grdcut - Extract subregion from a grid
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 is 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 given point. Complementary to grdcut there is grdpaste, which will join together two grid files along a common edge.
- This is the input grid file.
- This is the output grid file.
- Allow grid to be extended if new -R exceeds existing boundaries. Append nodata value to initialize nodes outside current region [Default is NaN].
- -R[unit]xmin/xmax/ymin/ymax[r] (more ...)
- Specify the region of interest. This defines the subregion to be cut out.
- 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 -Sn is used we set all nodes outside the circle to NaN.
- -V[level] (more ...)
- Select verbosity level [c].
- Determine the 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, specify a hyphen (-). Normally, any NaNs encountered are simply skipped and not considered in the decision. Use -Zn to consider a NaN to be outside the z-range. This means the new subset will be NaN-free. Alternatively, use -Zr 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].
- -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 exits (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 exits.
- -? or no arguments
- Print a complete usage (help) message, including the explanation of all options, then exits.
For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for meter [Default], 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. Prepend - to a distance (or the unit is no distance is given) to perform “Flat Earth” calculations (quicker but less accurate) or prepend + to perform exact geodesic calculations (slower but more accurate).
Grid File Formats¶
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. (more ...)
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.
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 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