grdtrack

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

SYNOPSIS

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

DESCRIPTION

       grdtrack reads a grdfile and a table (from file  or  stan­
       dard  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, requir­
       ing 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 .gmt­
              defaults  file.  If  used,  GMT default is 1 header
              record.

       -L     Boundary condition flag may be x or y or  xy  indi­
              cating  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  longi­
              tudes,  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 inter­
              polation.

       -R     west,  east, south, and north specify the Region of
              Append r if lower left and upper right map  coordi­
              nates 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"].

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

       -:     Toggles between (lon,lat) and (lat,lon)  input/out­
              put. [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  preci­
              sion [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 surround­
       ing  the  point  will  yield  an interpolated NaN. Bicubic
       interpolation [default] yields  continuous  first  deriva­
       tives  but  requires a neighborhood of 4 nodes by 4 nodes.
       Bilinear interpolation [-Q] uses only a 2 by  2  neighbor­
       hood, 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,  lati­
       tude,  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)