MAP <FORMAT_CLOCK_MAP>`.

Text alignment and font
~~~~~~~~~~~~~~~~~~~~~~~

Like the **Sl** symbol in :doc:`psxy`, you can change the current
font by appending to **l** the modifier **+f**\ *font* and the text justification
by appending the modifier **+j**\ *justify*.

Conditional statements
~~~~~~~~~~~~~~~~~~~~~~

There are two types of conditional statements in the macro language: A
simple condition preceding a single command, or a more elaborate
if-then-elseif-else construct. In any test you may use one (and only
one) of many logical operators, as listed in Table :ref:`custop <tbl-custop>`.

.. _tbl-custop:

+----------------+----------------------------------------------------------+
| **Operator**   | **Purpose**                                              |
+================+==========================================================+
| <              | Is *var* less than *constant*?                           |
+----------------+----------------------------------------------------------+
| <=             | Is *var* less than or equal to *constant*?               |
+----------------+----------------------------------------------------------+
| ==             | Is *var* equal to *constant*?                            |
+----------------+----------------------------------------------------------+
| !=             | Is *var* not equal to *constant*?                        |
+----------------+----------------------------------------------------------+
| >=             | Is *var* greater than or equal to *constant*?            |
+----------------+----------------------------------------------------------+
| >              | Is *var* greater than *constant*?                        |
+----------------+----------------------------------------------------------+
| %              | Does *var* have a remainder with *constant*?             |
+----------------+----------------------------------------------------------+
| !%             | Is *var* an exact multiple of *constant*?                |
+----------------+----------------------------------------------------------+
| <>             | Is *var* within the exclusive range of *constant*?       |
+----------------+----------------------------------------------------------+
| []             | Is *var* within the inclusive range of *constant*?       |
+----------------+----------------------------------------------------------+
| <]             | Is *var* within the in/ex-clusive range of *constant*?   |
+----------------+----------------------------------------------------------+
| [>             | Is *var* within the ex/in-clusive range of *constant*?   |
+----------------+----------------------------------------------------------+

Simple conditional test
^^^^^^^^^^^^^^^^^^^^^^^

The simple if-test uses a one-line format, defined as

    **if** *var OP constant* **then** *command*

where *var* must be one of the symbol parameters, specified as $1, $2,
$3, etc. You must document what these additional parameters control. For
example, to plot a small cyan circle at (0.2, 0.3) with diameter 0.4
only if $2 exceeds 45 you would write

    ::

     if $2 > 45 then 0.2 0.3 0.4 c -Gcyan

Note that this form of the conditional test has no mechanism for an
**else** branch, but this can be accomplished by repeating the test but
reversing the logic for the second copy, e.g.,

    ::

     if $1 > 10 then 0 0 0.5 c -Gred
     if $1 <= 10 then 0 0 0.5 c -Gblue

or you may instead consider the complete conditional construct below.

Complete conditional test
^^^^^^^^^^^^^^^^^^^^^^^^^

The complete conditional test uses a multi-line format, such as

| **if** *var* *OP constant* **then** {
|  <one or more lines with commands>
| } **elseif** *var OP constant* **then** {
|  <one or more lines with commands>
| } **else** {
|  <one or more lines with commands>
| }

The **elseif** (one or more) and **else** branches are optional. Note
that the syntax is strictly enforced, meaning the opening brace must
appear after **then** with nothing following it, and the closing brace
must appear by itself with no other text, and that the **elseif** and
**else** statements must have both closing and opening braces on the
same line (and nothing else). You may nest tests as well (up to 10
levels deep), e.g.,

   ::

    if $1 > 45 then {
            if $2 [> 0:10 then 0 0 0.5 c -Gred
    } elseif $1 < 15 then {
            if $2 [> 0:10 then 0 0 0.5 c -Ggreen
    } else {
            if $2 [> 10:20 then {
                    0 0 M -W1p,blue
                    0.3 0.3 D
                    S
                    0.3 0.3 0.3 c -Gcyan
            }
    }

N. Annotation of Contours and "Quoted Lines"
============================================


The GMT programs :doc:`grdcontour` (for
data given as 2-dimensional grids) and
:doc:`pscontour` (for *x,y,z* tables) allow
for contouring of data sets, while :doc:`psxy`
and :doc:`psxyz` can plot lines based on *x,y*-
and *x,y,z*-tables, respectively. In both cases it may be necessary to
attach labels to these lines. Clever or optimal placements of labels is
a very difficult topic, and GMT provides several algorithms for this
placement as well as complete freedom in specifying the attributes of
the labels. Because of the richness of these choices we present this
Appendix which summarizes the various options and gives several examples
of their use.

Label Placement
---------------

While the previous GMT versions 1--3 allowed for a single algorithm
that determined where labels would be placed, GMT 4 allows for five
different algorithms. Furthermore, a new "symbol" option (**-Sq** for
"quoted line") has been added to :doc:`psxy` and
:doc:`psxyz` and hence the new label placement
mechanisms apply to those programs as well. The contouring programs
expect the algorithm to be specified as arguments to **-G** while the
line plotting programs expect the same arguments to follow **-Sq**. The
information appended to these options is the same in both cases and is
of the form [**code**]\ *info*. The five algorithms correspond to the
five codes below (some codes will appear in both upper and lower case;
they share the same algorithm but differ in some other ways). In what
follows, the phrase "line segment" is taken to mean either a contour or
a line to be labeled. The codes are:

d:
    Full syntax is
    **d**\ *dist*\ [**c\ \|\ i\ \|\ p**][/\ *frac*].
    Place labels according to the distance measured along the projected
    line on the map. Append the unit you want to measure distances in
    [Default is taken from :ref:`PROJ_LENGTH_UNIT <PROJ_LENGTH_UNIT>`]. Starting at the
    beginning of a line, place labels every *dist* increment of distance
    along the line. To ensure that closed lines whose total length is
    less than *dist* get annotated, we may append *frac* which will
    place the first label at the distance *d = dist*
    :math:`\times` *frac* from the start of a closed line (and every
    *dist* thereafter). If not given, *frac* defaults to 0.25.

