1. The first allows the user to add a <FileSource
name="foo">/home/bar/baz/</FileSource> to the beginning of the file
and then in any of the symbolisers you can say:
<FooSymboliser base="foo" name="bridge">
It it will refer to the file /home/bar/baz/bridge.
2. The second allows you to create Datasource templates at the top
level, which can be used later in the actual layers like so:
<Map>
<Datasource name="db">
<Paramaeter name="host">/tmp</Parameter>
</Datasource>
<Layer name="lay">
<Datasource base="db">
<Parameter name="table">points</Parameter>
</Datasource>
</Layer>
</Map>
And the host parameter will be used in the layer.
3. The third adds the "base" parameter to the raster and shape input
plugins. All it does is specify a path to prefix to the filename prior
to using it. Together with the above feature it allows things like:
<Map>
<Datasource name="shapes">
<Paramaeter name="base">/home/foo/shapes</Parameter>
</Datasource>
<Layer name="lay">
<Datasource base="shapes">
<Parameter name="file">places</Parameter>
</Datasource>
</Layer>
</Map>
And it will use the shapefile /home/foo/shapes/places
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