<Areal Feature> and <Feature Face>
An <Areal Feature>'s topology is represented by an associated <Feature Face>, which is usually a regular <Feature Face>, through a <Face Direction> link object.
The <Face Direction> is necessary because in topology involving 3D <Feature Face> instances, an <Areal Feature> may involve only one side of a <Feature Face>.
An <Areal Feature> may represent such environmental objects as
- the "footprint" of a building
- a lake
- a forest