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<H2><A NAME="SECTION001333000000000000000">
5.3.3 Orthographic Projection (<B>-Jg </B> <B>-JG</B>)</A>
</H2>

<P>
<A NAME="7148"></A>
<A NAME="7161"></A>
<A NAME="7162"></A>

<P>
The orthographic azimuthal projection is a perspective projection
from infinite distance.  It is therefore often used to give the
appearance of a globe viewed from space.  As with Lambert's
equal-areal and the stereographic projection, only one hemisphere
can be viewed at any time.  The projection is neither equal-area
nor conformal, and much distortion is introducted near the edge
of the hemisphere.  The directions from the center of projection
are true.  The projection was known to the Egyptians and Greeks
more than 2,000 years ago.  Because it is mainly used for
pictoral views at a small scale, only the spherical form is necessary.

<P>
To specify the orthographic 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>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>-Jg</B>), or map
width (<B>-JG</B>).
</DD>
</DL>

<P>
Our example of a perspective view centered on 75<IMG
 WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img61.gif"
 ALT="$^{o}$">W/40<IMG
 WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img61.gif"
 ALT="$^{o}$">N
can therefore be generated by the following <A NAME="tex2html319"
  HREF="../pscoast.html"><I><B>pscoast</B></I></A><A NAME="7186"></A> command: 

<P>

<P>

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<PRE>
#!/bin/sh
#    $Id: GMT_orthographic.sh,v 1.1 2001/03/21 04:10:21 pwessel Exp $
#

pscoast -R0/360/-90/90 -JG-75/40/4.5i -B15g15 -Dc -A5000 -G0 -P &gt; GMT_orthographic.ps
</PRE>

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<DIV ALIGN="CENTER"><A NAME="fig:GMT_orthographic"></A><A NAME="7301"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 5.15:</STRONG>
Hemisphere map using the Orthographic projection.</CAPTION>
<TR><TD><IMG
 WIDTH="520" HEIGHT="520" BORDER="0"
 SRC="img77.gif"
 ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_orthographic.eps}\end{figure}"></TD></TR>
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<ADDRESS>
Paul Wessel
2001-04-18
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