Extract subregion from a grid or image
Note: No space is allowed between the option flag and the associated arguments.
grdcut will produce a new outgrid file which is a subregion of ingrid. The subregion may be specified with -R as in other programs; the specified range must not exceed the range of ingrid (but see -N). If in doubt, run grdinfo to check range. Alternatively, define the subregion indirectly via a range check on the node values or via distances from a fixed point. Finally, you can use -J for oblique projections to determine the corresponding rectangular -R setting that will give a subregion that fully covers the oblique domain. Note: If the input grid is actually an image (gray-scale, RGB, or RGBA), then options -N and -Z are unavailable, while for multi-layer Geotiff files only options -R, -S and -G are supported, i.e., you can cut out a sub-region only (which we do via gdal_translate if you have multiple bands). Complementary to grdcut there is grdpaste, which will join together two grid files (not images) along a common edge.
Input grid file. Optionally, append =ID for reading a specific file format [Default is =nf] or ?varname for a specific netCDF variable [Default is the first 2-D grid found by GMT] (See full description). The following modifiers are supported:
+b - Select a band (for images only) [Default is 0].
+d - Divide data values by the given divisor [Default is 1].
+n - Replace data values matching invalid with NaN.
+o - Offset data values by the given offset [Default is 0].
+s - Scale data values by the given scale [Default is 1].
Note: Any offset is added after any scaling.
Give the name of the output grid file. Optionally, append =ID for writing a specific file format (See full description). The following modifiers are supported:
+d - Divide data values by given divisor [Default is 1].
+n - Replace data values matching invalid with a NaN.
+o - Offset data values by the given offset, or append a for automatic range offset to preserve precision for integer grids [Default is 0].
+s - Scale data values by the given scale, or append a for automatic scaling to preserve precision for integer grids [Default is 1].
Note: Any offset is added before any scaling. +sa also sets +oa (unless overridden). To write specific formats via GDAL, use =gd and supply driver (and optionally dataType) and/or one or more concatenated GDAL -co options using +c. See the “Writing grids and images” cookbook section for more details.
A “dry run”: Simply report the region and increment of what would be the extracted grid given the selected options. No grid is created (-G is disallowed) and instead we write a single data record with west east south north xinc yinc to standard output. The increments will reflect the input grid unless it is a remote gridded data set without implied resolution. Append +t to instead receive the information as the trailing string “-Rwest/east/south/north -Ixinc/yinc”.
Specify a multisegment closed polygon file. All grid nodes outside the polygon will be set to NaN. Append +i to invert that and set all nodes inside the polygon to NaN instead. Optionally, append +c to crop the grid region to reflect the bounding box of the polygon.
Allow grid to be extended if new -R exceeds existing boundaries. Append nodata value to initialize nodes outside current region [Default is NaN].
Specify an origin and radius; append a distance unit (see Units) and we determine the corresponding rectangular region so that all grid nodes on or inside the circle are contained in the subset. If +n is appended we set all nodes outside the circle to NaN.
Determine a new rectangular region so that all nodes outside this region are also outside the given z-range [-inf/+inf]. To indicate no limit on min or max only, specify a hyphen (-). Normally, any NaNs encountered are simply skipped and not considered in the range-decision. Append +n to consider a NaN to be outside the given z-range. This means the new subset will be NaN-free. Alternatively, append +r to consider NaNs to be within the data range. In this case we stop shrinking the boundaries once a NaN is found [Default simply skips NaNs when making the range decision]. Finally, if your core subset grid is surrounded by rows and/or columns that are all NaNs, append +N to strip off such columns before (optionally) considering the range of the core subset for further reduction of the area.
- -f[i|o]colinfo (more …)
Specify data types of input and/or output columns.
- -^ or just -
Print a short message about the syntax of the command, then exit (Note: on Windows just use -).
- -+ or just +
Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.
- -? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exit.
Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.
For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for meter [Default unless stated otherwise], f for foot, k for km, M for statute mile, n for nautical mile, and u for US survey foot. By default we compute such distances using a spherical approximation with great circles (-jg) using the authalic radius (see PROJ_MEAN_RADIUS). You can use -jf to perform “Flat Earth” calculations (quicker but less accurate) or -je to perform exact geodesic calculations (slower but more accurate; see PROJ_GEODESIC for method used).
Geographical And Time Coordinates¶
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.
Note: Below are some examples of valid syntax for this module.
The examples that use remote files (file names starting with
can be cut and pasted into your terminal for testing.
Other commands requiring input files are just dummy examples of the types
of uses that are common but cannot be run verbatim as written.
To obtain data for an oblique Mercator projection map we need to extract more data that is actually used. This is necessary because the output of grdcut has edges defined by parallels and meridians, while the oblique map in general does not. Hence, to get all the data from the ETOPO2 data needed to make a contour map for the region defined by its lower left and upper right corners and the desired projection, use:
gmt grdcut @earth_relief_02m -R160/20/220/30+r -Joc190/25.5/292/69/1 -Gdata.nc
Suppose you have used surface to grid ship gravity in the region between 148E - 162E and 8N - 32N, and you do not trust the gridding near the edges, so you want to keep only the area between 150E - 160E and 10N - 30N, then:
gmt grdcut grav_148_162_8_32.nc -Ggrav_150_160_10_30.nc -R150/160/10/30 -V
To return the subregion of a grid such that any boundary strips where all values are entirely above 0 are excluded, try:
gmt grdcut bathy.nc -Gtrimmed_bathy.nc -Z-/0 -V
To return the subregion of a grid such that any boundary rows or columns that are all NaNs, try:
gmt grdcut bathy.nc -Gtrimmed_bathy.nc -Z+N -V
To return the subregion of a grid that contains all nodes within a distance of 500 km from the point 45,30 try:
gmt grdcut bathy.nc -Gsubset_bathy.nc -S45/30/500k -V
To create a topography grid with data only inside France and set it to NaN outside France, based on the 10x10 minute DEM, try:
gmt coast -EFR -M > FR.txt gmt grdcut @earth_relief_10m -FFR.txt+c -GFR_only.grd gmt grdimage FR_only.grd -B -pdf map
To determine what grid region and resolution (in text format) most suitable for a 24 cm wide map that is using an oblique projection to display the remote Earth Relief data grid, try:
gmt grdcut @earth_relief -R270/20/305/25+r -JOc280/25.5/22/69/24c -D+t -V
If the input file is a geotiff with multiple data bands then the output format will depend on your selection (if any) of the bands to keep: If you do not specify any bands (which means we consider all the available bands) or you select more than one band, then the output file can either be another geotiff (if you give a .tif[f] extension) or it can be a multiband netCDF file (if you give a .nc or .grd extension). If you select a single band from the input geotiff then GMT will normally read that in as a single grid layer and thus write a netCDF grid (unless you append another grid format specifier). However, if your output filename has a .tif[f] extension then we will instead write it as a one-band geotiff. All geotiff output operations are done via GDAL’s gdal_translate.