D:
    Full syntax is
    **D**\ *dist*\ [**d\ \|\ m\ \|\ s\ \|\ e\ \|\ f\ \|\ k\ \|\ M\ \|\ n**][/\ *frac*].
    This option is similar to **d** except the original data must be
    referred to geographic coordinates (and a map projection must have
    been chosen) and actual Earth [39]_ surface distances along the
    lines are considered. Append the unit you want to measure distances
    in; choose among arc **d**\ egree, **m**\ inute, and **s**\ econd,
    or m\ **e**\ ter [Default], **f**\ eet, **k**\ ilometer, statute
    **M**\ iles, or **n**\ autical miles. Other aspects are similar to
    code **d**.

f:
    Full syntax is
    **f**\ *fix.txt*\ [/*slop*\ [**c\ \|\ i\ \|\ p**]].
    Here, an ASCII file *fix.txt* is given which must contain records
    whose first two columns hold the coordinates of points along the
    lines at which locations the labels should be placed. Labels will
    only be placed if the coordinates match the line coordinates to
    within a distance of *slop* (append unit or we use
    :ref:`PROJ_LENGTH_UNIT <PROJ_LENGTH_UNIT>`). The default *slop* is zero, meaning only
    exact coordinate matches will do.

l:
    Full syntax is **l**\ *line1*\ [,\ *line2*\ [, ...]]. One or more
    straight line segments are specified separated by commas, and labels
    will be placed at the intersections between these lines and our line
    segments. Each *line* specification implies a *start* and *stop*
    point, each corresponding to a coordinate pair. These pairs can be
    regular coordinate pairs (i.e., longitude/latitude separated by a
    slash), or they can be two-character codes that refer to
    predetermined points relative to the map region. These codes are
    taken from the :doc:`pstext` justification keys
    [**L\ \|\ C\ \|\ R**][**B\ \|\ M\ \|\ T**]
    so that the first character determines the *x*-coordinate and
    the second determines the *y*-coordinate. In
    :doc:`grdcontour`, you can also use
    the two codes **Z+** and **Z-** as shorthands for the location of
    the grid's global maximum and minimum, respectively. For example,
    the *line* **LT**/**RB** is a diagonal from the upper left to the
    lower right map corner, while **Z-**/135W/15S is a line from the
    grid minimum to the point (135W, 15S).

L:
    Same as **l** except we will treat the lines given as great circle
    start/stop coordinates and fill in the points between before looking
    for intersections.

n:
    Full syntax is
    **n**\ *number*\ [/*minlength*\ [**c\ \|\ i\ \|\ p**]].
    Place *number* of labels along each line regardless of total line
    length. The line is divided into *number* segments and the labels
    are placed at the centers of these segments. Optionally, you may
    give a *minlength* distance to ensure that no labels are placed
    closer than this distance to its neighbors.

N:
    Full syntax is
    **N**\ *number*\ [/*minlength*\ [**c\ \|\ i\ \|\ p**]].
    Similar to code **n** but here labels are placed at the ends of each
    segment (for *number* >= 2). A special case arises for
    *number = 1* when a single label will be placed according to
    the sign of *number*: -1 places one label justified at the
    start of the line, while +1 places one label justified at
    the end of the line.

x:
    Full syntax is **x**\ *cross.d*. Here, an ASCII file *cross.d* is a
    multi-segment file whose lines will intersect our segment lines;
    labels will be placed at these intersections.

X:
    Same as **x** except we treat the lines given as great circle
    start/stop coordinates and fill in the points between before looking
    for intersections.

Only one algorithm can be specified at any given time.

Label Attributes
----------------

Determining where to place labels is half the battle. The other half is
to specify exactly what are the attributes of the labels. It turns out
that there are quite a few possible attributes that we may want to
control, hence understanding how to specify these attributes becomes
important. In the contouring programs, one or more attributes may be
appended to the **-A** option using the format +\ *code*\ [*args*\ ] for
each attribute, whereas for the line plotting programs these attributes
are appended to the **-Sq** option following a colon (:) that separates
the label codes from the placement algorithm. Several of the attributes
do not apply to contours so we start off with listing those that apply
universally. These codes are:

+a:
    Controls the angle of the label relative to the angle of the line.
    Append **n** for normal to the line, give a fixed *angle* measured
    counter-clockwise relative to the horizontal. or append **p** for
    parallel to the line [Default]. If using
    :doc:`grdcontour` the latter option
    you may further append **u** or **d** to get annotations whose upper
    edge always face the next higher or lower contour line.

+c:
    Surrounding each label is an imaginary label "textbox" which defines
    a region in which no segment lines should be visible. The initial
    box provides an exact fit to the enclosed text but clearance may be
    extended in both the horizontal and vertical directions (relative to
    the label baseline) by the given amounts. If these should be
    different amounts please separate them by a slash; otherwise the
    single value applies to both directions. Append the distance units
    of your choice (**c\ \|\ i\ \|\ m\ \|\ p**), or
    give % to indicate that the clearance should be this fixed
    percentage of the label font size in use. The default is 15%.

