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
-I1
m -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)