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<H2><A NAME="SECTION00523000000000000000">
3.2.3 Preprocessing</A>
</H2>
<P>
The <A NAME="tex2html223"
HREF="../surface.html"><I><B>surface</B></I></A><A NAME="3026"></A> program assumes that the data have been
preprocessed to eliminate aliasing, hence we must ensure that
this step is completed prior to gridding. <A NAME="tex2html224"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> comes with
three preprocessors, called <A NAME="tex2html225"
HREF="../blockmean.html"><I><B>blockmean</B></I></A><A NAME="3038"></A>, <A NAME="tex2html226"
HREF="../blockmedian.html"><I><B>blockmedian</B></I></A><A NAME="3047"></A>,
and <A NAME="tex2html227"
HREF="../blockmode.html"><I><B>blockmode</B></I></A><A NAME="3056"></A>. The first averages values inside the
grid-spacing boxes, the second returns median values, wile the
latter returns modal values. As a rule of thumb, we use means for
most smooth data (such as potential fields) and medians (or modes)
for rough, non-Gaussian data (such as topography). In addition
to the required <B>-R</B> and <B>-I</B> switches, these preprocessors
takes the same options:
<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="1348"></A>
<TABLE>
<CAPTION><STRONG>Table 3.4:</STRONG>
Some of the preprocessing options</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><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-N</B> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Choose pixel registration [Default is gridline] </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-W</B>[</FONT><FONT SIZE="-1"><B>i</B><IMG
WIDTH="8" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img1.gif"
ALT="$\vert$"></FONT><FONT SIZE="-1"><B>o</B>] </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Append </FONT><FONT SIZE="-1"><B>i</B> or </FONT><FONT SIZE="-1"><B>o</B> to read or write weights in the 4th column </FONT></TD>
</TR>
</TABLE><FONT SIZE="-1">
<A NAME="tbl:preprocess"></A></FONT></DIV></TD></TR>
</TABLE>
</DIV><P></P>
<BR>
<P>
With respect to our ship data we preprocess it using the median method:
<P>
<PRE>
blockmedian -R245/255/20/30 -I5m -V ship.xyz >! ship_5m.xyz
</PRE>
<P>
The output data can now be used with surface:
<P>
<PRE>
surface ship_5m.xyz -R245/255/20/30 -I5m -Gship.grd -V
</PRE>
<P>
If you rerun <A NAME="tex2html228"
HREF="../grdcontour.html"><I><B>grdcontour</B></I></A><A NAME="3069"></A> on the new grid file (try it!)
you will notice a big difference compared to the grid made by
<A NAME="tex2html229"
HREF="../nearneighbor.html"><I><B>nearneighbor</B></I></A><A NAME="3078"></A>: since <A NAME="tex2html230"
HREF="../surface.html"><I><B>surface</B></I></A><A NAME="3087"></A> is a global method
it will evaluate the solution at all nodes, even if there are no
data constraints. There are numerous options available to us at
this point:
<P>
<OL>
<LI>We can reset all nodes too far from a data constraint to the
NaN value.
<P>
</LI>
<LI>We can pour white paint over those regions where contours
are unreliable.
<P>
</LI>
<LI>We can plot the landmass which will cover most (but not all)
of the unconstrained areas.
<P>
</LI>
<LI>We can set up a clip path so that only the contours in the
constrained region will show.
<P>
</LI>
</OL>
<P>
Here we have only time to explore the latter approach. The <A NAME="tex2html231"
HREF="../psmask.html"><I><B>psmask</B></I></A><A NAME="3096"></A>
program can read the same preprocessed data and set up a contour mask
based on the data distribution. Once the clip path is activated we can
contour the final grid; we finally deactivate the clipping with a second
call to <A NAME="tex2html232"
HREF="../psmask.html"><I><B>psmask</B></I></A><A NAME="3105"></A>. Here's the recipe:
<P>
<PRE>
psmask -R245/255/20/30 -I5m ship_5m.xyz -JM6i -B2 -P -K -V >! map.ps
grdcontour ship.grd -JM -O -K -C250 -A1000 >> map.ps
psmask -C -O >> map.ps
</PRE>
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<ADDRESS>
Paul Wessel
2001-04-18
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