+d:
    Debug mode. This is useful when testing contour placement as it will
    draw the normally invisible helper lines and points in the label
    placement algorithms above.

+d:
    Delayed mode, to delay the plotting of the text as text clipping is set instead.

+f:
    Specifies the desired label font, including size or color. See
    :doc:`pstext` for font names or numbers.
    The default font is given by :ref:`FONT_ANNOT_PRIMARY <FONT_ANNOT_PRIMARY>`.

+g:
    Selects opaque rather than the default transparent text boxes. You
    may optionally append the color you want to fill the label boxes;
    the default is the same as :ref:`PS_PAGE_COLOR <PS_PAGE_COLOR>`.

+j:
    Selects the justification of the label relative to the placement
    points determined above. Normally this is center/mid justified
    (**CM** in :doc:`pstext` justification
    parlance) and this is indeed the default setting. Override by using
    this option and append another justification key code from
    [**L\ \|\ C\ \|\ R**\ ][**B\ \|\ M\ \|\ T**\ ].
    Note for curved text (**+v**) only vertical justification will be
    affected.

+o:
    Request a rounded, rectangular label box shape; the default is
    rectangular. This is only manifested if the box is filled or
    outlined, neither of which is implied by this option alone (see
    **+g** and **+p**). As this option only applies to straight text, it
    is ignored if **+v** is given.

+p:
    Selects the drawing of the label box outline; append your preferred
    *pen* unless you want the default GMT pen [0.25p,black].

+r:
    Do not place labels at points along the line whose local radius of
    curvature falls below the given threshold value. Append the radius
    unit of your choice (**c\ \|\ i\ \|\ p**) [Default is 0].

+u:
    Append the chosen *unit* to the label. Normally a space will
    separate the label and the unit. If you want to close this gap,
    append a *unit* that begins with a hyphen (-). If you are contouring
    with :doc:`grdcontour` and you specify
    this option without appending a unit, the unit will be taken from
    the *z*-unit attribute of the grid header.

+v:
    Place curved labels that follow the wiggles of the line segments.
    This is especially useful if the labels are long relative to the
    length-scale of the wiggles. The default places labels on an
    invisible straight line at the angle determined.

+w:
    The angle of the line at the point of straight label placement is
    calculated by a least-squares fit to the *width* closest points. If
    not specified, *width* defaults to 10.

+=:
    Similar in most regards to **+u** but applies instead to a label
    *prefix* which you must append.

For contours, the label will be the value of the contour (possibly
modified by **+u** or **+=**). However, for quoted lines other options apply:

+l:
    Append a fixed *label* that will be placed at all label locations.
    If the label contains spaces you must place it inside matching
    quotes.

+L:
    Append a code *flag* that will determine the label. Available codes
    are:

    +Lh:
        Take the label from the current multi-segment header (hence it
        is assumed that the input line segments are given in the
        multi-segment file format; if not we pick the single label from
        the file's header record). We first scan the header for an
        embedded **-L**\ *label* option; if none is found we instead use
        the first word following the segment marker [>].

    +Ld:
        Take the Cartesian plot distances along the line as the label;
        append **c\ \|\ i\ \|\ p** as the unit [Default is
        :ref:`PROJ_LENGTH_UNIT <PROJ_LENGTH_UNIT>`]. The label will be formatted according
        to the :ref:`FORMAT_FLOAT_OUT <FORMAT_FLOAT_OUT>` string, *unless* label placement
        was determined from map distances along the segment lines, in
        which case we determine the appropriate format from the distance
        value itself.

    +LD:
        Calculate actual Earth surface distances and use the distance at
        the label placement point as the label; append
        **d\ \|\ e\ \|\ f\ \|\ k\ \|\ m\ \|\ M\ \|\ n\ \|\ s**
        to specify the unit [If not given we default to **d**\ egrees,
        *unless* label placement was determined from map distances along
        the segment lines, in which case we use the same unit specified
        for that algorithm]. Requires a map projection to be used.

    +Lf:
        Use all text after the 2nd column in the fixed label location
        file *fix.txt* as labels. This choice obviously requires the
        fixed label location algorithm (code **f**) to be in effect.

    +Ln:
        Use the running number of the current multi-segment as label.

    +LN:
        Use a slash-separated combination of the current file number and
        the current multi-segment number as label.

    +Lx:
        As **h** but use the multi-segment headers in the *cross.d* file
        instead. This choice obviously requires the crossing segments
        location algorithm (code **x\ \|\ X**) to be in effect.

Examples of Contour Label Placement
-----------------------------------

We will demonstrate the use of these options with a few simple examples.
First, we will contour a subset of the global geoid data used in
GMT Example 01; the region selected encompasses the world's strongest
"geoid dipole": the Indian Low and the New Guinea High.

Equidistant labels
~~~~~~~~~~~~~~~~~~

Our first example uses the default placement algorithm. Because of the
size of the map we request contour labels every 1.5 inches along the
lines:

As seen in Figure :ref:`Contour label 1 <Contour_label_1>`, the contours are
placed rather arbitrary. The string of contours for -40 to
60 align well but that is a fortuitous consequence of reaching
the 1.5 inch distance from the start at the bottom of the map.

.. _Contour_label_1:

.. figure:: /_images/GMT_App_O_1.*
   :width: 500 px
   :align: center

   Equidistant contour label placement with **-Gd**, the only algorithm
   available in previous GMT versions.


Fixed number of labels
~~~~~~~~~~~~~~~~~~~~~~

We now exercise the option for specifying exactly how many labels each
contour line should have:

By selecting only one label per contour and requiring that labels only
be placed on contour lines whose length exceed 1 inch, we achieve the
effect shown in Figure :ref:`Contour label 2 <Contour_label_2>`.

