Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
grdsample - Resample a grid onto a new lattice
SYNOPSIS
grdsample in_grdfile -Gout_grdfile [ -Iincrement ] [ -Rregion ] [ -T ] [ -V[level] ] [ -fflags ] [
-nflags ] [ -rreg ] [ -x[[-]n] ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
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.
REQUIRED ARGUMENTS
in_grdfile
The name of the input 2-D binary grid file. (See GRID FILE FORMAT below.)
-Gout_grdfile
The name of the output grid file. (See GRID FILE FORMAT below.)
OPTIONAL ARGUMENTS
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
x_inc [and optionally y_inc] is the grid spacing. Optionally, append a suffix modifier.
Geographical (degrees) coordinates: Append m to indicate arc minutes or s to indicate arc seconds.
If one of the units e, f, k, M, n or u is appended instead, the increment is assumed to be given
in meter, foot, km, Mile, nautical mile or US survey foot, respectively, and 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: If +e is appended then the corresponding max x
(east) or y (north) may be slightly adjusted to fit exactly the given increment [by default the
increment may be adjusted slightly to fit the given domain]. Finally, instead of giving an
increment you may specify the number of nodes desired by appending +n to the supplied integer
argument; the increment is then recalculated from the number of nodes and the domain. The
resulting increment value depends on whether you have selected a gridline-registered or
pixel-registered grid; see App-file-formats for details. Note: if -Rgrdfile is used then the grid
spacing has already been initialized; use -I to override the values.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest.
-T Translate between grid and pixel registration; if the input is grid-registered, the output will be
pixel-registered and vice-versa.
-V[level] (more …)
Select verbosity level [c].
-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 (more …)
Set pixel 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 exits (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 exits.
-? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exits.
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.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF file format.
However, GMT is able to produce grid files in many other commonly used grid file formats and also
facilitates so called “packing” of grids, writing out floating point data as 1- or 2-byte integers. (more
…)
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.
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 [-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.
EXAMPLES
To resample the 5 x 5 minute grid in hawaii_5by5_topo.nc onto a 1 minute grid:
gmt grdsample hawaii_5by5_topo.nc -I1m -Ghawaii_1by1_topo.nc
To translate the gridline-registered file surface.nc to pixel registration while keeping the same region
and grid interval:
gmt grdsample surface.nc -T -Gpixel.nc
SEE ALSO
gmt, grdedit, grdfft, grdfilter
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe