grdsample (1gmt)

grdsample in_grdfile -Gout_grdfile [ -F ] [ -Idx[m|c][/dy[m|c]] ] [ -Lflag ] [ -Nnx/ny ] [ -Q ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V ]

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.

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"].

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.

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

gmt(1gmt), grdedit(1gmt), grdfft(1gmt), grdfilter(1gmt)