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<H1><A NAME="SECTION00510000000000000000">
3.1 Contouring gridded data sets</A>
</H1>
<P>
<A NAME="tex2html195"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> comes with several utilities that can create gridded data
sets; we will discuss two such programs later this session. First,
we will assume that we already have gridded data sets. In the
supplemental <A NAME="tex2html196"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> archive there is a program that serves as a data
extractor from several public domain global gridded data sets.
Among these data are ETOPO5, crustal ages, gravity and geoid,
and DEM for the continental US. Here, we will use <A NAME="tex2html197"
HREF="../grdraster.html"><I><B>grdraster</B></I></A><A NAME="2857"></A>
to extract a <A NAME="tex2html198"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A>-ready grid that we will next use for contouring:
<P>
<PRE>
grdraster 1 -R-66/-60/30/35 -Gbermuda.grd -V
</PRE>
<P>
We first use the <A NAME="tex2html199"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> program <A NAME="tex2html200"
HREF="../grdinfo.html"><I><B>grdinfo</B></I></A><A NAME="2872"></A> to see what's in this file:
<P>
<PRE>
grdinfo bermuda.grd
</PRE>
<P>
The file contains bathymetry for the Bermuda region and has depth
values from -5475 to -89 meters. We want to make a contour map of
this data; this is a job for <A NAME="tex2html201"
HREF="../grdcontour.html"><I><B>grdcontour</B></I></A><A NAME="2881"></A>. As with previous
plot commands we need to set up the map projection with <B>-J</B>.
Here, however, we do not have to specify the region since that is by
default assumed to be the extent of the grid file.
To generate any plot we will in addition need to supply information
about which contours to draw. Unfortunately, <A NAME="tex2html202"
HREF="../grdcontour.html"><I><B>grdcontour</B></I></A><A NAME="2891"></A>
is a complicated program with too many options. We put a positive
spin on this situation by touting its flexibility. Here are the most
useful options:
<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="1737"></A>
<TABLE>
<CAPTION><STRONG>Table 3.1:</STRONG>
The most useful options in <A NAME="tex2html192"
HREF="../grdcontour.html"><I><B>grdcontour</B></I></A><A NAME="2910"></A></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>-A</B></FONT><FONT SIZE="-1"><I>anot_int</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Annotation interval </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-C</B></FONT><FONT SIZE="-1"><I>cont_int</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Contour interval </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-G</B></FONT><FONT SIZE="-1"><I>gap</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Sets distance between contour annotations </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-L</B></FONT><FONT SIZE="-1"><I>low</I>/</FONT><FONT SIZE="-1"><I>high</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Only draw contours within the </FONT><FONT SIZE="-1"><I>low</I> to </FONT><FONT SIZE="-1"><I>high</I> range </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-N</B></FONT><FONT SIZE="-1"><I>unit</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Append </FONT><FONT SIZE="-1"><I>unit</I> to contour annotations </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-Q</B></FONT><FONT SIZE="-1"><I>cut</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Do not draw contours with fewer than </FONT><FONT SIZE="-1"><I>cut</I> points </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-S</B></FONT><FONT SIZE="-1"><I>smooth</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Resample contours every </FONT><FONT SIZE="-1"><I>x_inc</I>/</FONT><FONT SIZE="-1"><I>smooth</I> increment </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-T</B>[</FONT><FONT SIZE="-1"><B>+</B><IMG
WIDTH="8" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img1.gif"
ALT="$\vert$"></FONT><FONT SIZE="-1"><B>-</B>][</FONT><FONT SIZE="-1"><I>gap</I>/</FONT><FONT SIZE="-1"><I>length</I>][:</FONT><FONT SIZE="-1"><I>LH</I>] </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Draw tick-marks in downhill direction for innermost closed contours </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Add tick spacing and length, and characters to plot at the center of closed contours. </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-W</B>[</FONT><FONT SIZE="-1"><B>a</B><IMG
WIDTH="8" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img1.gif"
ALT="$\vert$"></FONT><FONT SIZE="-1"><B>c</B>]</FONT><FONT SIZE="-1"><I>pen</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Set contour and annotation pens </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-Z</B></FONT><FONT SIZE="-1"><I>factor</I>[/</FONT><FONT SIZE="-1"><I>offset</I>] </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> [Subtract </FONT><FONT SIZE="-1"><I>offset</I>] and multiply data by </FONT><FONT SIZE="-1"><I>factor</I> prior to processing </FONT></TD>
</TR>
</TABLE><FONT SIZE="-1">
<A NAME="tbl:grdcontour"></A></FONT></DIV></TD></TR>
</TABLE>
</DIV><P></P>
<BR>
<P>
We will first make a plain contour map using 1 km as annotation
interval and 250 m as contour interval. We choose a 7-inch-wide
Mercator plot and annotate the borders every 2<IMG
WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img25.gif"
ALT="$^{o}$">:
<P>
<PRE>
grdcontour bermuda.grd -JM7i -C250 -A1000 -P -B2 | ghostview -
</PRE>
<P>
<BR><HR>
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<ADDRESS>
Paul Wessel
2001-04-18
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