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<HTML>
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<TITLE>grdsample</TITLE>
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<H1>grdsample</H1>
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<PRE>
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grdsample - Resample a grd file onto a new grid
</PRE>
<H2>SYNOPSIS</H2><PRE>
<B>grdsample</B> <I>in</I><B>_</B><I>grdfile</I> <B>-G</B><I>out</I><B>_</B><I>grdfile</I> [ <B>-F</B> ] [
<B>-I</B><I>dx</I>[<B>m|c</B>][/<I>dy</I>[<B>m|c</B>]] ] [ <B>-L</B><I>flag</I> ] [ <B>-N</B><I>nx/ny</I> ] [ <B>-Q</B> ] [
<B>-R</B><I>west/east/south/north</I>[<B>r</B>] ] [ <B>-T</B> ] [ <B>-V</B> ]
</PRE>
<H2>DESCRIPTION</H2><PRE>
<B>grdsample</B> reads a grdfile and interpolates it to create a
new grdfile with either: a different registration (<B>-T</B>);
or, a new grid-spacing (<B>-I</B>) or number of nodes (<B>-N</B>), and
perhaps also a new sub-region (<B>-R</B>). Interpolation is bicu
bic [Default] or bilinear (<B>-Q</B>) and uses boundary condi
tions (<B>-L</B>). Note that using (<B>-R</B>) only is equivalent to
<B><A HREF="grdcut.html">grdcut</A></B> or <B><A HREF="grdedit.html">grdedit</A></B> <B>-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>.
<I>in</I><B>_</B><I>grdfile</I>
The name of the input 2-D binary grd file.
<B>-G</B> The name of the output grd file.
</PRE>
<H2>OPTIONS</H2><PRE>
No space between the option flag and the associated argu
ments.
<B>-F</B> Force pixel registration. [Default is grid regis
tration].
<B>-I</B> <I>x</I><B>_</B><I>inc</I> [and optionally <I>y</I><B>_</B><I>inc</I>] is the grid spacing.
Append <B>m</B> to indicate minutes or <B>c</B> to indicate sec
onds.
<B>-L</B> Boundary condition <I>flag</I> may be <I>x</I> or <I>y</I> or <I>xy</I> indi
cating 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).]
<B>-Q</B> Quick mode, use bilinear rather than bicubic inter
polation.
<B>-R</B> <I>west,</I> <I>east,</I> <I>south,</I> and <I>north</I> specify the Region of
interest. To specify boundaries in degrees and min
utes [and seconds], use the dd:mm[:ss] format.
Append <B>r</B> if lower left and upper right map coordi
nates are given instead of wesn.
<B>-T</B> Translate between grid and pixel registration while
keeping <B>-R</B> and <B>-I</B> the same; if input is grid-regis
no other options are necessary (except possibly <B>-L</B>
or <B>-Q</B>).
<B>-V</B> Selects verbose mode, which will send progress
reports to stderr [Default runs "silently"].
</PRE>
<H2>HINTS</H2><PRE>
If an interpolation point is not on a node of the input
grid, then a NaN at any node in the neighborhood surround
ing the point will yield an interpolated NaN. Bicubic
interpolation [default] yields continuous first deriva
tives but requires a neighborhood of 4 nodes by 4 nodes.
Bilinear interpolation [<B>-Q</B>] uses only a 2 by 2 neighbor
hood, but yields only zeroth-order continuity. Use bicubic
when smoothness is important. Use bilinear to minimize the
propagation of NaNs.
</PRE>
<H2>EXAMPLES</H2><PRE>
To resample the 5 x 5 minute grid in hawaii_5by5_topo.grd
onto a 1 minute grid, try
grdsample hawaii_5by5_topo.grd <B>-I</B>1<B>m</B> <B>-G</B>hawaii_1by1_topo.grd
To translate the gridline-registered file surface.grd to
pixel registration, try
grdsample surface.grd <B>-T</B> <B>-G</B>pixel.grd
</PRE>
<H2>SEE ALSO</H2><PRE>
<I>gmt</I>(l), <I><A HREF="grdedit.html">grdedit</A></I>(l), <I><A HREF="grdfft.html">grdfft</A></I>(l), <I><A HREF="grdfilter.html">grdfilter</A></I>(l)
</PRE>
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