NAME

      grd2cpt - Read a grdfile and make a color palette file

SYNOPSIS

      grd2cpt grdfile [ -Ccptmaster ] [ -I ] [ -Lminlimit/maxlimit ] [
      -Szstart/zstop/zinc ] [ -V ] [ -Z ]

DESCRIPTION

      grd2cpt reads a grdfile and writes a color palette (cpt) file to
      standard output.  The cpt file is based on an existing master cptfile
      of your choice, and the mapping from data value to colors is through
      the data's cumulative distribution 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.  Let z be the data values
      in the grdfile.  Define CDF(Z) = (# of z < Z) / (# of z in grdfile).
      (NaNs are ignored).  These z-values are then normalized to the master
      cptfile and colors are sampled at the desired intervals.

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

OPTIONS

      -C   Selects the master color table to use in the interpolation.
           Choose among the built-in tables (type grd2cpt to see the list)
           or give the name of an existing cptfile [Default gives a rainbow
           cpt file].

      -I   Reverses the sense of color progression in the master cptfile.

      -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.]

      -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.] .TP -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 the built-in
      master cptfile relief, try

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

SEE ALSO

      gmtdefaults, gmt, grdhisteq, grdinfo, makecpt