grdraster - Extract subregion from a binary raster and save as a
GMT grid
grdraster [ filenumber | "text
pattern" ]
-Rregion [ -Ggrdfile ] [
-Iincrement ] [ -Jparameters ] [
-V[level] ] [ -bobinary ] [ -donodata ] [
-oflags ]
Note: No space is allowed between the option flag and the
associated arguments.
grdraster reads a file called grdraster.info from
the current working directory, the directories pointed to by the environment
variables $GMT_USERDIR and $GMT_DATADIR, or in
$GMT_SHAREDIR/dbase (in that order). The file grdraster.info
defines binary arrays of data stored in scan-line format in data files. Each
file is given a filenumber in the info file. grdraster figures
out how to load the raster data into a grid file spanning a region defined
by -R. By default the grid spacing equals the raster spacing. The
-I option may be used to sub-sample the raster data. No filtering or
interpolating is done, however; the x_inc and y_inc of the
grid must be multiples of the increments of the raster file and
grdraster simply takes every n'th point. The output of
grdraster is either grid or pixel registered depending on the
registration of the raster used. It is up to the GMT system person to
maintain the grdraster.info file in accordance with the available
rasters at each site. Raster data sets are not supplied with GMT but can be
obtained by anonymous ftp and on CD-ROM (see README page in dbase
directory). grdraster will list the available files if no arguments
are given. Finally, grdraster will write xyz-triplets to stdout if no
output gridfile name is given
- filenumber
- If an integer matching one of the files listed in the
grdraster.info file is given we will use that data set, else we
will match the given text pattern with the data set description in order
to determine the data set.
- -Rwest/east/south/north[/zmin/zmax][+r][+uunit]
- 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 for grid creation, give
Rcodelon/lat/nx/ny, where
code is a 2-character combination of L, C, R (for left, center, or
right) and T, M, B for top, middle, or bottom. e.g., BL for lower left.
This indicates which point on a rectangular region the
lon/lat coordinate refers to, and the grid dimensions
nx and ny with grid spacings via -I is used to create
the corresponding region. Alternatively, specify the name of an existing
grid file and the -R settings (and grid spacing, if applicable) are
copied from the grid. Appending +uunit expects projected
(Cartesian) coordinates compatible with chosen -J and we inversely
project to determine actual rectangular geographic region. For perspective
view (-p), optionally append /zmin/zmax. In case of
perspective view (-p), a z-range (zmin, zmax) can be
appended to indicate the third dimension. This needs to be done only when
using the -Jz option, not when using only the -p option. In
the latter case a perspective view of the plane is plotted, with no third
dimension. If r is appended, you may also specify a map projection
to define the shape of your region. The output region will be rounded off
to the nearest whole grid-step in both dimensions.
- -Ggrdfile
- Name of output grid file. If not set, the grid will be written as ASCII
(or binary; see -bo) xyz-triplets to stdout instead.
- -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
- x_inc [and optionally y_inc] is the grid spacing.
Optionally, append a suffix modifier. Geographical (degrees)
coordinates: Append m to indicate arc minutes or s to
indicate arc seconds. If one of the units e, f, k,
M, n or u is appended instead, the increment is
assumed to be given in meter, foot, km, Mile, nautical mile or US survey
foot, respectively, and will be converted to the equivalent degrees
longitude at the middle latitude of the region (the conversion depends on
PROJ_ELLIPSOID). If y_inc is given but set to 0 it will be reset
equal to x_inc; otherwise it will be converted to degrees latitude.
All coordinates: If +e is appended then the corresponding
max x (east) or y (north) may be slightly
adjusted to fit exactly the given increment [by default the increment may
be adjusted slightly to fit the given domain]. Finally, instead of giving
an increment you may specify the number of nodes desired by
appending +n to the supplied integer argument; the increment is
then recalculated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a gridline-registered
or pixel-registered grid; see App-file-formats for details. Note: if
-Rgrdfile is used then the grid spacing has already been
initialized; use -I to override the values.
This option applies only if no -G option has been set.
- -^ 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.
To extract data from raster 1, taking one point every 30 minutes,
in an area extended beyond 360 degrees to allow later filtering, run
gmt grdraster 1 -R-4/364/-62/62 -I30m -Gdata.nc
To obtain data for an oblique Mercator projection we need to
extract more data that is actually used. This is necessary because the
output of grdraster 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 grdraster ETOPO2 -R160/20/220/30r -Joc190/25.5/292/69/1 -Gdata.nc
To extract data from the 2 min Geoware relief blend and write it
as binary double precision xyz-triplets to standard output:
gmt grdraster "2 min Geoware" -R20/25/-10/5 -bo > triplets.b
gmtdefaults, gmt, grdsample, grdfilter
2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F.
Wobbe