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  argu-
     ments.

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

     -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" condi-
          tions (second partial  derivative  normal  to  edge  is
          zero).]

     -Q   Quick mode, use bilinear rather than bicubic interpola-
          tion.

     -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  interpo-
     lation  [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 pro-
     pagation 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(l), grdedit(l), grdfft(l), grdfilter(l)