# 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] ] [ −N ] [ −Q[b|c|l|n][[/]threshold] ] [ −R west/east/south/north[r] ] [ −S ] [ −V ] [ −:[i|o] ] [ −b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ −m[flag] ]

## 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). If used, the default number of header records is N_HEADER_RECS. Use −Hi if only input data should have header records [Default will write out header records if the input data have them]. Blank lines and lines starting with # are always skipped.
| −N |  | Do Not output the rotated polygon outline [Default will write it to stdout].
| −Q |  | Quick mode, use bilinear rather than bicubic interpolation [Default]. Alternatively, select the interpolation mode by adding b for B-spline smoothing, c for bicubic interpolation, l for bilinear interpolation or n for nearest-neighbor value. Optionally, append threshold in the range [0,1]. This parameter controls how close to nodes with NaN values the interpolation will go. E.g., a threshold of 0.5 will interpolate about half 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 (4 or 16) nearby nodes may be NaN]. −Q0 will just return the value of the nearest node instead of interpolating. This is the same as using −Qn.
| −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.
| −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 input file if it exceeds the columns needed by the program. Or append c if the input file is netCDF. Optionally, append var1/var2/... to specify the variables to be read. [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].
| −m |  | Multiple segment file(s). Segments are separated by a special record. For ASCII files the first character must be flag [Default is ’>’]. For binary files all fields must be NaN and −b must set the number of output columns explicitly. By default the −m setting applies to both input and output. Use −mi and −mo to give separate settings to input and output.

## 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(1), hotspotter(1), originator(1) rotconverter(1)