.. _Contour_label_2:

.. figure:: /_images/GMT_App_O_2.*
   :width: 500 px
   :align: center

   Placing one label per contour that exceed 1 inch in length, centered on the segment with **-Gn**.


Prescribed label placements
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here, we specify four points where we would like contour labels to be
placed. Our points are not exactly on the contour lines so we give a
nonzero "slop" to be used in the distance calculations: The point on the
contour closest to our fixed points and within the given maximum
distance will host the label.

The angle of the label is evaluated from the contour line geometry, and
the final result is shown in Figure :ref:`Contour label 3 <Contour_label_3>`.
To aid in understanding the algorithm we chose to specify "debug" mode
(**+d**) which placed a small circle at each of the fixed points.

.. _Contour_label_3:

.. figure:: /_images/GMT_App_O_3.*
   :width: 500 px
   :align: center

   Four labels are positioned on the points along the contours that are
   closest to the locations given in the file fix.txt in the **-Gf** option.


Label placement at simple line intersections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Often, it will suffice to place contours at the imaginary intersections
between the contour lines and a well-placed straight line segment. The
**-Gl** or **-GL** algorithms work well in those cases:

The obvious choice in this example is to specify a great circle between
the high and the low, thus placing all labels between these extrema.

.. _Contour_label_4:

.. figure:: /_images/GMT_App_O_4.*
   :width: 500 px
   :align: center

   Labels are placed at the intersections between contours and the great circle specified in the **-GL** option.


The thin debug line in Figure :ref:`Contour label 4 <Contour_label_4>` shows
the great circle and the intersections where labels are plotted. Note
that any number of such lines could be specified; here we are content
with just one.

Label placement at general line intersections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If (1) the number of intersecting straight line segments needed to pick
the desired label positions becomes too large to be given conveniently
on the command line, or (2) we have another data set or lines whose
intersections we wish to use, the general crossing algorithm makes more
sense:

.. _Contour_label_5:

.. figure:: /_images/GMT_App_O_5.*
   :width: 500 px
   :align: center

   Labels are placed at the intersections between contours and the
   multi-segment lines specified in the **-GX** option.


In this case, we have created three strands of lines whose intersections
with the contours define the label placements, presented in
Figure :ref:`Contour label 5 <Contour_label_5>`.

Examples of Label Attributes
----------------------------

We will now demonstrate some of the ways to play with the label
attributes. To do so we will use :doc:`psxy` on
a great-circle line connecting the geoid extrema, along which we have
sampled the ETOPO5 relief data set. The file thus contains *lon, lat,
dist, geoid, relief*, with distances in km.

Label placement by along-track distances, 1
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This example will change the orientation of labels from along-track to
across-track, and surrounds the labels with an opaque, outlined text box
so that the label is more readable. We choose the place the labels every
1000 km along the line and use that distance as the label. The labels
are placed normal to the line:

.. _Contour_label_6:

.. figure:: /_images/GMT_App_O_6.*
   :width: 500 px
   :align: center

   Labels attributes are controlled with the arguments to the **-Sq** option.


The composite illustration in Figure :ref:`Contour label 6 <Contour_label_6>`
shows the new effects. Note that the line connecting the extrema does
not end exactly at the ‘-' and ‘+' symbols. This is because the
placements of those symbols are based on the mean coordinates of the
contour and not the locations of the (local or global) extrema.

Label placement by along-track distances, 2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A small variation on this theme is to place the labels parallel to the
line, use spherical degrees for placement, append the degree symbol as a
unit for the labels, choose a rounded rectangular text box, and
inverse-video the label:

The output is presented as Figure :ref:`Contour label 7 <Contour_label_7>`.

.. _Contour_label_7:

.. figure:: /_images/GMT_App_O_7.*
   :width: 500 px
   :align: center

   Another label attribute example.


Using a different data set for labels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In the next example we will use the bathymetry values along the transect
as our label, with placement determined by the distance along track. We
choose to place labels every 1500 km. To do this we need to pull out
those records whose distances are multiples of 1500 km and create a
"fixed points" file that can be used to place labels and specify the
labels. This is done with **awk**.

The output is presented as Figure :ref:`Contour label 8 <Contour_label_8>`.

.. _Contour_label_8:

.. figure:: /_images/GMT_App_O_8.*
   :width: 500 px
   :align: center

   Labels based on another data set (here bathymetry) while the placement is based on distances.


Putting it all together
-----------------------

Finally, we will make a more complex composite illustration that uses
several of the label placement and label attribute settings discussed in
the previous sections. We make a map showing the tsunami travel times
(in hours) from a hypothetical catastrophic landslide in the Canary
Islands [40]_. We lay down a color map based on the travel times and the
shape of the seafloor, and travel time contours with curved labels as
well as a few quoted lines. The final script is

with the complete illustration presented as Figure
:ref:`Contour label 9 <Contour_label_9>`.

.. _Contour_label_9:

.. figure:: /_images/GMT_App_O_9.*
   :width: 500 px
   :align: center

   Tsunami travel times from the Canary Islands to places in the Atlantic,
   in particular New York. Should a catastrophic landslide occur it is possible
   that New York will experience a large tsunami about 8 hours after the event.


