.TH GRDSAMPLE 1gmt "Feb 27 2014" "GMT 4.5.13 (SVN)" "Generic Mapping Tools"
.SH NAME
grdsample \- Resample a grid file onto a new grid
.SH SYNOPSIS
\fBgrdsample\fP \fIin_grdfile\fP \fB\-G\fP\fIout_grdfile\fP [ \fB\-F\fP ] [ \fB\-I\fP\fIxinc\fP[\fIunit\fP][\fB=\fP|\fB+\fP][/\fIyinc\fP[\fIunit\fP][\fB=\fP|\fB+\fP]] ] 
[ \fB\-L\fP\fIflag\fP ] [ \fB\-Q\fP[\fBb\fP|\fBc\fP|\fBl\fP|\fBn\fP][[\fB/\fP]\fIthreshold\fP] ]
[ \fB\-R\fP\fIwest\fP/\fIeast\fP/\fIsouth\fP/\fInorth\fP[\fBr\fP] ] [ \fB\-T\fP ] [ \fB\-V\fP ] [ \fB\-f\fP\fIcolinfo\fP ]
.SH DESCRIPTION
\fBgrdsample\fP reads a grid file and interpolates it to create a new grid file with either:  a different registration 
(\fB\-F\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 (\fB\-Q\fP) is used, requiring boundary conditions (\fB\-L\fP).  
Note that using \fB\-R\fP only is equivalent to \fBgrdcut\fP or \fBgrdedit \-S\fP.  \fBgrdsample\fP safely 
creates a fine mesh from a coarse one; the converse may suffer aliasing unless the data are filtered using
\fBgrdfft\fP or \fBgrdfilter\fP.
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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\-F\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.
.TP
\fIin_grdfile\fP
The name of the input 2-D binary grid file.
(See GRID FILE FORMAT below.)
.TP
\fB\-G\fP
The name of the output grid file.
(See GRID FILE FORMAT below.)
.SH OPTIONS
.TP
\fB\-F\fP
Force pixel node registration on output grid.  [Default is same registration as input grid].
.TP
\fB\-I\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 \fBc\fP to indicate arc seconds.  If one of the units \fBe\fP, \fBk\fP, \fBi\fP,
or \fBn\fP is appended instead, the increment is assumed to be given in meter, km, miles, or
nautical miles, respectively, and will be converted to the equivalent degrees longitude at
the middle latitude of the region (the conversion depends on \fBELLIPSOID\fP).  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 Appendix B for details.  Note: if \fB\-R\fP\fIgrdfile\fP is used then
grid spacing has already been initialized; use \fB\-I\fP to override the values.
.TP
\fB\-L\fP
Boundary condition \fIflag\fP may be \fIx\fP or \fIy\fP or \fIxy\fP indicating data is periodic in range of 
x or y or both set by \fB\-R\fP, or \fIflag\fP may be \fIg\fP indicating geographical conditions (x and y are 
lon and lat).  [Default uses "natural" conditions (second partial derivative normal to edge is zero) unless the grid is
automatically recognised as periodic.]  
.TP
\fB\-Q\fP
Quick mode, use bilinear rather than bicubic interpolation [Default].
Alternatively, select the interpolation mode by adding \fBb\fP for B-spline smoothing,
\fBc\fP for bicubic interpolation, \fBl\fP for bilinear interpolation or \fBn\fP for
nearest-neighbor value.
Optionally, append \fIthreshold\fP in the range [0,1].
This parameter controls how close to nodes with NaN values the
interpolation will go.  E.g., a \fIthreshold\fP of 0.5 will interpolate about half way from a non-NaN to a
NaN node, whereas 0.1 will go about 90% of the way, etc. [Default is 1, which means none of the (4 or 16) nearby
nodes may be NaN].  \fB\-Q0\fP will just return the value of the nearest node instead of interpolating.
This is the same as using \fB\-Qn\fP.
