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<h1 align="center">GRDSAMPLE</h1>
<a href="#NAME">NAME</a><br>
<a href="#SYNOPSIS">SYNOPSIS</a><br>
<a href="#DESCRIPTION">DESCRIPTION</a><br>
<a href="#OPTIONS">OPTIONS</a><br>
<a href="#GRID VALUES PRECISION">GRID VALUES PRECISION</a><br>
<a href="#GRID FILE FORMATS">GRID FILE FORMATS</a><br>
<a href="#HINTS">HINTS</a><br>
<a href="#EXAMPLES">EXAMPLES</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<hr>
<h2>NAME
<a name="NAME"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">grdsample
− Resample a grid file onto a new grid</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdsample</b>
<i>in_grdfile</i> <b>−G</b><i>out_grdfile</i> [
<b>−F</b> ] [
<b>−I</b><i>xinc</i>[<i>unit</i>][<b>=</b>|<b>+</b>][/<i>yinc</i>[<i>unit</i>][<b>=</b>|<b>+</b>]]
] [ <b>−L</b><i>flag</i> ] [
<b>−Q</b>[<b>b</b>|<b>c</b>|<b>l</b>|<b>n</b>][[<b>/</b>]<i>threshold</i>]
] [
<b>−R</b><i>west</i>/<i>east</i>/<i>south</i>/<i>north</i>[<b>r</b>]
] [ <b>−T</b> ] [ <b>−V</b> ] [
<b>−f</b><i>colinfo</i> ]</p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdsample</b>
reads a grid file and interpolates it to create a new grid
file with either: a different registration (<b>−F</b>
or <b>−T</b>); or, a new grid-spacing or number of
nodes (<b>−I</b>), and perhaps also a new sub-region
(<b>−R</b>). A bicubic [Default], bilinear, B-spline
or nearest-neighbor interpolation (<b>−Q</b>) is used,
requiring boundary conditions (<b>−L</b>). Note that
using <b>−R</b> only is equivalent to <b><A HREF="grdcut.html">grdcut</A></b> or
<b>grdedit −S</b>. <b>grdsample</b> safely creates a
fine mesh from a coarse one; the converse may suffer
aliasing unless the data are filtered using <b><A HREF="grdfft.html">grdfft</A></b> or
<b><A HREF="grdfilter.html">grdfilter</A></b>.</p>
<p style="margin-left:11%; margin-top: 1em">When
<b>−R</b> is omitted, the output grid will cover the
same region as the input grid. When <b>−I</b> is
omitted, the grid spacing of the output grid will be the
same as the input grid. Either <b>−F</b> or
<b>−T</b> can be used to change the grid registration.
When omitted, the output grid will have the same
registration as the input grid. <i><br>
in_grdfile</i></p>
<p style="margin-left:22%;">The name of the input 2-D
binary grid file. (See GRID FILE FORMAT below.)</p>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−G</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>The name of the output grid file. (See GRID FILE FORMAT
below.)</p> </td></tr>
</table>
<h2>OPTIONS
<a name="OPTIONS"></a>
</h2>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em"><b>−F</b></p></td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em">Force pixel node registration on
output grid. [Default is same registration as input
grid].</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−I</b></p></td>
<td width="8%"></td>
<td width="78%">
<p><i>x_inc</i> [and optionally <i>y_inc</i>] is the grid
spacing. Optionally, append a suffix modifier.
<b>Geographical (degrees) coordinates</b>: Append <b>m</b>
to indicate arc minutes or <b>c</b> to indicate arc seconds.