O. Special Operations
=====================

.. _Isolation mode:

Running GMT in *isolation mode*
-------------------------------

In Chapter `General features`_ it is described how GMT creates
several (temporary) files to communicate between the different commands
that make up the script that finally creates a plot. Among those files
are:

    **gmt.conf** This file covers about 100 different settings that influence the
       layout of your plot, from font sizes to tick lengths and date
       formats (See Section `GMT defaults`_). Those settings can be altered
       by editing the file, or by running the
       :doc:`gmtset` command. A problem may arise
       when those settings are changed half-way through the script: the
       next time you run the script it will start with the modified
       settings and hence might alter your scripts results. It is therefore
       often necessary to revert to the original ``gmt.conf`` file. *Isolation mode*
       avoids that issue.

    **gmt.history** This file is created to communicate the command line history from
       one command to the next (Section `Command line history`_) so that
       shorthands like **-R** or **-J** can be used once it has been set in
       a previous GMT command. The existence of this file makes if
       impossible to run two GMT scripts simultaneously in the same
       directory, since those ``gmt.history`` files may clash (contain different histories)
       and adversely affect the results of both scripts.

A cure to all these woes is the *isolation mode* introduced in
GMT version 4.2.2. This mode allows you to run a GMT script without
leaving any traces other than the resulting PostScript  or data files,
and not altering the ``gmt.conf`` or ``gmt.history`` files. Those files will be placed in a temporary
directory instead. And if properly set up, this temporary directory will
only be used by a single script, even if another GMT script is running
simultaneously. This also provides the opportunity to create any other
temporary files that the script might create in the same directory.

The example below shows how *isolation mode* works.

.. figure:: /_images/GMT_App_P_2.*
   :width: 500 px
   :align: center

   Example created in isolation mode


The files ``gmt.conf`` and ``gmt.history`` are automatically created in the temporary directory
``$GMT_TMPDIR``. The script is also adjusted such that the temporary grid file ``lat.nc`` and colormap
``lat.cpt`` are created in that directory as well. To make things even more easy,
GMT now provides a set of handy shell functions in :doc:`gmt_shell_functions.sh`:
simply include that file in the script and the creation and the removal
of the temporary directory is reduced to a single command.


P. The GMT Vector Data Format for OGR Compatibility
===================================================


Background
----------

The National Institute for Water and Atmospheric Research (NIWA) in New
Zealand has funded the implementation of a GMT driver (read and write)
for the OGR package. OGR is an Open Source toolkit for accessing or
reformatting vector (spatial) data stored in a variety of formats and is
part of the. The intention was to enable the easy rendering (using
GMT) of spatial data held in non-\ GMT formats, and the export of
vector data (e.g., contours) created by GMT for use with other GIS and
mapping packages. While **ogr2ogr** has had the capability to write
this new format since 2009, GMT 4 did not have the capability to use
the extra information.

GMT 5 now allows for more advanced vector data, including donut
polygons (polygons with holes) and aspatial attribute data. At the same
time, the spatial data implementation will not disrupt older GMT 4
programs since all the new information are written via comments.

The identification of spatial feature types in GMT files generally
follows the technical description, (which is largely consistent with the
OGC SFS specification). This specification provides for non-topological
point, line and polygon (area) features, as well as multipoint,
multiline and multipolygon features, and was written by
`Brent Wood <http://www.niwa.co.nz/key-contacts/brent-wood/>`_
based on input from Paul Wessel and others on the GMT list.

The OGR/GMT format
------------------

Several key properties of the OGR/GMT format is summarized below:

-  All new data fields are stored as comment lines, i.e., in lines
   starting with a "#". OGR/GMT files are therefore compatible with
   GMT 4 binaries, which will simply ignore this new information.

-  To be consistent with current practice in GMT, data fields are
   represented as whitespace-separated strings within the comments, each
   identified by the "@" character as a prefix, followed by a single
   character identifying the content of the field. To avoid confusion
   between words and strings, the word (field) separator within strings
   will be the "\|" (pipe or vertical bar) character.

-  Standard UNIX "\\" escaping is used, such as \\n for newline in a string.

-  All new data are stored before the spatial data (coordinates) in the
   file, so when any GMT 5 program is processing the coordinate data
   for a feature, it will already have parsed any non-spatial
   information for each feature, which may impact on how the spatial
   data is treated (e.g., utilizing the aspatial attribute data for a
   feature to control symbology).

-  The first comment line must specify the version of the OGR/GMT data
   format, to allow for future changes or enhancements to be supported
   by future GMT programs. This document describes v1.0.

-  For consistency with other GIS formats (such as shapefiles) the
   OGR/GMT format explicitly contains a field specifying whether the
   features are points, linestrings or polygons, or the "multi" versions
   of these. (Other shapefile feature types will not be supported at
   this stage). At present, GMT programs are informed of this via
   command line parameters. This will now be explicit in the data file,
   but does not preclude command line switches setting symbologies for
   plotting polygons as lines (perimeters) or with fills, as is
   currently the practice.

-  Note that what is currently called a "multiline" (multi-segment) file
   in GMT parlance is generally a set of "lines" in shapefile/OGR
   usage. A multiline in this context is a single feature comprising
   multiple lines. For example, all the transects from a particular
   survey may be stored as lines, each with it's own attribute set, such
   as transect number, date/time, etc. They may also be stored as a
   single multiline feature with one attribute set, such as trip ID.
   This difference is explicitly stored in the data in OGR/shapefiles,
   but currently specified only on the command line in GMT. This
   applies also to points and polygons. The GMT equivalent to
   {multipoint, multiline, multipolygon} datatypes is multiple
   GMT files, each comprising a single {multipoint, multiline,
   multipolygon} feature.

-  The new GMT vector data files includes a header comment specifying
   the type of spatial features it contains, as well as the description
   of the aspatial attribute data to be associated with each feature.
   Unlike the shapefile format, which stores the spatial and aspatial
   attribute data in separate files, the GMT format will store all
   data in a single file.

