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<H2><A NAME="SECTION001335000000000000000">
5.3.5 Gnomonic Projection (<B>-Jf</B> <B>-JF</B>)</A>
</H2>
<P>
<A NAME="7415"></A>
<A NAME="7428"></A>
<A NAME="7429"></A>
<P>
The Gnomonic azimuthal projection is a perspective projection
from the center onto a plane tangent to the surface. Its origin
goes back to the old Greeks who used it for star maps almost 2500
years ago. The projection is neither equal-area nor conformal,
and much distortion is introducted near the edge of the hemisphere;
in fact, less than a hemisphere may be shown around a given center.
The directions from the center of projection are true. Great circles
project onto straight lines. Because it is mainly used for pictoral
views at a small scale, only the spherical form is necessary.
<P>
To specify the Gnomonic projection you must supply:
<P>
<DL>
<DT><STRONG><IMG
WIDTH="12" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img54.gif"
ALT="$\bullet$"></STRONG></DT>
<DD>Longitude and latitude of the projection center.
<P>
</DD>
<DT><STRONG><IMG
WIDTH="12" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img54.gif"
ALT="$\bullet$"></STRONG></DT>
<DD>The horizon, i.e., the number of degrees from
the center to the edge. This must be <IMG
WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img42.gif"
ALT="$<$"> 90<IMG
WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img61.gif"
ALT="$^{o}$">.
<P>
</DD>
<DT><STRONG><IMG
WIDTH="12" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img54.gif"
ALT="$\bullet$"></STRONG></DT>
<DD>Scale as 1:xxxxx or as radius/latitude where
radius is distance on map in inches from projection center to
a particular [possibly oblique] latitude (<B>-Jf</B>), or map
width (<B>-JF</B>).
</DD>
</DL>
<P>
Using a horizon of 60<IMG
WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img61.gif"
ALT="$^{o}$">, our example of this projection
centered on 120<IMG
WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img61.gif"
ALT="$^{o}$">W/35<IMG
WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img61.gif"
ALT="$^{o}$">N can therefore be generated by
the following <A NAME="tex2html323"
HREF="../pscoast.html"><I><B>pscoast</B></I></A><A NAME="7453"></A> command:
<P>
<P>
<BR>
<P>
<BR CLEAR="ALL">
<HR>
<BR>
<PRE>
#!/bin/sh
# $Id: GMT_gnomonic.sh,v 1.1 2001/03/21 04:10:21 pwessel Exp $
#
pscoast -R0/360/-90/90 -JF-120/35/60/4.5i -Bg15 -Dc -A10000 -G200 -W0.25p -P > GMT_gnomonic.ps
</PRE>
<P>
<BR>
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<HR>
<P>
<P></P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_gnomonic"></A><A NAME="7559"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 5.17:</STRONG>
Gnomonic azimuthal projection.</CAPTION>
<TR><TD><IMG
WIDTH="520" HEIGHT="520" BORDER="0"
SRC="img79.gif"
ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_gnomonic.eps}\end{figure}"></TD></TR>
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<ADDRESS>
Paul Wessel
2001-04-18
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