## 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 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
               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 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.

## 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, surface, sample1d