OVERPROP very efficiently calculates the areas of the intersection polygons resulting from overlaying two maps. Unlike other polygon overlay algorithms, it uses no explicit global topology. There is no tracing chains of edges. Polygons completely contained inside other polygons are not a problem, the correct answer is produced with the area of the outer polygon excluding the inner area. Polygons may also have multiple separate components. These ideas are also applicable to implementation on parallel machines.
OVERPROP is a result of an investigation into how little topology we actually need explicitly to store, and concurrently, by how much can special cases be reduced. Sometimes a more complete topology is needed, for instance to draw the output polygons, but for mass calculations it is not.