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.