<sect2 id="calc-geodetic">
<title>Geodetic Coordinates module</title>
<indexterm><primary>Tools</primary>
<secondary>Astrocalculator</secondary>
<tertiary>Geodetic Coordinates module</tertiary>
</indexterm>

<screenshot>
<screeninfo>
The Geodetic Coordinates calculator module
</screeninfo>
<mediaobject>
  <imageobject>
    <imagedata fileref="calc-geodetic.png" format="PNG"/>
  </imageobject>
  <textobject>
    <phrase>Geodetic Coordinates</phrase>
  </textobject>
</mediaobject>
</screenshot>

<para>
The normal <link linkend="ai-geocoords">geographic coordinate 
system</link> assumes that the Earth is a perfect sphere.  This is 
nearly true, so for most purposes geographic coordinates are fine.
If very high precision is required, then we must take the true shape 
of the Earth into account.  The Earth is an ellipsoid; the distance
around the equator is about 0.3% longer than a <link 
linkend="ai-greatcircle">Great Circle</link> that passes through the 
poles.  The <firstterm>Geodetic Coordinate system</firstterm> takes
this ellipsoidal shape into account, and expresses the position
on the Earth's surface in Cartesian coordinates (X, Y, and Z).
</para>
<para>
To use the module, first select which coordinates you will use as 
input in the <guilabel>Input Selection</guilabel> section.  Then, fill 
in the input coordinates in either the <guilabel>Cartesian
Coordinates</guilabel> section or the <guilabel>Geographic 
Coordinates</guilabel> section.  When you press the 
<guibutton>Compute</guibutton> button, the corresponding 
coordinates will be filled in.
</para>
</sect2>