NAME

     grd2cpt - Read a grdfile and make a color palette file

SYNOPSIS

     grd2cpt grdfile [ -Cminhue/maxhue ] [ -Lminlimit/maxlimit ]
      [ -Mmingray/maxgray ] [ -Szstart/zstop/zinc ] [ -V ]

DESCRIPTION

     grd2cpt reads a grdfile and writes  a  color  palette  (cpt)
     file  to  standard  output.  The  cpt  file has continuously
     changing hue (or gray level) and the mapping from data value
     to hue (or gray level) is through the data's cumulative dis-
     tribution function (CDF), so that the colors  are  histogram
     equalized.   Thus if the resulting cpt file is used with the
     grdfile and grdimage with a linear  projection,  the  colors
     will be uniformly distributed in area on the plot.  A simple
     linear transform from data to hue (or  gray  level)  is  not
     provided  here because that can be made with makecpt or awk.
     Let z be the data values in the grdfile.  Define CDF(Z) = (#
     of  z < Z) / (# of z in grdfile).  (NaNs are ignored).  Then
     hue(Z) = minhue  +  (maxhue  -  minhue)  *  CDF(Z),  or  the
     equivalent with mingray/maxgray.

     grdfile
          The 2-D binary grdfile used to derive the color palette
          table.

OPTIONS

     -C    Color option.  Set hues to use at lowest (minhue)  and
          highest  (maxhue) end of cpt range.  [Default is 300/0,
          i.e. magenta/red].

     -L    Limit range of cptfile to minlimit/maxlimit, and don't
          count   data  outside  range  when  estimating  CDF(Z).
          [Default uses min and max of data.]

     -M    Monochrome option.  Set gray levels (on a  scale  from
          0=black  to  1=white)  to  use  at lowest (mingray) and
          highest (maxgray) end of cpt range.  [Default is  color
          output.]

     -S    Set steps in cpt file.  Calculate entries  in  cptfile
          from  zstart  to  zstop  in  steps of (zinc).  [Default
          chooses arbitrary values by a crazy scheme.]

     -V    Verbose operation.  This will write  CDF(Z)  estimates
          to stderr.  [Default is silent.]

EXAMPLES

     Sometimes you don't want to make a cpt file (yet) but  would
     find it helpful to know that 90% of your data lie between z1
     and z2, something you cannot learn from grdinfo. So you  can
     do  this  to  see  some  points  on the CDF(Z) curve (use -V
     option to see more):

     grd2cpt mydata.grd -V > /dev/null

     To make a cpt file with entries from 0 to 200  in  steps  of
     20,  and ignore data below zero in computing CDF(Z), and use
     hues from dark purple (260) to orange (30), try

     grd2cpt  mydata.grd  -  C260/30   -L0/10000   -S0/200/20   >
     mydata.cpt

SEE ALSO

     gmtdefaults, gmt, grdhisteq, grdinfo, makecpt

REFERENCES

     Wessel, P., and W. H. F. Smith, 1995,  The  Generic  Mapping
     Tools  (GMT)  version  3.0  Technical  Reference & Cookbook,
     SOEST/NOAA.
     Wessel, P., and W. H. F. Smith, 1995,  New  Version  of  the
     Generic Mapping Tools Released, EOS Trans. AGU, 76, 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, p. 441.