grd2cpt - Read a grdfile and make a color palette file
grd2cpt grdfile [ -Ccptmaster ] [ -Enlevels ] [ -I ] [ -Lmin-
limit/maxlimit ] [ -N ] [ -Q[i|o] ] [ -Rw/e/s/n ] [ -Szstart/zstop/zinc
] [ -V ] [ -Z ]
grd2cpt reads a grdfile and writes a color palette (cpt) file to stan-
dard 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 uni-
formly 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
-E Create a linear color table by dividing the grid z-range into
nlevels equidistant slices.
-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.]
-N Do Not write out the background, foreground, and NaN-color
fields [Default will write the parameters COLOR_BACKGROUND,
COLOR_FOREGROUND, and COLOR_NAN in your .gmtdefaults4 file.
-Q Selects a logarithmic interpolation scheme [Default is linear].
-Qi expects input z-values to be log10(z), assigns colors, and
writes out z [Default]. -Qo takes log10(z) first, assigns col-
ors, and writes out z.
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of wesn. The
two shorthands -Rg -Rd stand for global domain (0/360 or
-180/+180 in longitude respectively, with -90/+90 in latitude).
For calendar time coordinates you may either give relative time
(relative to the selected TIME_EPOCH and in the selected
TIME_UNIT; append t to -JX|x), or absolute time of the form
[date]T[clock] (append T to -JX|x). At least one of date and
clock must be present; the T is always required. The date string
must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or
yyyy[-Www[-d]] (ISO week calendar), while the clock string must
be of the form hh:mm:ss[.xxx]. The use of delimiters and their
type and positions must be as indicated (however, input/output
and plotting formats are flexible).
-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.]
-Z Will create a continuous color palette. [Default is discontinu-
ous, i.e., constant color intervals]
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, run
grd2cpt mydata.grd -Crelief -L0/10000 -S0/200/20 > mydata.cpt
gmtdefaults(l), gmt(l), grdhisteq(l), grdinfo(l), makecpt(l)
GMT4.0 1 Oct 2004 GRD2CPT(l)
Man(1) output converted with