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.TH "GRDSAMPLE" "1gmt" "Nov 05, 2016" "5.3.1" "GMT"
.SH NAME
grdsample \- Resample a grid onto a new lattice
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.SH SYNOPSIS
.sp
\fBgrdsample\fP \fIin_grdfile\fP  \fB\-G\fP\fIout_grdfile\fP
[  \fB\-I\fP\fIincrement\fP ]
[  \fB\-R\fP\fIregion\fP ]
[  \fB\-T\fP ]
[  \fB\-V\fP[\fIlevel\fP] ]
[ \fB\-f\fPflags ]
[ \fB\-n\fPflags ]
[ \fB\-r\fPreg ]
[ \fB\-x\fP[[\-]\fIn\fP] ]
.sp
\fBNote:\fP No space is allowed between the option flag and the associated arguments.
.SH DESCRIPTION
.sp
\fBgrdsample\fP reads a grid file and interpolates it to create a new grid
file with either: a different registration (\fB\-r\fP or \fB\-T\fP); or, a new
grid\-spacing or number of nodes (\fB\-I\fP), and perhaps also a new
sub\-region (\fB\-R\fP). A bicubic [Default], bilinear, B\-spline or
nearest\-neighbor interpolation is used; see \fB\-n\fP for settings. Note
that using \fB\-R\fP only is equivalent to grdcut or grdedit \fB\-S\fP\&.
\fBgrdsample\fP safely creates a fine mesh from a coarse one; the converse
may suffer aliasing unless the data are filtered using grdfft or grdfilter\&.
.sp
When \fB\-R\fP is omitted, the output grid will cover the same region as
the input grid. When \fB\-I\fP is omitted, the grid spacing of the output
grid will be the same as the input grid. Either \fB\-r\fP or \fB\-T\fP can be
used to change the grid registration. When omitted, the output grid will
have the same registration as the input grid.
.SH REQUIRED ARGUMENTS
.INDENT 0.0
.TP
.B \fIin_grdfile\fP
The name of the input 2\-D binary grid file. (See GRID FILE FORMAT below.)
.UNINDENT
.INDENT 0.0
.TP
\fB\-G\fP\fIout_grdfile\fP
The name of the output grid file. (See GRID FILE FORMAT below.)
.UNINDENT
.SH OPTIONAL ARGUMENTS
.INDENT 0.0
.TP
\fB\-I\fP\fIxinc\fP[\fIunit\fP][\fB=\fP|\fB+\fP][/\fIyinc\fP[\fIunit\fP][\fB=\fP|\fB+\fP]]
\fIx_inc\fP [and optionally \fIy_inc\fP] is the grid spacing. Optionally,
append a suffix modifier. \fBGeographical (degrees) coordinates\fP: Append
\fBm\fP to indicate arc minutes or \fBs\fP to indicate arc seconds. If one
of the units \fBe\fP, \fBf\fP, \fBk\fP, \fBM\fP, \fBn\fP or \fBu\fP 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 \fIy_inc\fP is given
but set to 0 it will be reset equal to \fIx_inc\fP; otherwise it will be
converted to degrees latitude. \fBAll coordinates\fP: If \fB=\fP is appended
then the corresponding max \fIx\fP (\fIeast\fP) or \fIy\fP (\fInorth\fP) 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 \fInumber of nodes\fP desired by
appending \fB+\fP 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 \fB\-R\fP\fIgrdfile\fP is used then the grid spacing has
already been initialized; use \fB\-I\fP to override the values.
.UNINDENT
.INDENT 0.0
.TP
\fB\-R\fP[\fIunit\fP]\fIxmin\fP/\fIxmax\fP/\fIymin\fP/\fIymax\fP[\fBr\fP] (more ...)
Specify the region of interest.  
.UNINDENT
.INDENT 0.0
.TP
\fB\-T\fP
Translate between grid and pixel registration; if the input is
grid\-registered, the output will be pixel\-registered and vice\-versa.
.UNINDENT
.INDENT 0.0
.TP
\fB\-V\fP[\fIlevel\fP] (more ...)
Select verbosity level [c].  
.UNINDENT
.INDENT 0.0
.TP
\fB\-f\fP[\fBi\fP|\fBo\fP]\fIcolinfo\fP (more ...)
Specify data types of input and/or output columns.  
.UNINDENT
.INDENT 0.0
.TP
\fB\-n\fP[\fBb\fP|\fBc\fP|\fBl\fP|\fBn\fP][\fB+a\fP][\fB+b\fP\fIBC\fP][\fB+c\fP][\fB+t\fP\fIthreshold\fP] (more ...)
Select interpolation mode for grids.
.UNINDENT
.INDENT 0.0
.TP
\fB\-r\fP (more ...)
Set pixel node registration [gridline].  
.UNINDENT
.INDENT 0.0
.TP
\fB\-x\fP[[\-]\fIn\fP] (more ...)
Limit number of cores used in multi\-threaded algorithms (OpenMP required).
.UNINDENT
.INDENT 0.0
.TP
\fB\-^\fP or just \fB\-\fP
Print a short message about the syntax of the command, then exits (NOTE: on Windows use just \fB\-\fP).
.TP
\fB\-+\fP or just \fB+\fP
Print an extensive usage (help) message, including the explanation of
any module\-specific option (but not the GMT common options), then exits.
.TP
\fB\-?\fP or no arguments
Print a complete usage (help) message, including the explanation of options, then exits.
.UNINDENT
.SH GRID VALUES PRECISION
.sp
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.
.SH GRID FILE FORMATS
.sp
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 ...)
.SH CONSEQUENCES OF GRID RESAMPLING
.sp
Resample or sampling of grids will use various algorithms (see \fB\-n\fP) 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 \fB+c\fP to
your \fB\-n\fP option.
.SH HINTS
.sp
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 [\fB\-n\fP] 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.
.SH EXAMPLES
.sp
To resample the 5 x 5 minute grid in hawaii_5by5_topo.nc onto a 1 minute grid:
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.sp
.nf
.ft C
gmt grdsample hawaii_5by5_topo.nc \-I1m \-Ghawaii_1by1_topo.nc
.ft P
.fi
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.UNINDENT
.UNINDENT
.sp
To translate the gridline\-registered file surface.nc to pixel
registration while keeping the same region and grid interval:
.INDENT 0.0
.INDENT 3.5
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
gmt grdsample surface.nc \-T \-Gpixel.nc
.ft P
.fi
.UNINDENT
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.SH SEE ALSO
.sp
gmt, grdedit,
grdfft, grdfilter
.SH COPYRIGHT
2016, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
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