[OSM-dev] Multipolygons as members of a multipolygon

[Igor Brejc]

Hi,

While working on some multipolygon processing code, I noticed some cases
where multipolygons contained members which were themselves multipolygons
(with an inner role): http://www.openstreetmap.org/browse/relation/1450360

Since the Wiki docs don't mention such cases at all, I'm wondering about
what should be the appropriate interpretation of such cases. I can see that
OSM Mapnik rendering treats these as "top-level" multipolygons (
http://www.openstreetmap.org/browse/relation/1452648), probably ignoring
the multipolygon -> submultipolygon relationship (I assume).

Some questions:

   1. What is the purpose of the "inner" role in this case?
   2. Does the inner multipolygon represent a hole of the main polygon (as
   is the case with member ways)?
   3. Are there any "outer" submultipolygons out there? If not, does the
   inner role even make sense?

Thanks.
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