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). Interpolation is bicubic [Default]  or  bilinear  (-Q)
       and  uses  boundary conditions (-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 registration, try

       grdsample surface.grd -T -Gpixel.grd

SEE ALSO

       gmt(1gmt), grdedit(1gmt), grdfft(1gmt), grdfilter(1gmt)

                                            1 Jan 2004                               GRDSAMPLE(l)