.TP
\fB\-R\fP
\fIxmin\fP, \fIxmax\fP, \fIymin\fP, and \fIymax\fP specify the Region of interest.  For geographic
regions, these limits correspond to \fIwest, east, south,\fP and \fInorth\fP and you may specify them
in decimal degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format.  Append \fBr\fP if lower left and upper right
map coordinates are given instead of w/e/s/n.  The two shorthands \fB\-Rg\fP and \fB\-Rd\fP stand for global domain
(0/360 and -180/+180 in longitude respectively, with -90/+90 in latitude).  Alternatively, specify the name
of an existing grid file and the \fB\-R\fP settings (and grid spacing, if applicable) are copied from the grid.
For calendar time coordinates you may either give (a) relative
time (relative to the selected \fBTIME_EPOCH\fP and in the selected \fBTIME_UNIT\fP; append \fBt\fP to
\fB\-JX\fP|\fBx\fP), or (b) absolute time of the form [\fIdate\fP]\fBT\fP[\fIclock\fP]
(append \fBT\fP to \fB\-JX\fP|\fBx\fP).  At least one of \fIdate\fP and \fIclock\fP
must be present; the \fBT\fP is always required.  The \fIdate\fP string must be of the form [-]yyyy[-mm[-dd]]
(Gregorian calendar) or yyyy[-Www[-d]] (ISO week calendar), while the \fIclock\fP string must be of
the form hh:mm:ss[.xxx].  The use of delimiters and their type and positions must be exactly as indicated
(however, input, output and plot formats are customizable; see \fBgmtdefaults\fP). 
.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.
.TP
\fB\-V\fP
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
.TP
\fB\-f\fP
Special formatting of input and/or output columns (time or geographical data).
Specify \fBi\fP or \fBo\fP to make this apply only to input or output [Default applies to both].
Give one or more columns (or column ranges) separated by commas.
Append \fBT\fP (absolute calendar time), \fBt\fP (relative time in chosen \fBTIME_UNIT\fP since \fBTIME_EPOCH\fP),
\fBx\fP (longitude), \fBy\fP (latitude), or \fBf\fP (floating point) to each column
or column range item.  Shorthand \fB\-f\fP[\fBi\fP|\fBo\fP]\fBg\fP means \fB\-f\fP[\fBi\fP|\fBo\fP]0\fBx\fP,1\fBy\fP
(geographic coordinates).
.SH 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.
.SH GRID FILE FORMATS
By default \fBGMT\fP writes out grid as single precision floats in a COARDS-complaint netCDF file format.
However, \fBGMT\fP 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 2- or 4-byte integers. To specify the precision, scale and offset, the user should add the suffix
\fB=\fP\fIid\fP[\fB/\fP\fIscale\fP\fB/\fP\fIoffset\fP[\fB/\fP\fInan\fP]], where \fIid\fP is a two-letter identifier of the grid type and precision, and \fIscale\fP and \fIoffset\fP are optional scale factor
and offset to be applied to all grid values, and \fInan\fP is the value used to indicate missing data.
When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names.
See \fBgrdreformat\fP(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information.
.P
When reading a netCDF file that contains multiple grids, \fBGMT\fP will read, by default, the first 2-dimensional grid that can find in that
file. To coax \fBGMT\fP into reading another multi-dimensional variable in the grid file, append \fB?\fP\fIvarname\fP to the file name, where
\fIvarname\fP is the name of the variable. Note that you may need to escape the special meaning of \fB?\fP 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 \fB?\fP\fIvarname\fP suffix can also be used for output grids to specify a variable name different from the default: "z".
See \fBgrdreformat\fP(1) and Section 4.18 of the GMT Technical Reference and Cookbook for more information,
particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
.SH 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 [\fB\-Q\fP]
uses only a 2 by 2 neighborhood, but yields only zeroth-order continuity.  Use bicubic when smoothness is 
important.  Use bilinear to minimize the propagation of NaNs.
.SH EXAMPLES
To resample the 5 x 5 minute grid in hawaii_5by5_topo.grd onto a 1 minute grid:
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\fBgrdsample\fP hawaii_5by5_topo.grd \fB\-I\fP 1\fBm \-G\fPhawaii_1by1_topo.grd
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To translate the gridline-registered file surface.grd to pixel registration while
keeping the same region and grid interval:
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\fBgrdsample\fP surface.grd \fB\-T\fP \fB\-G\fP pixel.grd
.SH "SEE ALSO"
.IR GMT (1),
.IR grdedit (1),
.IR grdfft (1),
.IR grdfilter (1)