NAME

     grdtrack - Sampling of a 2-D grdfile along 1-D trackline

SYNOPSIS

     grdtrack xyfile -Ggrdfile [ -H ] [ -L ] [ -M[flag] ] [ -Q  ]
     [ -V ] [ -: ]

DESCRIPTION

     grdtrack reads a 2-D binary grdfile and an ASCII table (from
     file  or  standard  input) with (x,y) positions in the first
     two columns (more columns may be present). It  then  samples
     the  gridded data set at the positions in the ASCII file and
     outputs the ASCII file with the sampled values  added  as  a
     new column. A bicubic [Default] or bilinear interpolation is
     used to sample  the  grid.   Free  boundary  conditions  are
     assumed (curvature and 3rd normal derivative is zero).

     xyfile
          This is an ASCII file where the first  2  columns  hold
          the  (x,y) positions where the user wants to sample the
          2-D data set.

     -G    grdfile is a 2-D binary grd  file  with  the  function
          f(x,y).

OPTIONS

     No space between the option flag and  the  associated  argu-
     ments.

     -H    Input file(s) has Header record(s).  Number of  header
          records  can  be  changed  by editing your .gmtdefaults
          file.  If used, GMT default is 1 header record.

     -L    Indicates that the x column contains longitudes, which
          may  differ  from  the  region  specified in grdfile by
          [multiples of] 360 degrees [Default assumes no periodi-
          city].

     -M    Multiple segment file.  Segments are  separated  by  a
          record  whose  first  character  is  flag.  [Default is
          '>'].

     -Q    Quick mode.  Use bilinear rather than bicubic interpo-
          lation.

     -V    Selects verbose mode, which will send progress reports
          to stderr [Default runs "silently"].

       -   :    Toggles    between    (longitude,latitude)    and
          (latitude,longitude)    input/output.     [Default   is
          (longitude,latitude)]

EXAMPLES

     To sample the file hawaii_topo.grd along  the  SEASAT  track
     track_4.xyg  (An ASCII table containing longitude, latitude,
     and SEASAT-derived gravity, preceeded by one header record),
     try

     grdtrack track_4.xyg -Ghawaii_topo.grd -H > track_4.xygt

BUGS

     All the peripheral nodes needed to evaluate  the  interpola-
     tion  must  be  real in order to return a sampled value.  If
     any of these are NaN then NaN is returned.

SEE ALSO

     gmt, surface, sample1d

REFERENCES

     Wessel, P., and W. H. F. Smith, 1995,  The  Generic  Mapping
     Tools  (GMT)  version  3.0  Technical  Reference & Cookbook,
     SOEST/NOAA.
     Wessel, P., and W. H. F. Smith, 1995,  New  Version  of  the
     Generic Mapping Tools Released, EOS Trans. AGU, 76, p. 329.
     Wessel, P., and W. H. F. Smith, 1995,  New  Version  of  the
     Generic           Mapping           Tools          Released,
     http://www.agu.org/eos_elec/95154e.html, Copyright  1995  by
     the American Geophysical Union.
     Wessel, P., and W. H. F. Smith, 1991,  Free  Software  Helps
     Map and Display Data, EOS Trans. AGU, 72, p. 441.