man grdraster () - extract subregion from a binary raster and write a grd file


grdraster - extract subregion from a binary raster and write a grd file


grdraster [ filenumber ] -Rwest/east/south/north[r] [ -Ggrdfilename ] [ -Ix_inc[m|c][/y_inc[m|c]] ] [ -Jparams ] [ -V ] [ -bo[s] ]


grdraster reads a file called from the directory pointed to by the environment parameter $GMT_GRIDDIR (if this parameter is not set it defaults to $GMTHOME/share/dbase). The info file 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 grd 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 grd file 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 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

An integer matching one of the files listed in the file.
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[][W|E|S|N] format. Append r if lower left and upper right map coordinates are given instead of wesn. The two shorthands -Rg -Rd stand for global domain (0/360 or -180/+180 in longitude respectively, with -90/+90 in latitude). For calendar time coordinates you may either give relative time (relative to the selected TIME_EPOCH and in the selected TIME_UNIT; append t to -JX|x), or 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 as indicated (however, input/output and plotting formats are flexible). 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.


Name of output grid file. If not set, the grid will be written as ASCII (or binary; see -bo xyz-triplets to stdout instead.
x_inc [and optionally y_inc] is the grid spacing. Append m to indicate minutes or c to indicate seconds.
Selects the map projection. Scale is UNIT/degree, 1:xxxxx, or width in UNIT (upper case modifier). UNIT is cm, inch, or m, depending on the MEASURE_UNIT setting in .gmtdefaults4, but this can be overridden on the command line by appending c, i, or m to the scale/width value. For map height, max dimension, or min dimension, append h, +, or - to the width, respectively.


-Jclon0/lat0/scale (Cassini)

-Jjlon0/scale (Miller)

-Jmscale (Mercator - Greenwich and Equator as origin)

-Jmlon0/lat0/scale (Mercator - Give meridian and standard parallel)

-Joalon0/lat0/azimuth/scale (Oblique Mercator - point and azimuth)

-Joblon0/lat0/lon1/lat1/scale (Oblique Mercator - two points)

-Joclon0/lat0/lonp/latp/scale (Oblique Mercator - point and pole)

-Jqlon0/scale (Equidistant Cylindrical Projection (Plate Carree))

-Jtlon0/scale (TM - Transverse Mercator, with Equator as y = 0)

-Jtlon0/lat0/scale (TM - Transverse Mercator, set origin)

-Juzone/scale (UTM - Universal Transverse Mercator)

-Jylon0/lats/scale (Basic Cylindrical Projection)


-Jalon0/lat0/scale (Lambert).

-Jelon0/lat0/scale (Equidistant).

-Jflon0/lat0/horizon/scale (Gnomonic).

-Jglon0/lat0/scale (Orthographic).

-Jslon0/lat0/[slat/]scale (General Stereographic)


-Jblon0/lat0/lat1/lat2/scale (Albers)

-Jdlon0/lat0/lat1/lat2/scale (Equidistant)

-Jllon0/lat0/lat1/lat2/scale (Lambert)


-Jhlon0/scale (Hammer)

-Jilon0/scale (Sinusoidal)

-Jk[f|s]lon0/scale (Eckert IV (f) and VI (s))

-Jnlon0/scale (Robinson)

-Jrlon0/scale (Winkel Tripel)

-Jvlon0/scale (Van der Grinten)

-Jwlon0/scale (Mollweide)


-Jp[a]scale[/origin] (polar (theta,r) coordinates, optional a for azimuths and offset theta [0])

-Jxx-scale[l|ppow][/y-scale[l|ppow]][d] (Linear, log, and power scaling)

More details can be found in the psbasemap man pages.

Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
Selects binary output. Append s for single precision [Default is double]. This option applies only if no -G option has been set.


To extract data from raster 1, taking one point every 30 minutes, in an area extended beyond 360 degrees to allow later filtering, run

grdraster 1 -R-4/364/-62/62 -I30m -Gdata.grd

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 data set 3 needed to make a contour map for the region defined by its lower left and upper right corners and the desired projection, use

grdraster 3 -R160/20/220/30r -Joc190/25.5/292/69/1 -Gdata.grd

To extract data from raster 3 and write it as binary double precision xyz-triplets to standard output:

grdraster 3 -R20/25/-10/5 -bo >! triplets.b



Wessel, P., and W. H. F. Smith, 2004, The Generic Mapping Tools (GMT) version 4 Technical Reference & Cookbook, SOEST/NOAA.

Wessel, P., and W. H. F. Smith, 1998, New, Improved Version of Generic Mapping Tools Released, EOS Trans., AGU, 79 (47), p. 579.

Wessel, P., and W. H. F. Smith, 1995, New Version of the Generic Mapping Tools Released, EOS Trans., AGU, 76 (33), p. 329.

Wessel, P., and W. H. F. Smith, 1995, New Version of the Generic Mapping Tools Released, http: American Geophysical Union.

Wessel, P., and W. H. F. Smith, 1991, Free Software Helps Map and Display Data, EOS Trans., AGU, 72 (41), p. 441.