## 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