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GRDCUT(1) Generic Mapping Tools GRDCUT(1)
NAME
grdcut - Extract a subregion out of a grid file
SYNOPSIS
grdcut input_file.grd -Goutput_file.grd -Rwest/east/south/north[r] [ -V
] [ -Z[n]min/max] ] [ -f[i|o]colinfo ]
DESCRIPTION
grdcut will produce a new output_file.grd file which is a subregion of
input_file.grd. The subregion is specified with -R as in other
programs; the specified range must not exceed the range of
input_file.grd. If in doubt, run grdinfo to check range.
Alternatively, define the subregion indirectly via a range check on the
node values. Complementary to grdcut there is grdpaste, which will
join together two grid files along a common edge.
input_file.grd
this is the input .grd format file.
-Goutput_file.grd
this is the output .grd format file.
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north 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. For calendar time coordinates you may
either give (a) relative time (relative to the selected
TIME_EPOCH and in the selected TIME_UNIT; append t to -JX|x), or
(b) absolute time of the form [date]T[clock] (append T to
-JX|x). At least one of date and clock must be present; the T
is always required. The date string must be of the form
[-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]] (ISO
week calendar), while the clock string must be of the form
hh:mm:ss[.xxx]. The use of delimiters and their type and
positions must be exactly as indicated (however, input, output
and plot formats are customizable; see gmtdefaults). This
defines the subregion to be cut out.
OPTIONS
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-Z 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. Use -Zn to
consider a NaN to be outside the z-range.
-f Special formatting of input and/or output columns (time or
geographical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T
(absolute calendar time), t (relative time in chosen TIME_UNIT
since TIME_EPOCH), x (longitude), y (latitude), or f (floating
point) to each column or column range item. Shorthand -f[i|o]g
means -f[i|o]0x,1y (geographic coordinates).
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 2- or 4-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. When
reading grids, the format is generally automatically recognized. If
not, the same suffix can be added to input grid file names. See
grdreformat(1) and Section 4.17 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 grdreformat(1) and Section
4.18 of the GMT Technical Reference and Cookbook for more information,
particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
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 -R 90w/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 .gmtdefaults file or on the command line. In addition, the unit
attribute of the time variable will indicate both this unit and epoch.
EXAMPLES
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:
grdcut grav_148_162_8_32.nc -G grav_150_160_10_30.nc -R 150/160/10/30
-V To return the subregion of a grid such that any boundary strips
where all values are entirely above 0, try
grdcut bathy.nc -G trimmed_bathy.nc -Z-/0 -V
SEE ALSO
grdpaste(1), grdinfo(1), GMT(1)
GMT 4.5.14 1 Nov 2015 GRDCUT(1)