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<H2><A NAME="SECTION00521000000000000000">
3.2.1 Nearest neighbor gridding</A>
</H2>
<P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_nearneighbor"></A><A NAME="2351"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.1:</STRONG>
Search geometry for <A NAME="tex2html174"
HREF="../man/nearneighbor.html"><I><B>nearneighbor</B></I></A><A NAME="2357"></A>.</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><IMG
WIDTH="232" HEIGHT="232" ALIGN="BOTTOM" BORDER="0"
SRC="img34.png"
ALT="\includegraphics{scripts/GMT_nearneighbor}"></DIV></TD></TR>
</TABLE>
</DIV>
<P>
The <A NAME="tex2html180"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> program <A NAME="tex2html181"
HREF="../man/nearneighbor.html"><I><B>nearneighbor</B></I></A><A NAME="2365"></A> implements a simple
``nearest neighbor'' averaging operation. It is the preferred
way to grid data when the data density is high. <A NAME="tex2html182"
HREF="../man/nearneighbor.html"><I><B>nearneighbor</B></I></A><A NAME="2370"></A>
is a local procedure which means it will only consider the control
data that is close to the desired output grid node.
Only data points inside a specified search radius will
be used, and we may also impose the condition that each of the <I>n</I>
sectors must have at least one data point in order to assign the nodal
value. The nodal value is computed as a weighted average of the nearest
data point per sector inside the search radius, with each point weighted
according to its distance from the node as follows:
<P>
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
\bar{z} = \frac{\sum_{i=1}^{n} z_{i} w_{i}}{\sum_{i=1}^{n} w_{i}} \quad
w_{i} =
\left( 1 + \frac{9 r_{i}^{2}}{R^{2}} \right) ^{-1}
\end{displaymath}
-->
<IMG
WIDTH="224" HEIGHT="50" BORDER="0"
SRC="img35.png"
ALT="\begin{displaymath}
\bar{z} = \frac{\sum_{i=1}^{n} z_{i} w_{i}}{\sum_{i=1}^{n} w...
...d
w_{i} =
\left( 1 + \frac{9 r_{i}^{2}}{R^{2}} \right) ^{-1} \end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<P>
<A NAME="1158"></A>
<P>
The most important switches are listed in Table <A HREF="#tbl:nearneighbor">3.2</A>.
<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="1607"></A>
<TABLE>
<CAPTION><STRONG>Table 3.2:</STRONG>
Switches used with the <A NAME="tex2html177"
HREF="../man/nearneighbor.html"><I><B>nearneighbor</B></I></A><A NAME="2379"></A> program.</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><TABLE CELLPADDING=3 BORDER="1">
<TR><TD ALIGN="CENTER" COLSPAN=1><FONT SIZE="-1">
<I>Option</I></FONT></TD>
<TD ALIGN="CENTER" COLSPAN=1><FONT SIZE="-1"> <I>Purpose</I></FONT></TD>
</TR>
<TR><TH ALIGN="LEFT"><FONT SIZE="-1">
<B>-S</B></FONT><FONT SIZE="-1"><I>radius</I>[</FONT><FONT SIZE="-1"><B>k</B>] </FONT></TH>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Sets search radius. Append </FONT><FONT SIZE="-1"><B>k</B> to indicate radius in kilometers [Default is </FONT><FONT SIZE="-1"><I>x</I>-units] </FONT></TD>
</TR>
<TR><TH ALIGN="LEFT"><FONT SIZE="-1">
<B>-E</B></FONT><FONT SIZE="-1"><I>empty</I> </FONT></TH>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Assign this value to unconstrained nodes [Default is NaN] </FONT></TD>
</TR>
<TR><TH ALIGN="LEFT"><FONT SIZE="-1">
<B>-N</B></FONT><FONT SIZE="-1"><I>sectors</I> </FONT></TH>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Sector search, indicate number of sectors [Default is 4] </FONT></TD>
</TR>
<TR><TH ALIGN="LEFT"><FONT SIZE="-1">
<B>-W</B> </FONT></TH>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Read relative weights from the 4th column of input data </FONT></TD>
</TR>
</TABLE><FONT SIZE="-1">
<A NAME="tbl:nearneighbor"></A></FONT></DIV></TD></TR>
</TABLE>
</DIV><P></P>
<BR>
<P>
We will grid the data in the file <U>ship.xyz</U> which contains
ship observations of bathymetry off Baja California. You can find the
file in the sub-directory for example 15.
We desire to make a 5' by 5' grid. Running <A NAME="tex2html183"
HREF="../man/minmax.html"><I><B>minmax</B></I></A><A NAME="2385"></A> on the file yields
<P>
<PRE>
ship.xyz: N = 82970 <245/254.705> <20/29.99131> <-7708/-9>
</PRE>
<P>
so we choose the region accordingly:
<P>
<PRE>
nearneighbor -R245/255/20/30 -I5m -S40k -Gship.nc -V ship.xyz
</PRE>
<P>
We may get a view of the contour map using
<P>
<PRE>
grdcontour ship.nc -JM6i -P -B2 -C250 -A1000 | ghostview -
</PRE>
<P>
Since the grid <U>ship.nc</U> is stored in netCDF format that is supported by a host of other programs,
you can try one of those as well on the same grid.
<P>
<BR><HR>
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<LI><A NAME="tex2html1022"
HREF="node45.html">3.2.1.1 Exercises</A>
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<ADDRESS>
Paul Wessel
2010-01-14
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