-  All the features in a GMT file must be of the same type.

OGR/GMT Metadata
----------------

Several pieces of metadata information must be present in the header of
the OGR/GMT file, followed by both spatial and aspatial data. In this
section we look at the metadata.

Format version
~~~~~~~~~~~~~~

The comment header line will include a version identifier providing for
possible different versions in future. It is indicated by the **@V**
sequence.

+-----------+---------------+--------------------------------------------------------------------+
| **Code**  | **Argument**  | **Description**                                                    |
+===========+===============+====================================================================+
| V         | GMT1.0        | Data in this file is stored using v1.0 of the OGR/GMT data format  |
+-----------+---------------+--------------------------------------------------------------------+

An OGR/GMT file must therefore begin with the line

   ::

    # @VGMT1.0

Parsing of the OGR/GMT format is only activated if the version
code-sequence has been found.

Geometry types
~~~~~~~~~~~~~~

The words and characters used to specify the geometry type (preceded by
the **@G** code sequence on the header comment line), are listed in
Table :ref:`geometries <tbl-geometries>`.

.. _tbl-geometries:

+----------+-----------------+---------------------------------------------------------------------------------------+
| **Code** | **Geometry**    | **Description**                                                                       |
+==========+=================+=======================================================================================+
| G        | POINT           | File with point features                                                              |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | (Each point will have it's own attribute/header line preceding the point coordinates) |
+----------+-----------------+---------------------------------------------------------------------------------------+
| G        | MULTIPOINT      | File with a single multipoint feature                                                 |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | (All the point features are a single multipoint, with the same attribute/header       |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | information)                                                                          |
+----------+-----------------+---------------------------------------------------------------------------------------+
| G        | LINESTRING      | File with features comprising multiple single lines                                   |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | (Effectively the current GMT multiline file, each line feature will have it's own     |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | attribute and header data)                                                            |
+----------+-----------------+---------------------------------------------------------------------------------------+
| G        | MULTILINESTRING | File with features comprising a multiline                                             |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | (All the line features in the file are a single multiline feature, only one attribute |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | and header which applies to all the lines)                                            |
+----------+-----------------+---------------------------------------------------------------------------------------+
| G        | POLYGON         | File with one or more polygons                                                        |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | (Similar to a line file, except the features are closed polygons)                     |
+----------+-----------------+---------------------------------------------------------------------------------------+
| G        | MULTIPOLYGON    | File with a single multipolygon                                                       |
+----------+-----------------+---------------------------------------------------------------------------------------+
|          |                 | (Similar to a GMT multiline file, except the feature is a closed multipolygon)        |
+----------+-----------------+---------------------------------------------------------------------------------------+

An example GMT polygon file header using this specification (in format 1.0) is

   ::

    # @VGMT1.0 @GPOLYGON

Domain and map projections
~~~~~~~~~~~~~~~~~~~~~~~~~~

The new format will also support region and projection information. The
region will be stored in GMT **-R** format (i.e., **-R**\ *W/E/S/N*,
where the *W/E/S/N* values represent the extent of features); the **@R**
code sequence marks the domain information. A sample region header is:

   ::

    # @R150/190/-45/-54

Projection information will be represented as four optional strings,
prefixed by **@J** (J being the GMT character for projection
information. The **@J** code will be followed by a character identifying
the format, as shown in Table :ref:`projectspec <tbl-projectspec>`.

.. _tbl-projectspec:

+------------+-------------------------------------------------------------------------------------------------+
| **Code**   | **Projection Specification**                                                                    |
+============+=================================================================================================+
| @Je        | EPSG code for the projection                                                                    |
+------------+-------------------------------------------------------------------------------------------------+
| @Jg        | A string representing the projection parameters as used by GMT                                  |
+------------+-------------------------------------------------------------------------------------------------+
| @Jp        | A string comprising the Proj.4 parameters representing the projection parameters                |
+------------+-------------------------------------------------------------------------------------------------+
| @Jw        | A string comprising the OGR WKT (well known text) representation of the projection parameters   |
+------------+-------------------------------------------------------------------------------------------------+

Sample projection strings are:

   ::

    # @Je4326 @JgX @Jp"+proj=longlat +ellps=WGS84+datum=WGS84 +no_defs"
    # @Jw"GEOGCS[\"WGS84\",DATUM[\"WGS_1984\",SPHEROID\"WGS84\",6378137,\
    298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],TOWGS84[0,0,0,0,0,0,0],
    AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0,\
    AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.01745329251994328,\
    AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\"EPSG\",\"4326\"]]"

Note that an OGR-generated file will not have a **@Jg** string, as OGR
does not have any knowledge of the GMT projection specification
format. GMT supports at least one of the other formats to provide
interoperability with other Open Source related GIS software packages.
One relatively simple approach, (with some limitations), would be a
lookup table matching EPSG codes to GMT strings.

Declaration of aspatial fields
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The string describing the aspatial field names associated with the
features is flagged by the **@N** prefix.

+------------+-----------------------------+-----------------------------------------------------------------+
| **Code**   | **Argument**                | **Description**                                                 |
+============+=============================+=================================================================+
| N          | word\ \|\ word\ \|\ word    | A "\|" -separated string of names of the attribute field names  |
+------------+-----------------------------+-----------------------------------------------------------------+

Any name containing a space must be quoted. The **@N** selection must be
combined with a matching string specifying the data type for each of the
named fields, using the **@T** prefix.

+------------+-----------------------------+-------------------------------------------------------------+
| **Code**   | **Argument**                | **Description**                                             |
+============+=============================+=============================================================+
| T          | word\ \|\ word\ \|\ word    | A "\|" -separated string of the attribute field data types  |
+------------+-----------------------------+-------------------------------------------------------------+

Available datatypes should largely follow the shapefile (DB3)
specification, including **string**, **integer**, **double**,
**datetime**, and **logical** (boolean). In OGR/GMT vector files, they
will be stored as appropriately formatted text strings.

An example header record containing all these is

   ::

    # @VGMT1.0 @GPOLYGON @Nname|depth|id @Tstring|double|integer

OGR/GMT Data
------------

All generic fields must be at the start of the file before any
feature-specific content (feature attribute data follow the metadata, as
do the feature coordinates, separated by a comment line comprising "#
FEATURE_DATA". Provided each string is formatted as specified, and
occurs on a line prefixed with "#" (i.e., is a comment), the format is
free form, in that as many comment lines as desired may be used, with
one or more parameter strings in any order in any line. E.g., one
parameter per line, or all parameters on one line.

Embedding aspatial data
~~~~~~~~~~~~~~~~~~~~~~~

