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)