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


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


      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

           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).


      No space between the option flag and the associated arguments.

      -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" conditions (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 interpolation.

      -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

      -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


      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 interpolation [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 propagation of NaNs.


      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


      gmt, surface, sample1d

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