NAME

     grdtrack - Sampling of a 2-D grdfile along 1-D trackline  (a
     sequence of x,y points)


SYNOPSIS

     grdtrack xyfile -Ggrdfile [ -H[nrec] ] [ -Lflag ] [ -M[flag]
     ]  [ -Q ] [ -Rwest/east/south/north ] [ -S ] [ -V ] [ -: ] [
     -bi[s][n] ] [ -bo[s] ]


DESCRIPTION

     grdtrack reads a grdfile and a table (from file or  standard
     input)  with  (x,y) positions in the first two columns (more
     columns may be present). It interpolates  the  grid  at  the
     positions  in  the  table  and writes out the table with the
     interpolated values  added  as  a  new  column.   A  bicubic
     [Default]  or bilinear [-Q] interpolation is used, requiring
     boundary conditions at the limits of the region (see  -Lflag
     option).

     xyfile
          This is an ASCII [or binary, see  -b]  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   Boundary condition flag may be x or y or xy  indicating
          data  is periodic in range of x or y or both set by -R,
          or flag may be g indicating geographical conditions  (x
          and y are lon and lat).  [Default uses "natural" condi-
          tions (second partial  derivative  normal  to  edge  is
          zero).]

     -M   Multiple segment file.  Segment separator is  a  record
          beginning with flag.  [Default is '>'].

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

     -R   west, east, south, and  north  specify  the  Region  of
          interest.  To specify boundaries in degrees and minutes
          [and seconds], use the dd:mm[:ss] format.  Append r  if
          lower  left  and  upper right map coordinates are given
          instead of wesn.

     -S   Suppress the output of interpolated points that  result
          in NaN values.

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

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

     -bi  Selects binary input.  Append s  for  single  precision
          [Default  is  double].   Append  n  for  the  number of
          columns in the binary file(s).   [Default  is  2  input
          columns].

     -bo  Selects binary output.  Append s for  single  precision
          [Default is double].


HINTS

     If an interpolation point is not on  a  node  of  the  input
     grid, then a NaN at any node in the neighborhood surrounding
     the point will yield an interpolated NaN.  Bicubic  interpo-
     lation  [default]  yields  continuous  first derivatives but
     requires a neighborhood of 4 nodes  by  4  nodes.   Bilinear
     interpolation  [-Q]  uses  only  a  2 by 2 neighborhood, but
     yields  only  zeroth-order  continuity.   Use  bicubic  when
     smoothness  is important.  Use bilinear to minimize the pro-
     pagation of NaNs.


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


SEE ALSO

     gmt(l), surface(l), sample1d(l)