grdedit - Modify header or content of a grid
grdedit grid [ -A ] [ -Dxname/yname/zname/scale/offset/invalid/title/remark ] [ -E[a|h|l|r|t|v] ] [ -Goutgrid ] [ -Jparameters ] [ -Ntable ] [ -Rregion ] [ -S ] [ -T ] [ -V[level] ] [ -bi<binary> ] [ -di<nodata> ] [ -f<flags> ] [ -i<flags> ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated arguments.
grdedit reads the header information in a binary 2-D grid file and replaces the information with values provided on the command line [if any]. As an option, global, geographical grids (with 360 degrees longitude range) can be rotated in the east-west direction, and individual nodal values can be replaced from a table of x, y, z values. grdedit only operates on files containing a grid header. Note: If it is important to retain the original data you should use -G to save the modified grid to a new file.
By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF file format. However, GMT is able to produce grid files in many other commonly used grid file formats and also facilitates so called “packing” of grids, writing out floating point data as 1- or 2-byte integers. To specify the precision, scale and offset, the user should add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid type and precision, and scale and offset are optional scale factor and offset to be applied to all grid values, and nan is the value used to indicate missing data. In case the two characters id is not provided, as in =/scale than a id=nf is assumed. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. See grdconvert and Section Grid file format specifications of the GMT Technical Reference and Cookbook for more information.
When reading a netCDF file that contains multiple grids, GMT will read, by default, the first 2-dimensional grid that can find in that file. To coax GMT into reading another multi-dimensional variable in the grid file, append ?varname to the file name, where varname is the name of the variable. Note that you may need to escape the special meaning of ? in your shell program by putting a backslash in front of it, or by placing the filename and suffix between quotes or double quotes. The ?varname suffix can also be used for output grids to specify a variable name different from the default: “z”. See grdconvert and Sections Modifiers for COARDS-compliant netCDF files and Grid file format specifications of the GMT Technical Reference and Cookbook for more information, particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
When the output grid type is netCDF, the coordinates will be labeled “longitude”, “latitude”, or “time” based on the attributes of the input data or grid (if any) or on the -f or -R options. For example, both -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid. When the x, y, or z coordinate is time, it will be stored in the grid as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH in the gmt.conf file or on the command line. In addition, the unit attribute of the time variable will indicate both this unit and epoch.
Let us assume the file data.nc covers the area 300/310/10/30. We want to change the boundaries from geodetic longitudes to geographic and put a new title in the header. We accomplish this by
gmt grdedit data.nc -R-60/-50/10/30 -D//////"Gravity Anomalies"
The grid world.nc has the limits 0/360/-72/72. To shift the data so that the limits would be -180/180/-72/72, use
gmt grdedit world.nc -R-180/180/-72/72 -S
The file junk.nc was created prior to GMT 3.1 with incompatible -R and -I arguments. To reset the x- and y-increments we run
gmt grdedit junk.nc -A
The file junk.nc was created prior to GMT 4.1.3 and does not contain the required information to indicate that the grid is geographic. To add this information, run
gmt grdedit junk.nc -fg
To rotate the grid oblique.nc 90 degrees counter-clockwise and write out the rotated grid to a new file, run
gmt grdedit oblique.nc -El -Goblique_rot.nc