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 directory
      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 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.

      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 inch/degree, 1:xxxxx, or
           width in inch (upper case modifier).

           CYLINDRICAL PROJECTIONS:

           -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)

           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:

           -Jpscale (Linear projection for polar (theta,r) coordinates)
           -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 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


SEE ALSO

      gmtdefaults, gmt, grdsample, grdfilter


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.




























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