Following this header line is the data itself, both aspatial and
spatial. For line and polygon (including multiline and multipolygon)
data, features are separated using a predefined character, by default
">". For point (and multipoint) data, no such separator is required. The
comment line containing the aspatial data for each feature will
immediately precede the coordinate line(s). Thus in the case of lines
and polygons, it will immediately follow the ">" line. The data line
will be a comment flag ("#") followed by **@D**, followed by a string of
"\|"-separated words comprising the data fields defined in the
header record.

To allow for names and values containing spaces, such string items among
the **@N** or **@D** specifiers must be enclosed in double quotes.
(Where double quotes or pipe characters are included in the string, they
must be escaped using "\\"). Where any data values are
null, they will be represented as no characters between the field
separator, (e.g., #@D\ \|\ \|\ \|). A Sample header
and corresponding data line for points are

   ::

    # @VGMT1.0 @GPOINT @Nname|depth|id @Tstring|double|integer
    # @D"Point 1"|-34.5|1

while for a polygon it may look like

   ::

    # @VGMT1.0 @GPOLYGON @Nname|depth|id @Tstring|double|integer
    >
    # @D"Area 1"|-34.5|1

Polygon topologies
~~~~~~~~~~~~~~~~~~

New to GMT is the concept of polygon holes. Most other formats do
support this structure, so that a polygon is specified as a sequence of
point defining the perimeter, optionally followed by similar coordinate
sequences defining any holes (the "donut" polygon concept).

To implement this in a way which is compatible with previous
GMT versions, each polygon feature must be able to be identified as
the outer perimeter, or an inner ring (hole). This is done using a
**@P** or **@H** on the data comment preceding the polygon coordinates.
The **@P** specifies a new feature boundary (perimeter), any following
**@H** polygons are holes, and must be within the preceding **@P**
polygon (as described in the shapefile specification). Any **@H**
polygons will NOT have any **@D** values, as the aspatial attribute data
pertain to the entire feature, the **@H** polygons are not new polygons,
but are merely a continuation of the definition of the same feature.

Examples
--------

Sample point, line and polygon files are (the new data structures are in
lines starting with "#" in strings prefixed with "@"). Here is a typical
point file:

   ::

    # @VGMT1.0 @GPOINT @Nname|depth|id
    # @Tstring|double|integer
    # @R178.43/178.5/-57.98/-34.5
    # @Je4326
    # @Jp"+proj=longlat +ellps=WGS84 +datum=WGS84+no_defs"
    # FEATURE_DATA
    # @D"point 1"|-34.5|1
    178.5 -45.7
    # @D"Point 2"|-57.98|2
    178.43 -46.8
    ...

Next is an example of a line file:

   ::

    # @VGMT1.0 @GLINESTRING @Nname|depth|id
    # @Tstring|double|integer
    # @R178.1/178.6/-48.7/-45.6
    # @Jp"+proj=longlat +ellps=WGS84 +datum=WGS84+no_defs"
    # FEATURE_DATA
    > -W0.25p
    # @D"Line 1"|-50|1
    178.5 -45.7
    178.6 -48.2
    178.4 -48.7
    178.1 -45.6
    > -W0.25p
    # @D"Line 2"|-57.98|$
    178.43 -46.8
    ...

Finally we show an example of a polygon file:

   ::

    # @VGMT1.0 @GPOLYGON @N"Polygon name"|substrate|id @Tstring|string|integer
    # @R178.1/178.6/-48.7/-45.6
    # @Jj@Jp"+proj=longlat +ellps=WGS84 +datum=WGS84+no_defs"
    # FEATURE_DATA
    > -Gblue -W0.25p
    # @P
    # @D"Area 1"|finesand|1
    178.1 -45.6
    178.1 -48.2
    178.5 -48.2
    178.5 -45.6
    178.1 -45.6
    >
    # @H
    # First hole in the preceding perimeter, so is technically still
    # part of the same geometry, despite the preceding > character.
    # No attribute data is provided, as this is inherited.
    178.2 -45.4
    178.2 -46.5
    178.4 -46.5
    178.4 -45.4
    178.2 -45.4
    >
    # @P
    ...

.. [1]
   Version 1.0 was then informally released at the Lamont-Doherty Earth Observatory.

.. [2]
   See GNU Lesser General Public License (`<http://www.gnu.org/copyleft/gpl.html>`_)
   for terms on redistribution and modifications.

.. [3]
   The tools can also be installed on other platforms (see Appendix [app:L]).

.. [4]
   One public-domain RIP is ghostscript, available from `<http://www.gnu.org/>`_.

.. [5]
   Programs now also allow for fast, binary multicolumn file i/o.

.. [6]
   While the netCDF format is the default, many other formats are also possible.

.. [7]
   Vicenty, T. (1975), Direct and inverse solutions of geodesics on the
   ellipsoid with application of nested equations, *Surv. Rev.,
   XXII(176)*, 88--93.

.. [8]
   PostScript definition. In the typesetting industry a slightly
   different definition of point (1/72.27 inch) is used, presumably to
   cause needless trouble.

.. [9]
   Choose between SI and US default units by modifying in the
   GMT share directory.

.. [10]
   To remain backwards compatible with GMT 4 we will also look for
