NAME

       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][n] ]


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).]  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 "silently"].

       -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].

       -O     Selects Overlay plot mode [Default initializes a new  plot  sys-
              tem].


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



GMT3.4.6                          1 Jan 2005                       GRDTRACK(l)

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