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<H1><A NAME="SECTION0014130000000000000000"></A>
<A NAME="11200"></A>
<BR>
6.13 Plotting of vector fields
</H1>
<P>
In many areas, such as fluid dynamics and elasticity,
it is desirable to plot vector fields of various kinds.
<A NAME="tex2html415"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A> provides a way to illustrate 2-component vector fields
using the <A NAME="tex2html416"
HREF="../grdvector.html"><I><B>grdvector</B></I></A><A NAME="11216"></A> utility. The two components of
the field (Cartesian or polar components) are stored in
separate grdfiles. In this example we use <A NAME="tex2html417"
HREF="../grdmath.html"><I><B>grdmath</B></I></A><A NAME="11225"></A>
to generate a surface <!-- MATH
$z(x, y) = x \cdot \exp(-x^2 -y^2)$
-->
<IMG
WIDTH="170" HEIGHT="35" ALIGN="MIDDLE" BORDER="0"
SRC="img114.gif"
ALT="$z(x, y) = x \cdot \exp(-x^2 -y^2)$">
and to calculate <IMG
WIDTH="22" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
SRC="img115.gif"
ALT="$\nabla z$"> by
returning the <I>x</I>- and <I>y</I>-derivatives separately.
We superpose the gradient vector field and the surface
<I>z</I> and also plot the components of the gradient
in separate windows:
<P>
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<P>
<BR CLEAR="ALL">
<HR>
<BR>
<PRE>
grdmath -R-2/2/-2/2 -I0.1 X Y R2 NEG EXP X MUL = z.grd
grdmath z.grd DDX = dzdx.grd
grdmath z.grd DDY = dzdy.grd
grdcontour dzdx.grd -JX3i -B1/1WSne -C0.1 -A0.5 -K -P -G2i/10 -S4 -T0.1i/0.03i \
-U"Example 13 in Cookbook" >! example_13.ps
grdcontour dzdy.grd -JX -B1/1WSne -C0.05 -A0.2 -O -K -G2i/10 -S4 -T0.1i/0.03i -X3.45i >> example_13.ps
grdcontour z.grd -JX -B1/1WSne -C0.05 -A0.1 -O -K -G2i/10 -S4 -T0.1i/0.03i -X-3.45i -Y3.45i \
>> example_13.ps
grdcontour z.grd -JX -B1/1WSne -C0.05 -O -K -G2i/10 -S4 -X3.45i >> example_13.ps
grdvector dzdx.grd dzdy.grd -I0.2 -JX -O -K -Q0.03i/0.1i/0.09in0.25i -G0 -S5i >> example_13.ps
echo "3.2 3.6 40 0 6 2 z(x,y) = x * exp(-x@+2@+-y@+2@+)" | pstext -R0/6/0/4.5 -Jx1i -O -X-3.45i \
>> example_13.ps
\rm -f z.grd dzdx.grd dzdy.grd .gmtcommands
</PRE>
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<P>
A <A NAME="tex2html418"
HREF="../pstext.html"><I><B>pstext</B></I></A><A NAME="11294"></A> call to place a header finishes the plot
(Figure <A HREF="node86.html#fig:GMT_example_13">6.13</A>.
<P>
<P></P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_example_13"></A><A NAME="11303"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 6.13:</STRONG>
Display of vector fields in <A NAME="tex2html412"
HREF="http://www.soest.hawaii.edu/gmt"><B>GMT</B></A></CAPTION>
<TR><TD><IMG
WIDTH="394" HEIGHT="448" BORDER="0"
SRC="img116.gif"
ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_example_13.eps}\end{figure}"></TD></TR>
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<ADDRESS>
Paul Wessel
2001-04-18
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