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<H1><A NAME="SECTION0014180000000000000000"></A>
<A NAME="11898"></A>
<BR>
6.18 Volumes and Spatial Selections
</H1>

<P>
To demonstrate potential usage of the new programs
<A NAME="tex2html465"
  HREF="../grdvolume.html"><I><B>grdvolume</B></I></A><A NAME="11913"></A> and <A NAME="tex2html466"
  HREF="../gmtselect.html"><I><B>gmtselect</B></I></A><A NAME="11922"></A> we extract a subset
of the Sandwell &amp; Smith altimetric gravity field<A NAME="tex2html463"

HREF="footnode.html#foot11901"><SUP>6.3</SUP></A>
for the northern Pacific and decide to isolate all seamounts that
(1) exceed 50 mGal in amplitude and (2) are within 200 km
of the Pratt seamount.  We do this by dumping the 50 mGal
contours to disk, then making a simple <I>$AWK</I><A NAME="11930"></A> script
<U>center.awk</U> that returns the mean location of the points
making up each closed polygon, and then pass these locations
to <A NAME="tex2html467"
  HREF="../gmtselect.html"><I><B>gmtselect</B></I></A><A NAME="11937"></A> which retains only the points within 200
km of Pratt.  We then mask out all the data outside this
radius and use <A NAME="tex2html468"
  HREF="../grdvolume.html"><I><B>grdvolume</B></I></A><A NAME="11946"></A> to determine the combined
area and volumes of the chosen seamounts.

<P>

<P>

<BR>

<P>
<BR CLEAR="ALL">
<HR>
<BR>
<PRE>
gmtset ELLIPSOID Sphere
echo "-142.65    56.25" &gt;! pratt.d
makecpt -Crainbow -T-60/60/10 -Z &gt;! grav.cpt
grdgradient AK_gulf_grav.grd -Nt1 -A45 -GAK_gulf_grav_i.grd
grdimage AK_gulf_grav.grd -IAK_gulf_grav_i.grd -JM5.5i -Cgrav.cpt -B2f1 -P -K -X1.5i -Y5.85i &gt;! example_18.ps
pscoast -R-149/-135/52.5/58 -JM -O -K -Di -G160 -W0.25p &gt;&gt; example_18.ps
psscale -D2.75i/-0.4i/4i/0.15ih -Cgrav.cpt -B20f10/:mGal: -O -K &gt;&gt; example_18.ps
$AWK '{print $1, $2, 12, 0, 1, "LB", "Pratt"}' pratt.d | pstext -R -JM -O -K -D0.1i/0.1i \
   &gt;&gt; example_18.ps
$AWK '{print $1, $2, 0, 200, 200}' pratt.d | psxy -R -JM -O -K -SE -W0.25p &gt;&gt; example_18.ps
grdcontour AK_gulf_grav.grd -JM -C20 -B2f1WSEn -O -K -Y-4.85i -U/-1.25i/-0.75i/"Example 18 in Cookbook" \
   &gt;&gt; example_18.ps
grdcontour AK_gulf_grav.grd -JM -C10 -L49/51 -O -K -Dsm -Wc0.75p/0/255/0 &gt;&gt; example_18.ps
pscoast -R -JM -O -K -Di -G160 -W0.25p &gt;&gt; example_18.ps
$AWK '{print $1, $2, 0, 200, 200}' pratt.d | psxy -R -JM -O -K -SE -W0.25p &gt;&gt; example_18.ps
\rm -f sm_*[0-9].xyz    # Only consider closed contours
cat &lt;&lt; EOF &gt;! center.awk
BEGIN {
    x = 0
    y = 0
    n = 0
}
{
    x += \$1
    y += \$2
    n++
}
END {
    print x/n, y/n
}
EOF
\rm -f centers.d
foreach file (sm_*.xyz)
    $AWK -f center.awk $file &gt;&gt;! centers.d
end
gmtselect -R -JM -C200/pratt.d centers.d &gt;! $$
psxy $$ -R -JM -O -K -SC0.04i -G255/0/0 -W0.25p &gt;&gt; example_18.ps
psxy -R -JM -O -K -ST0.1i -G255/255/0 -W0.25p pratt.d &gt;&gt; example_18.ps
grdmath -R -I2m -F -142.65 56.25 GDIST = mask.grd
grdclip mask.grd -Sa200/NaN -Sb200/1 -Gmask.grd
grdmath AK_gulf_grav.grd mask.grd MUL = tmp.grd
set info = `grdvolume tmp.grd -C50 -Sk`
psxy -R -JM -A -O -K -L -W0.75p -G255 &lt;&lt; EOF &gt;&gt; example_18.ps
-148.5 52.75
-140.5    52.75
-140.5    53.75
-148.5    53.75
EOF
pstext -R -JM -O &lt;&lt; EOF &gt;&gt; example_18.ps
-148 53.08 14 0 1 LM Areas: $info[2] km@+2@+
-148 53.42 14 0 1 LM Volumes: $info[3] mGal\264km@+2@+
EOF
\rm -f $$ grav.cpt sm_*.xyz *_i.grd tmp.grd mask.grd pratt.d center* .gmt*
</PRE>

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<BR>

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<HR>
<P>
Our illustration is presented in Figure&nbsp;<A HREF="node91.html#fig:GMT_example_18">6.18</A>.

<P>

<P></P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_example_18"></A><A NAME="12001"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 6.18:</STRONG>
Volumes and geo-spatial selections</CAPTION>
<TR><TD><IMG
 WIDTH="365" HEIGHT="531" BORDER="0"
 SRC="img124.gif"
 ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_example_18.eps}\end{figure}"></TD></TR>
</TABLE>
</DIV><P></P>

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<ADDRESS>
Paul Wessel
2001-04-18
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