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


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


       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).


       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).]  If no flag is supplied, it is assumed that
              the x column contains longitudes, which  may  differ  from  the  region  in  -R  by
              [multiples of] 360 degrees [Default assumes no periodicity].

       -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

       -Z     Only write out the sampled z-values [Default writes all columns].

       -:     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].


       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(1gmt), surface(1gmt), sample1d(1gmt)

                                            1 Jan 2004                                GRDTRACK(l)