grdraster
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 ] [ -bo[s] ]
DESCRIPTION
grdraster reads a file called grdraster.info 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 inter
polating 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 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 avail
able files if no arguments are given. Finally, grdraster
will write xyz-triplets to stdout if no output gridfile
name is 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 min
utes [and seconds], use the dd:mm[:ss] format.
Append r if lower left and upper right map coordi
nates are given instead of wesn. 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.
OPTIONS
-G Name of output grid file. If not set, the grid will
be written as ASCII (or binary; see -boxyz-triplets
to stdout instead.
Append m to indicate minutes or c to indicate sec
onds.
-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 MEA
SURE_UNIT setting 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 ori
gin)
-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 Equa
tor as y = 0)
-Jtlon0/lat0/scale (TM - Transverse Mercator, set
origin)
-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/[slat/]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))
-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 man
pages.
-V Selects verbose mode, which will send progress
reports to stderr [Default runs "silently"].
-bo Selects binary output. Append s for single preci
sion [Default is double]. This option applies only
if no -G option has been set.
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 neces
sary 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 projec
tion, try
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 dou
ble precision xyz-triplets to standard output, try
grdraster 3 -R20/25/-10/5 -bo >! triplets.b
gmtdefaults(l), gmt(l), grdsample(l), grdfilter(l)
REFERENCES
Wessel, P., and W. H. F. Smith, 2001, The Generic Mapping
Tools (GMT) version 3.4 Technical Reference & Cookbook,
SOEST/NOAA.
Wessel, P., and W. H. F. Smith, 1998, New, Improved Ver
sion of Generic Mapping Tools Released, EOS Trans., AGU,
79 (47), p. 579.
Wessel, P., and W. H. F. Smith, 1995, New Version of the
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.
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