grdsample(1) GMT grdsample(1)
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 FOR-
MAT 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, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude 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 coordi-
nates: 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 recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-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 a|)
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 a|)
Select verbosity level [c].
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more a|)
Select interpolation mode for grids.
-r (more a|)
Set pixel node registration [gridline].
-x[[-]n] (more a|)
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 explana-
tion 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 preci-
sion once GMT operates on the grid or writes out new grids. To limit
loss of precision when processing data you should always consider nor-
malizing 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 apackinga of grids, writing out floating point
data as 1- or 2-byte integers. (more a|)
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 lim-
its 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 registra-
tion while keeping the same region and grid interval:
gmt grdsample surface.nc -T -Gpixel.nc
SEE ALSO
gmt(1), grdedit(1), grdfft(1), grdfilter(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdsample(1)
gmt5 5.4.2 - Generated Thu Jun 29 08:08:49 CDT 2017