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<h3>The TRE and RGN files
</h3>
<p>These files are so inter dependant that it only makes sense to put
them into the same package.
</p>
<h4>The TRE file
</h4>
<p>This file describes the structure of the map. It contains the
zoom levels that exist in the map and how the map is divided up.
</p>
<p>There are in effect several copies of the map at different levels
of detail. Each level is divided up into several areas known as subdivisions.
These subdivisions form a tree, with more detailed ones being children of
the higher (less detailed) levels.
</p>
<h4>The RGN file
</h4>
<p>This file contains the detailed location and path of all the map
objects. Pointers from the TRE file indicate the start of the definition
of a map object and information from the TRE section needs to be used
to work out where the location of the map object is.
</p>
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