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
              cpt file].

       -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[][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)

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