sphtriangulate — GMT 5.1.1 documentation

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# sphtriangulate

sphtriangulate - Delaunay or Voronoi construction of spherical lon,lat data

## Synopsis

sphtriangulate [ table ] [ -A ] [ -C ] [ -D ] [ -Lunit ] [ -Nnfile ] [ -Qd|v ] [ -T ] [ -V[level] ] [ -b[i|o][ncol][type][w][+L|+B] ] [ -h[i|o][n][+c][+d][+rremark][+rtitle] ] [ -icols[l][sscale][ooffset][,...] ] [ -:[i|o] ]

Note: No space is allowed between the option flag and the associated arguments.

## Description

sphtriangulate reads one or more ASCII [or binary] files (or standard input) containing lon, lat and performs a spherical Delaunay triangulation, i.e., it find how the points should be connected to give the most equilateral triangulation possible on the sphere. Optionally, you may choose -Qv which will do further processing to obtain the Voronoi polygons. Normally, either set of polygons will be written as fillable segment output; use -T to write unique arcs instead. As an option, compute the area of each triangle or polygon. The algorithm used is STRIPACK.

## Required Arguments

None.

## Optional Arguments

table
  One or more ASCII (or binary, see -bi[ncols][type]) data table file(s) holding a number of data columns. If no tables are given then we read from standard input.
-A
  Compute the area of the spherical triangles (-Qd) or polygons (-Qv) and write the areas (in chosen units; see -L) in the output segment headers [no areas calculated].
-C
  For large data set you can save some memory (at the expense of more processing) by only storing one form of location coordinates (geographic or Cartesian 3-D vectors) at any given time, translating from one form to the other when necessary [Default keeps both arrays in memory].
-D
  Used with -m to skip the last (repeated) input vertex at the end of a closed segment if it equals the first point in the segment. Requires -m [Default uses all points].
-Lunit
  Specify the unit used for distance and area calculations. Choose among e (m), f (foot), k (km), m (mile), n (nautical mile), u (survey foot), or d (spherical degree). A spherical approximation is used unless PROJ_ELLIPSOID is set to an actual ellipsoid, in which case we convert latitudes to authalic latitudes before calculating areas. When degree is selected the areas are given in steradians.
-Nnfile
  Write the information pertaining to each polygon (for Delaunay: the three node number and the triangle area (if -A was set); for Voronoi the unique node lon, lat and polygon area (if -A was set)) to a separate file. This information is also encoded in the segment headers of ASCII output files]. Required if binary output is needed.
d|v
  Select between delaunay or voronoi mode [Delaunay].
-T
  Write the unique arcs of the construction [Default writes fillable triangles or polygons]. When used with -A we store arc lenght in the segment header in chosen unit (see -L).

-V[level] (more ...)
  Select verbosity level [c].

-bi[ncols][type] (more ...)
  Select binary input. [Default is 2 input columns].

-bo[ncols][type] (more ...)
  Select binary output. [Default is same as input].

-h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
  Skip or produce header record(s).

-:[i|o] (more ...)
  Swap 1st and 2nd column on input and/or output.

-r (more ...)
  Set pixel node registration [gridline].

-^
  Print a short message about the syntax of the command, then exits (NOTE: on Windows use just -).
+
  Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exits.
-?
  Print a complete usage (help) message, including the explanation of options, then exits.
--version
  Print GMT version and exit.
--show-datadir
  Print full path to GMT share directory and exit.

## Ascii Format Precision

The ASCII output formats of numerical data are controlled by parameters in your gmt.conf file. Longitude and latitude are formatted according to FORMAT_GEO_OUT, whereas other values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format in effect can lead to loss of precision in the output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (-bo if available) or specify more decimals using the FORMAT_FLOAT_OUT setting.

## Examples

To triangulate the points in the file testdata.txt, and make a Voronoi diagram via psxy, use

>     gmt sphtriangulate testdata.txt -Qv | psxy -Rg -JG30/30/6i -L -P -W1p -B0g30 | gv -

To compute the optimal Delaunay triangulation network based on the multiple segment file globalnodes.d and save the area of each triangle in the header record, try

>     gmt sphtriangulate globalnodes.d -Qd -A > global_tri.d

## See Also

gmt, triangulate, sphdistance

## References

Renka, R, J., 1997, Algorithm 772: STRIPACK: Delaunay Triangulation and Voronoi Diagram on the Surface of a Sphere, AMC Trans. Math. Software, 23(3), 416-434.

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