If one of the units <b>e</b>, <b>k</b>, <b>i</b>, or
<b>n</b> 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
<b><A HREF="gmtdefaults.html#ELLIPSOID">ELLIPSOID</A></b>). If /<i>y_inc</i> is given but set to 0 it
will be reset equal to <i>x_inc</i>; otherwise it will be
converted to degrees latitude. <b>All coordinates</b>: If
<b>=</b> is appended then the corresponding max <i>x</i>
(<i>east</i>) or <i>y</i> (<i>north</i>) 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
<i>number of nodes</i> desired by appending <b>+</b> 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
<b>−R</b><i>grdfile</i> is used then grid spacing has
already been initialized; use <b>−I</b> to override
the values.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−L</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Boundary condition <i>flag</i> may be <i>x</i> or
<i>y</i> or <i>xy</i> indicating data is periodic in range
of x or y or both set by <b>−R</b>, or <i>flag</i> may
be <i>g</i> 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.]</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−Q</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Quick mode, use bilinear rather than bicubic
interpolation [Default]. Alternatively, select the
interpolation mode by adding <b>b</b> for B-spline
smoothing, <b>c</b> for bicubic interpolation, <b>l</b> for
bilinear interpolation or <b>n</b> for nearest-neighbor
value. Optionally, append <i>threshold</i> in the range
[0,1]. This parameter controls how close to nodes with NaN
values the interpolation will go. E.g., a <i>threshold</i>
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]. <b>−Q0</b> will just return the
value of the nearest node instead of interpolating. This is
the same as using <b>−Qn</b>.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−R</b></p></td>
<td width="8%"></td>
<td width="78%">
<p><i>xmin</i>, <i>xmax</i>, <i>ymin</i>, and <i>ymax</i>
specify the Region of interest. For geographic regions,
these limits correspond to <i>west, east, south,</i> and
<i>north</i> and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append <b>r</b> if
lower left and upper right map coordinates are given instead
of w/e/s/n. The two shorthands <b>−Rg</b> and
<b>−Rd</b> 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 <b>−R</b> 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 <b><A HREF="gmtdefaults.html#TIME_EPOCH">TIME_EPOCH</A></b> and in the
selected <b><A HREF="gmtdefaults.html#TIME_UNIT">TIME_UNIT</A></b>; append <b>t</b> to
<b>−JX</b>|<b>x</b>), or (b) absolute time of the form
[<i>date</i>]<b>T</b>[<i>clock</i>] (append <b>T</b> to
<b>−JX</b>|<b>x</b>). At least one of <i>date</i> and
<i>clock</i> must be present; the <b>T</b> is always
required. The <i>date</i> string must be of the form
[-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]]
(ISO week calendar), while the <i>clock</i> 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
<b><A HREF="gmtdefaults.html">gmtdefaults</A></b>).</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−T</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Translate between grid and pixel registration; if the
input is grid-registered, the output will be
pixel-registered and vice-versa.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−V</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Selects verbose mode, which will send progress reports
to stderr [Default runs "silently"].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−f</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Special formatting of input and/or output columns (time
or geographical data). Specify <b>i</b> or <b>o</b> 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 <b>T</b> (absolute calendar time),
<b>t</b> (relative time in chosen <b><A HREF="gmtdefaults.html#TIME_UNIT">TIME_UNIT</A></b> since
<b><A HREF="gmtdefaults.html#TIME_EPOCH">TIME_EPOCH</A></b>), <b>x</b> (longitude), <b>y</b>
(latitude), or <b>f</b> (floating point) to each column or
column range item. Shorthand
<b>−f</b>[<b>i</b>|<b>o</b>]<b>g</b> means
<b>−f</b>[<b>i</b>|<b>o</b>]0<b>x</b>,1<b>y</b>
(geographic coordinates).</p></td></tr>
</table>
<h2>GRID VALUES PRECISION
<a name="GRID VALUES PRECISION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">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.</p>
<h2>GRID FILE FORMATS
<a name="GRID FILE FORMATS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">By default
<b><A HREF="GMT.html">GMT</A></b> writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, <b><A HREF="GMT.html">GMT</A></b> 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
<b>=</b><i>id</i>[<b>/</b><i>scale</i><b>/</b><i>offset</i>[<b>/</b><i>nan</i>]],
where <i>id</i> is a two-letter identifier of the grid type
and precision, and <i>scale</i> and <i>offset</i> are
optional scale factor and offset to be applied to all grid
values, and <i>nan</i> 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 <b><A HREF="grdreformat.html">grdreformat</A></b>(1)
and Section 4.17 of the GMT Technical Reference and Cookbook
for more information.</p>
<p style="margin-left:11%; margin-top: 1em">When reading a
netCDF file that contains multiple grids, <b><A HREF="GMT.html">GMT</A></b> will
read, by default, the first 2-dimensional grid that can find
in that file. To coax <b><A HREF="GMT.html">GMT</A></b> into reading another
multi-dimensional variable in the grid file, append
<b>?</b><i>varname</i> to the file name, where
<i>varname</i> is the name of the variable. Note that you
may need to escape the special meaning of <b>?</b> 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 <b>?</b><i>varname</i> suffix can also be used
for output grids to specify a variable name different from
the default: "z". See <b><A HREF="grdreformat.html">grdreformat</A></b>(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.</p>
<h2>HINTS
<a name="HINTS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">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
[<b>−Q</b>] 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.</p>
<h2>EXAMPLES
<a name="EXAMPLES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">To resample the
5 x 5 minute grid in hawaii_5by5_topo.grd onto a 1 minute
grid:</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdsample</b>
hawaii_5by5_topo.grd <b>−I</b> 1<b>m
−G</b>hawaii_1by1_topo.grd</p>
<p style="margin-left:11%; margin-top: 1em">To translate
the gridline-registered file surface.grd to pixel
registration while keeping the same region and grid
interval:</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdsample</b>
surface.grd <b>−T −G</b> pixel.grd</p>
<h2>SEE ALSO
<a name="SEE ALSO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><i><A HREF="GMT.html">GMT</A></i>(1),
<i><A HREF="grdedit.html">grdedit</A></i>(1), <i><A HREF="grdfft.html">grdfft</A></i>(1), <i><A HREF="grdfilter.html">grdfilter</A></i>(1)</p>
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