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).]

      -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, grdedit, grdfft, grdfilter