Resample a grid onto a new lattice
Note: No space is allowed between the option flag and the associated arguments.
grdsample reads a grid file and interpolates it to create a new grid file with either: a different registration (-r or -T); or, a new grid-spacing or number of nodes (-I), and perhaps also a new sub-region (-R). A bicubic [Default], bilinear, B-spline or nearest-neighbor interpolation is used; see -n for settings. 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.
When -R is omitted, the output grid will cover the same region as the input grid. When -I is omitted, the grid spacing of the output grid will be the same as the input grid. Either -r or -T can be used to change the grid registration. When omitted, the output grid will have the same registration as the input grid.
2-D gridded data file. Optionally, append =ID for reading a specific file format [Default is =nf] or ?varname for a specific netCDF variable [Default is the first 2-D grid found by GMT] (See full description). The following modifiers are supported:
+b - Select a band (for images only) [Default is 0].
+d - Divide data values by the given divisor [Default is 1].
+n - Replace data values matching invalid with NaN.
+o - Offset data values by the given offset [Default is 0].
+s - Scale data values by the given scale [Default is 1].
Note: Any offset is added after any scaling.
-Goutgrid[=ID][+ddivisor][+ninvalid] [+ooffset|a][+sscale|a] [:driver[dataType][+coptions]]
Give the name of the output grid file. Optionally, append =ID for writing a specific file format (See full description). The following modifiers are supported:
+d - Divide data values by given divisor [Default is 1].
+n - Replace data values matching invalid with a NaN.
+o - Offset data values by the given offset, or append a for automatic range offset to preserve precision for integer grids [Default is 0].
+s - Scale data values by the given scale, or append a for automatic scaling to preserve precision for integer grids [Default is 1].
Note: Any offset is added before any scaling. +sa also sets +oa (unless overridden). To write specific formats via GDAL, use = gd and supply driver (and optionally dataType) and/or one or more concatenated GDAL -co options using +c. See the “Writing grids and images” cookbook section for more details.
Set the grid spacing as x_inc [and optionally y_inc].
Geographical (degrees) coordinates: Optionally, append an increment unit. Choose among:
m to indicate arc minutes
s to indicate arc seconds
e (meter), f (foot), k (km), M (mile), n (nautical mile) or u (US survey foot), in which case the increment will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion depends on PROJ_ELLIPSOID). If y_inc is given but set to 0 it will be reset equal to x_inc; otherwise it will be converted to degrees latitude.
All coordinates: The following modifiers are supported:
+e to slightly adjust the max x (east) or y (north) to fit exactly the given increment if needed [Default is to slightly adjust the increment to fit the given domain].
+n to define the number of nodes rather than the increment, in which case the increment is recalculated from the number of nodes, the registration (see GMT File Formats), and the domain. Note: If -Rgrdfile is used then the grid spacing and the registration have already been initialized; use -I and -R to override these values.
Translate between grid and pixel registration; if the input is grid-registered, the output will be pixel-registered and vice-versa. This is a destructive grid change; see Switching registrations.
- -f[i|o]colinfo (more …)
Specify data types of input and/or output columns.
- -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more …)
Select interpolation mode for grids.
- -r[g|p] (more …)
Set node registration [gridline].
- -x[[-]n] (more …)
Limit number of cores used in multi-threaded algorithms (OpenMP required).
- -^ or just -
Print a short message about the syntax of the command, then exit (NOTE: on Windows just use -).
- -+ or just +
Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.
- -? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exit.
Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.
Grid Values Precision¶
Regardless of the precision of the input data, GMT programs that create grid files will internally hold the grids in 4-byte floating point arrays. This is done to conserve memory and furthermore most if not all real data can be stored using 4-byte floating point values. Data with higher precision (i.e., double precision values) will lose that precision once GMT operates on the grid or writes out new grids. To limit loss of precision when processing data you should always consider normalizing the data prior to processing.
Consequences of grid resampling¶
Resample or sampling of grids will use various algorithms (see -n) that may lead to possible distortions or unexpected results in the resampled values. One expected effect of resampling with splines is the tendency for the new resampled values to slightly exceed the global min/max limits of the original grid. If this is unacceptable, you can impose clipping of the resampled values values so they do not exceed the input min/max values by adding +c to your -n option.
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 [-n] uses only a 2 by 2 neighborhood, but yields only zero-order continuity. Use bicubic when smoothness is important. Use bilinear to minimize the propagation of NaNs.
As an alternative to bicubic spline, linear spline or nearest neighbor interpolation one can instead send the entire dataset through surface for re-gridding. This approach allows more control on aspects such as tension but it also leads to a solution that is not likely to have fully converged. The general approach would be something like
gmt grd2xyz old.grd | gmt surface -Rold.grd -Inewinc -Gnew.grd [other options]
For moderate data set one could also achieve an exact solution with greenspline, such as
gmt grd2xyz old.grd | gmt greenspline -Rold.grd -Inewinc -Gnew.grd [other options]
To resample a sub-region of the 5 x 5 minute remote grid earth_relief_05m onto a 1 minute grid:
gmt grdsample @earth_relief_05m -R0/20/0/20 -I1m -Gtopo_1m.nc
To translate the gridline-registered remote grid earth_relief_05m to pixel registration while keeping the same region and grid interval:
gmt grdsample @earth_relief_05m -T -Gpixel.nc
Marks, K. M., and W. H. F. Smith, 2007, Some remarks on resolving seamounts in satellite gravity, Geophys. Res. Lett., 34 (L03307), http://doi.org/10.1029/2006GL028857.