grd2cpt - Read a grdfile and make a color palette file
grd2cpt grdfile [ -Ccptmaster ] [ -I ] [ -Lminlimit/maxlimit ] [
-Szstart/zstop/zinc ] [ -V ] [ -Z ]
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.
The 2-D binary grdfile used to derive the color palette table.
-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
-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
-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.]
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
gmtdefaults, gmt, grdhisteq, grdinfo, makecpt
Man(1) output converted with