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# grdrotater

grdrotater - Finite rotation reconstruction of geographic grid

## Synopsis

grdrotate ingrdfile -Erotfile | -elon/lat/angle -Goutgrdfile [ -Fpolygonfile ] [ -N ] [ -R[unit]xmin/xmax/ymin/ymax[r] ] [ -S ] [ -Tage ] [ -V[level] ] [ -b[i|o][ncol][type][w][+L|+B] ] [ -h[i|o][n][+c][+d][+rremark][+rtitle] ] [ -icols[l][sscale][ooffset][,...] ] [ -n[b|c|l|n][+a][+bBC][+tthreshold] ] [ -:[i|o] ]

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

## Description

grdrotater reads a geographical grid and reconstructs it given a total reconstruction rotation. Optionally, the user may supply a clipping polygon in multiple-segment format; then, only the part of the grid inside the polygon is used to determine the return grid region. The outline of the projected region is returned on stdout provided the rotated region is not the entire globe.

## Required Arguments

ingrdfile
  Name of a grid file in geographical (lon, lat) coordinates.
-Erotfile

  Give file with rotation parameters. This file must contain one record for each rotation; each record must be of the following format:

  lon lat tstart [tstop] angle [ khat a b c d e f g df ]

  where tstart and tstop are in Myr and lon lat angle are in degrees. tstart and tstop are the ages of the old and young ends of a stage. If tstop is not present in the record then a total reconstruction rotation is expected and tstop is implicitly set to 0 and should not be specified for any of the records in the file. If a covariance matrix C for the rotation is available it must be specified in a format using the nine optional terms listed in brackets. Here, C = (g/khat)*[ a b d; b c e; d e f ] which shows C made up of three row vectors. If the degrees of freedom (df) in fitting the rotation is 0 or not given it is set to 10000. Blank lines and records whose first column contains # will be ignored. You may prepend a leading + to the filename to indicate you wish to invert the rotations. Alternatively, give the filename composed of two plate IDs separated by a hyphen (e.g., PAC-MBL) and we will instead extract that rotation from the GPlates rotation database. We return an error if the rotation cannot be found.

-elon/lat/angle
  Alternatively, specify the longitude, latitude, and opening angle (all in degrees and separated by /) for a single total reconstruction.
-Goutgrdfile
  Name of output grid. This is the grid with the data reconstructed according to the specified rotation.

## Optional Arguments

-Fpolygonfile
  Specify a multisegment closed polygon file that describes the inside area of the grid that should be projected [Default projects entire grid].
-N
  Do Not output the rotated polygon outline [Default will write it to stdout].

-Rwest/east/south/north[/zmin/zmax][r]
  west, east, south, and north specify the region of interest, and you may specify them in decimal degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and upper right map coordinates are given instead of w/e/s/n. The two shorthands -Rg and -Rd stand for global domain (0/360 and -180/+180 in longitude respectively, with -90/+90 in latitude). Alternatively, specify the name of an existing grid file and the -R settings (and grid spacing, if applicable) are copied from the grid. Using -Runit expects projected (Cartesian) coordinates compatible with chosen -J and we inversely project to determine actual rectangular geographic region. For perspective view (-p), optionally append /zmin/zmax. In case of perspective view (-p), a z-range (zmin, zmax) can be appended to indicate the third dimension. This needs to be done only when using the -Jz option, not when using only the -p option. In the latter case a perspective view of the plane is plotted, with no third dimension.
-S
  Skip the rotation of the grid, just rotate the polygon outline (requires -F).
-Tage
  Sets the desired age of reconstruction when -E is given.

-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.

-icols[l][sscale][ooffset][,...] (more ...)
  Select input columns.

-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more ...)
  Select interpolation mode for grids.

-^
  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.

## Consequences of grid resampling

Resample or sampling of grids will use various algorithms (see -n) that may lead to possible distortions or unexpected results in the resampled values. One expected effect of resampling with splines is the tendency for the new resampled values to slightly exceed the global min/max limits of the original grid. If this is unacceptable, you can impose clipping of the resampled values values so they do not exceed the input min/max values by adding +c to your -n option.

## Examples

To rotate the data defined by grid topo.nc and the polygon outline clip_path.d, using a total reconstruction rotation with pole at (135.5, -33.0) and a rotation angle of 37.3 degrees and bicubic interpolation, try

>     gmt grdrotater topo.nc -e135.5/-33/37.3 -V -Fclip_path.d -Grot_topo.nc > rot_clip_path.d

To rotate the entire grid faa.nc back to 32 Ma using the rotation file rotations.txt and a bilinear interpolation, try

>     gmt grdrotater faa.nc -Erotations.txt -T32 -V -Grot_faa.nc -nl > rot_faa_path.d

To just see how the outline of the grid large.nc will plot after the same rotation, try

>     gmt grdrotater large.nc -Erotations.txt -T32 -V -S \| psxy -Rg -JH180/6i -B30 -W0.5p \| gv -

Let say you have rotated gridA.nc and gridB.nc, restricting each rotation to nodes inside polygons polyA.d and polyB.d, respectively, using rotation A = (123W,22S,16,4) and rotation B = (108W, 16S, -14.5), yielding rotated grids rot_gridA.nc and rot_gridB.nc. To determine the region of overlap between the rotated grids, we use grdmath:

>     gmt grdmath 1 rot_gridA.nc ISNAN SUB 1 rot_gridB.nc ISNAN SUB 2 EQ = overlap.nc

The grid overlap.nc now has 1s in the regions of overlap and 0 elsewhere. You can use it as a mask or use grdcontour -D to extract a polygon (contour).

## See Also

backtracker, grdspotter, hotspotter, originator, rotconverter

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