NAME

       grdsample - Resample a grd file onto a new grid


SYNOPSIS

       grdsample  in_grdfile  -Gout_grdfile  [  -F ] [ -Idx[m|c][/dy[m|c]] ] [
       -Lflag ] [ -Nnx/ny ] [ -Q ] [ -Rwest/east/south/north[r] ] [ -T ] [  -V
       ]


DESCRIPTION

       grdsample  reads  a grdfile and interpolates it to create a new grdfile
       with either: a different registration (-T); or, a new grid-spacing (-I)
       or number of nodes (-N), and perhaps also a new sub-region (-R). Inter-
       polation is bicubic [Default] or bilinear (-Q) and uses boundary condi-
       tions  (-L).   Note  that  using  (-R)  only is equivalent to grdcut or
       grdedit -S. grdsample safely creates a fine mesh from a coarse one; the
       converse  may suffer aliasing unless the data are filtered using grdfft
       or grdfilter.

       in_grdfile
              The name of the input 2-D binary grd file.

       -G     The name of the output grd file.


OPTIONS

       No space between the option flag and the associated arguments.

       -F     Force pixel registration. [Default is grid registration].

       -I     x_inc [and optionally y_inc] is the grid spacing.  Append  m  to
              indicate minutes or c to indicate seconds.

       -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).]

       -N     Specify number of columns nx and rows ny of new grid.

       -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.

       -T     Translate between grid and pixel registration while  keeping  -R
              and  -I  the  same;  if input is grid-registered, output will be
              pixel-registered and vice-versa. The input file  determines  -R,
              -I  and -N so no other options are necessary (except possibly -L
              or -Q).

       -V     Selects verbose mode, which will send progress reports to stderr
              [Default runs "silently"].


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  resample  the  5  x  5 minute grid in hawaii_5by5_topo.grd onto a 1
       minute grid, try

       grdsample hawaii_5by5_topo.grd -I1m -Ghawaii_1by1_topo.grd

       To translate the gridline-registered file surface.grd to  pixel  regis-
       tration, try

       grdsample surface.grd -T -Gpixel.grd


SEE ALSO

       gmt(l), grdedit(l), grdfft(l), grdfilter(l)



GMT3.4.6                          1 Jan 2005                      GRDSAMPLE(l)

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