# GRDROTATER

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
EXAMPLES
SEE ALSO

## NAME

| grdrotater − Rotate a grid using a finite rotation

## SYNOPSIS

| grdrotate ingrdfile −G outgrdfile −T plon/plat/omega [ −F polygonfile ] [ −H[i][nrec] ] [ −M[flag] ] [ −N ] [ −Q[value] [ −R west/east/south/north[r] ] [ −S ] [ −V ] [ −:[i|o] ] [ −b[i|o][s|S|d|D][ncol] ]

## 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.

|  | No space between the option flag and the associated arguments. Use upper case for the option flags and lower case for modifiers.

| ingrdfile

| Name of a grid file in geographical (lon, lat) coordinates.

| −G |  | Name of output grid. This is the grid with the data reconstructed according to the specified rotation.
| −T |  | Finite rotation. Specify the longitude and latitude of the rotation pole and the opening angle, all in degrees.

## OPTIONS

| −F |  | Specify a multi-segment closed polygon file that describes the inside area of the grid that should be projected [Default projects entire grid].
| −H |  | Input file(s) has Header record(s). Number of header records can be changed by editing your .gmtdefaults4 file. If used, GMT default is 1 header record. Use −Hi if only input data should have header records [Default will write out header records if the input data have them].
| −M |  | Multiple segment file. Segment separator is a record beginning with flag. [Default is ’>’].
| −N |  | Do Not output the rotated polygon outline [Default will write it to stdout].

−Q

Quick mode, use bilinear rather than bicubic interpolation [Default]. Optionally, append value in the 0 <= value <= 1 range. This parameter controls how close to nodes with NaN values the interpolation will go. E.g., a value of 0.5 will interpolate about 1/2-way from a non-NaN to a NaN node, whereas 0.1 will go about 90% of the way, etc. [Default is 1, which means none of the four nearby nodes may be NaN]. A value of 0 will just return the value of the nearest node instead of interpolating.

| −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 wesn. The two shorthands −Rg −Rd stand for global domain (0/360 or -180/+180 in longitude respectively, with -90/+90 in latitude).
| −S |  | Skip the rotation of the grid, just rotate the polygon outline (requires −F).
| −V |  | Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
| −: |  | Toggles between (longitude,latitude) and (latitude,longitude) input/output. [Default is (longitude,latitude)].
| −bi |  | Selects binary input. Append s for single precision [Default is d (double)]. Uppercase S (or D) will force byte-swapping. Optionally, append ncol, the number of columns in your binary file if it exceeds the columns needed by the program. [Default is 2 input columns].
| −bo |  | Selects binary output. Append s for single precision [Default is d (double)]. Uppercase S (or D) will force byte-swapping. Optionally, append ncol, the number of desired columns in your binary output file. [Default is same as input].

## EXAMPLES

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

grdrotater topo.grd −T135.5/-33/37.3 −V −Fclip_path.d −Grot_topo.grd > rot_clip_path.d

To rotate the entire grid faa.grd using a finite rotation pole at (67:45W, 22:35S) and a rotation angle of 19.6 degrees using a bilinear interpolation, try

grdrotater faa.grd −T67:45W/22:35S/19.6 −V −Q −Grot_faa.grd > rot_faa_path.d

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

grdrotater large.grd −T67:45W/22:35S/19.6 −V −S | psxy −Rg −JH180/6i −B30 −M -W0.5p | gv -

Let say you have rotated gridA.grd and gridB.grd, 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.grd and rot_gridB.grd. To determine the region of overlap between the rotated grids, we use grdmath:

grdmath 1 rot_gridA.grd ISNAN SUB 1 rot_gridB.grd ISNAN SUB 2 EQ = overlap.grd

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

## SEE ALSO

| backtracker(l), hotspotter(l), originator(l) rotconverter(l)