   but only if cannot be found.

.. [11]
   The Gregorian Calendar is a revision of the Julian Calendar which was
   instituted in a papal bull by Pope Gregory XIII in 1582. The reason
   for the calendar change was to correct for drift in the dates of
   significant religious observations (primarily Easter) and to prevent
   further drift in the dates. The important effects of the change were
   (a) Drop 10 days from October 1582 to realign the Vernal Equinox with
   21 March, (b) change leap year selection so that not all years ending
   in "00" are leap years, and (c) change the beginning of the year to 1
   January from 25 March. Adoption of the new calendar was essentially
   immediate within Catholic countries. In the Protestant countries,
   where papal authority was neither recognized not appreciated,
   adoption came more slowly. England finally adopted the new calendar
   in 1752, with eleven days removed from September. The additional day
   came because the old and new calendars disagreed on whether 1700 was
   a leap year, so the Julian calendar had to be adjusted by one more
   day.

.. [12]
   While UTM coordinates clearly refer to points on the Earth, in this
   context they are considered "other". Thus, when we refer to
   "geographical" coordinates herein we imply longitude, latitude.

.. [13]
   Please consult the man page for *printf* or any book on C.

.. [14]
   For historical reasons, the GMT default is Landscape; see
   :doc:`gmt.conf` to change this.

.. [15]
   Ensures that boundary annotations do not fall off the page.

.. [16]
   To keep PostScript files small, such comments are by default turned
   off; see :ref:`PS_COMMENTS <PS_COMMENTS>` to enable them.

.. [17]
   For an overview of color systems such as HSV, see Appendix [app:I].

.. [18]
   Convert other graphics formats to Sun ras format using ImageMagick's
   **convert** program.

.. [19]
   Requires building GMT with GDAL.

.. [20]
   Snyder, J. P., 1987, Map Projections A Working Manual, U.S.
   Geological Survey Prof. Paper 1395.

.. [21]
   This is, however, not the shortest distance. It is given by the great
   circle connecting the two points.

.. [22]
   Robinson provided a table of *y*-coordinates for latitudes
   every 5. To project values for intermediate latitudes one must
   interpolate the table. Different interpolants may result in slightly
   different maps. GMT uses the
   interpolant selected by the parameter :ref:`GMT_INTERPOLANT <GMT_INTERPOLANT>` in the
   file.

.. [23]
   These data are available on CD-ROM from NGDC (`<http://www.ngdc.noaa.gov/>`_).

.. [24]
   See `<http://topex.ucsd.edu/marine_grav/mar_grav.html>`_.

.. [25]
   Pedants who wish to argue about the "other" arc going the long way
   should consider using it.

.. [26]
   While Quicktime is free you must upgrade to QuickTime Pro (USD 30) to
   use the authoring functions.

.. [27]
   QuickTime Pro can do this, as can most video-editing programs.

.. [28]
   For data bases, see `http://topex.ucsd.edu/marine_grav/mar_grav.html`_.

.. [29]
   The ASCII MGD77 data are available on CD-ROM from NGDC (`<http://www.ngdc.noaa.gov/>`_).

.. [30]
   Okabe, M., 1979, Analytical expressions for gravity anomalies due to
   polyhedral bodies and translation into magnetic anomalies,
   *Geophysics, 44*, 730--741.

.. [31]
   `Timothy J. Henstock <http://www.southampton.ac.uk/oes/research/staff/then.page>`_,
   University of Southampton

.. [32]
   If you chose SI units during the installation then the default
   encoding is ISOLatin1+, otherwise it is Standard+.

.. [33]
   R. Bracewell, *The Fourier Transform and its Applications*,
   McGraw-Hill, London, 444 p., 1965.

.. [34]
   `National Geophysical Data Center, Boulder, Colorado <http://www.ngdc.noaa.gov/>`_

.. [35]
   Douglas, D.H., and T. K. Peucker, 1973, Algorithms for the reduction
   of the number of points required to represent a digitized line or its
   caricature, *Canadian Cartographer*, 10, 112--122.

.. [36]
   The full and high resolution files are in separate archives because
   of their size. Not all users may need these files as the intermediate
   data set is better than the data provided with version 2.2.4.

.. [37]
   If you need complete polygons in a simpler format, see the article on
   GSHHG (Wessel, P., and W. H. F. Smith, 1996, A Global,
   self-consistent, hierarchical, high-resolution shoreline database,
   *J. Geophys. Res. 101*, 8741--8743).

.. [38]
   The 23rd palette is called *random* and produces a random set of
   colors suitable for categorical plots.

.. [39]
   or whatever planet we are dealing with.

.. [40]
   Travel times were calculated using Geoware's travel time calculator,
   **ttt**; see `<http://www.geoware-online.com/>`_.

.. |degree| unicode:: U+00B0 .. degree sign

.. vim: textwidth=78 noexpandtab tabstop=2 softtabstop=2 shiftwidth=2
