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+<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>