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<H1><A NAME="SECTION00520000000000000000">
3.2 Gridding of arbitrarily spaced data</A>
</H1>
<P>
Except in the situation above when a gridded file is available, we must
convert our data to the right format readable by <A NAME="tex2html203"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> before we can
make contour plots and color-coded images. We distinguish between
two scenarios:
<P>
<OL>
<LI>The (<I>x</I>, <I>y</I>, <I>z</I>) data are available on a regular
lattice grid.
<P>
</LI>
<LI>The (<I>x</I>, <I>y</I>, <I>z</I>) data are distributed unevenly
in the plane.
<P>
</LI>
</OL>
<P>
The former situation may require a simple reformatting (using
<A NAME="tex2html204"
HREF="../xyz2grd.html"><I><B>xyz2grd</B></I></A><A NAME="2927"></A>), while the latter must be interpolated onto a
regular lattice; this process is known as gridding.
<A NAME="tex2html205"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> supports three different approaches to gridding; here, we
will briefly discuss the two most common techniques.
<P>
All <A NAME="tex2html206"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> gridding programs have in common the requirement that the
user must specify the grid domain and output filename:
<BR>
<P>
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT"><B>-R</B><I>xmin</I>/<I>xmax</I>/<I>ymin</I>/<I>ymax</I></TD>
<TD ALIGN="LEFT">The desired grid extent</TD>
</TR>
<TR><TD ALIGN="LEFT"><B>-I</B><I>xinc</I>[<B>m</B><IMG
WIDTH="8" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img1.gif"
ALT="$\vert$"><B>c</B>][/<I>yinc</I>[<B>m</B><IMG
WIDTH="8" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img1.gif"
ALT="$\vert$"><B>c</B>]]</TD>
<TD ALIGN="LEFT">The grid spacing (append <B>m</B> or <B>c</B> for minutes or seconds of arc)</TD>
</TR>
<TR><TD ALIGN="LEFT"><B>-G</B><I>gridfile</I></TD>
<TD ALIGN="LEFT">The output grid filename</TD>
</TR>
</TABLE>
<P>
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<LI><A NAME="tex2html1288"
HREF="node62.html">3.2.1 Nearest neighbor gridding</A>
<UL>
<LI><A NAME="tex2html1289"
HREF="node63.html">3.2.1.1 Exercises</A>
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<LI><A NAME="tex2html1290"
HREF="node64.html">3.2.2 Gridding with Splines in Tension</A>
<LI><A NAME="tex2html1291"
HREF="node65.html">3.2.3 Preprocessing</A>
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<ADDRESS>
Paul Wessel
2001-04-18
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