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<H2><A NAME="SECTION001431000000000000000">
6.3.1 Mercator projection (<B>-Jm</B> <B>-JM</B>)</A>
</H2>
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<P>
Probably the most famous of the various map projections,
the Mercator projection takes its name from the Flemish cartographer Gheert Cremer, better known as Gerardus Mercator, who presented it in 1569.
The projection is a cylindrical and conformal, with no distortion along the equator. A major
navigational feature of the projection is that a line of
constant azimuth is straight. Such a line is called a
rhumb line or <I>loxodrome</I><A NAME="21957"></A>. Thus, to sail from one
point to another one only had to connect the points with
a straight line, determine the azimuth of the line, and
keep this constant course for the entire voyage<A NAME="tex2html1292"
HREF="footnode.html#foot21958"><SUP>6.2</SUP></A><A NAME="21959"></A>. The
Mercator projection has been used extensively for world
maps in which the distortion towards the polar regions
grows rather large, thus incorrectly giving the impression
that, for example, Greenland is larger than South America.
In reality, the latter is about eight times the size of
Greenland. Also, the Former Soviet Union looks much bigger
than Africa or South America. One may wonder whether this
illusion has had any influence on U.S. foreign policy.
<P>
In the regular Mercator projection, the cylinder touches
the globe along the equator. Other orientations like
vertical and oblique give rise to the Transverse and
Oblique Mercator projections, respectively. We will
discuss these generalizations following the regular
Mercator projection.
<P>
The regular Mercator projection requires a minimum of
parameters. To use it in <A NAME="tex2html1294"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> programs you supply this
information (the first two items are optional and have defaults):
<P>
<UL>
<LI>Central meridian [Middle of your map].
</LI>
<LI>Standard parallel for true scale [Equator]. When supplied, central meridian must be supplied as well.
</LI>
<LI>Scale along the equator in inch/degree or
1:xxxxx (<B>-Jm</B>), or map width (<B>-JM</B>).
</LI>
</UL>
<P>
Our example presents a world map at a scale of 0.012
inch pr degree which will give a map 4.32 inch wide.
It was created with the command:
<P>
<BR CLEAR="ALL">
<HR>
<BR>
<PRE>gmtset BASEMAP_TYPE fancy
pscoast -R0/360/-70/70 -Jm1.2e-2i -Ba60f30/a30f15 -Dc -A5000 -Gblack -P > GMT_mercator.ps
</PRE>
<BR CLEAR="ALL">
<HR>
<P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_mercator"></A><A NAME="22940"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 6.15:</STRONG>
Simple Mercator map.</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><IMG
WIDTH="595" HEIGHT="332" ALIGN="BOTTOM" BORDER="0"
SRC="img136.png"
ALT="\includegraphics{scripts/GMT_mercator}"></DIV></TD></TR>
</TABLE>
</DIV>
<P>
While this example is centered on the Dateline, one can
easily choose another configuration with the <B>-R</B> option.
A map centered on Greenwich would specify the region with
<B>-R</B>-180/180/-70/70.
<P>
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<ADDRESS>
Paul Wessel
2010-07-14
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