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[value] ] [ -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.
              optionally, append value in the 0  <  value  <=  1  range.  This
              parameter controls how close to nodes with NaN values the inter-
              polation will go. E.g., a valuefP of 0.5 will interpolate  about
              1/2-way  from a non-NaN to a NaN node, whereas 0.1 will go about
              90% of the way, etc. [Default is 1, which means none of the four
              nearby nodes may be NaN].

       -R     xmin,  xmax,  ymin, and ymax specify the Region of interest. For
              geographic regions,  these  limits  correspond  to  west,  east,
              south,  and north and you may specify them in decimal degrees or
              in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r  if  lower  left
              and  upper  right map coordinates are given instead of wesn. The
              two shorthands  -Rg  -Rd  stand  for  global  domain  (0/360  or
              -180/+180  in longitude respectively, with -90/+90 in latitude).
              For calendar time coordinates you may either give relative  time
              (relative  to  the  selected  TIME_EPOCH  and  in  the  selected
              TIME_UNIT; append t to -JX|x), or  absolute  time  of  the  form
              [date]T[clock]  (append  T  to  -JX|x). At least one of date and
              clock must be present; the T is always required. The date string
              must  be  of  the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or
              yyyy[-Www[-d]] (ISO week calendar), while the clock string  must
              be  of  the form hh:mm:ss[.xxx]. The use of delimiters and their
              type and positions must be as indicated  (however,  input/output
              and plotting formats are flexible).

       -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:

       grdsample hawaii_5by5_topo.grd -I1m -Ghawaii_1by1_topo.grd

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

       grdsample surface.grd -T -Gpixel.grd

SEE ALSO

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



GMT4.0                            1 Oct 2004                      GRDSAMPLE(l)