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PROGRAM:

NAME

grdsample - Resample a grid onto a new lattice

SYNOPSIS

grdsample in_grdfile out_grdfile [ increment ] [ region ] [ ] [ [level] ] [ -f<flags> ] [
-n<flags> ] [ -r ] [ -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][=|+][/yinc[unit][=|+]]
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 = 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 + 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.

-R[unit]xmin/xmax/ymin/ymax[r] (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
use just -).

-+ 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 options, then
exits.

--version
Print GMT version and exit.

--show-datadir
Print full path to GMT share directory and exit.

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. To specify the precision, scale and offset, the user should
add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid
type and precision, and scale and offset are optional scale factor and offset to be
applied to all grid values, and nan is the value used to indicate missing data. In case
the two characters id is not provided, as in =/scale than a id=nf is assumed. When
reading grids, the format is generally automatically recognized. If not, the same suffix
can be added to input grid file names. See grdconvert and Section grid-file-format of the
GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the
first 2-dimensional grid that can find in that file. To coax GMT into reading another
multi-dimensional variable in the grid file, append ?varname to the file name, where
varname is the name of the variable. Note that you may need to escape the special meaning
of ? in your shell program by putting a backslash in front of it, or by placing the
filename and suffix between quotes or double quotes. The ?varname suffix can also be used
for output grids to specify a variable name different from the default: "z". See
grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical
Reference and Cookbook for more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.

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

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