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 ] [ -bo[s][n] ]
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 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 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
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
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.
-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 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 origin)
-Jmlon0/lat0/scale (Mercator - Give meridian and standard paral-
lel)
-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 Car-
ree))
-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)
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))
-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"].
-O Selects Overlay plot mode [Default initializes a new plot sys-
tem]. 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 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, 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 double precision
xyz-triplets to standard output, try
grdraster 3 -R20/25/-10/5 -bo >! triplets.b
SEE ALSO
gmtdefaults(GMTMANSECTION), gmt(GMTMANSECTION), grdsample(GMTMANSEC-
TION), grdfilter(GMTMANSECTION)
REFERENCES
Wessel, P., and W. H. F. Smith, 2004, The Generic Mapping Tools (GMT)
version 3.4.5 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 Map-
ping Tools Released, EOS Trans., AGU, 76 (33), p. 329.
Wessel, P., and W. H. F. Smith, 1995, New Version of the Generic Map-
ping 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 Dis-
play Data, EOS Trans., AGU, 72 (41), p. 441.
VERSION DATE GRDRASTER(GMTMANSECTION)
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