NAME
grdraster - extract subregion from a binary raster and write
a grd file
SYNOPSIS
grdraster [ filenumber ] -Rwest/east/south/north[r]
-Ggrdfilename [ -Ix_inc[m|c][/y_inc[m|c]] ] [ -Jparams ] [
-V ]
DESCRIPTION
grdraster reads a file called grdraster.info from the direc-
tory pointed to by the environment parameter $GMT_GRIDDIR
(if this parameter is not set it defaults to
$GMTHOME/lib/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 multi-
ples of the increments of the raster file and grdraster sim-
ply takes every n'th point. The output of grdraster is
either grid or pixel registered depending on the registra-
tion 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.
filenumber
An integer matching one of the files listed in the
grdraster.info file.
-R west, east, south, and north specify the Region of
interest. To specify boundaries in degrees and minutes
[and seconds], use the dd:mm[:ss] format. Append r if
lower left and upper right map coordinates are given
instead of wesn. If r is appended, you must also
specify a map projection to define the shape of your
region.
OPTIONS
-I x_inc [and optionally y_inc] is the grid spacing.
Append m to indicate minutes or c to indicate seconds.
-J 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 set-
ting in .gmtdefaults, but this can be overridden on the
command line by appending the c, i, or m to the
scale/width value.
CYLINDRICAL PROJECTIONS:
-Jclon0/lat0/scale (Cassini)
-Jjlon0/scale (Miller)
-Jmscale (Mercator - Greenwich and Equator as origin)
-Jmlon0/lat0/scale (Mercator - Give meridian and stan-
dard 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 ori-
gin)
-Juzone/scale (UTM - Universal Transverse Mercator)
-Jylon0/lats/scale (Basic Cylindrical Projection)
AZIMUTHAL PROJECTIONS:
-Jalon0/lat0/scale (Lambert).
-Jelon0/lat0/scale (Equidistant).
-Jflon0/lat0/horizon/scale (Gnomonic).
-Jglon0/lat0/scale (Orthographic).
-Jslon0/lat0/scale (General Stereographic)
CONIC PROJECTIONS:
-Jblon0/lat0/lat1/lat2/scale (Albers)
-Jdlon0/lat0/lat1/lat2/scale (Equidistant)
-Jllon0/lat0/lat1/lat2/scale (Lambert)
MISCELLANEOUS PROJECTIONS:
-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)
NON-GEOGRAPHICAL PROJECTIONS:
-Jp[a]scale[/origin] (polar (theta,r) coordinates,
optional a for azimuths and offset theta [0])
-Jxx-scale[l|ppow][/y-scale[l|ppow]] (Linear, log, and
power scaling)
More details can be found in the psbasemap manpages.
-V Selects verbose mode, which will send progress reports
to stderr [Default runs "silently"].
EXAMPLES
To extract data from raster 1, taking one point every 30
minutes, in an area extended beyond 360 degrees to allow
later filtering, try
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 paral-
lels 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, try
grdraster 3 -R160/20/220/30r -Joc190/25.5/292/69/1
-Gdata.grd
SEE ALSO
gmtdefaults(l), gmt(l), grdsample(l), grdfilter(l)
REFERENCES
Wessel, P., and W. H. F. Smith, 1999, The Generic Mapping
Tools (GMT) version 3.2 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://www.agu.org/eos_elec/95154e.html, Copyright 1995 by
the 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.