[OSM-dev] Looking for guidance on interpreting multipolygon relations

[Jon Burgess]

On Sat, 2007-11-03 at 14:25 -0700, Karl Newman wrote:
> An area calculation would work to
> find the outermost ring, but after that it's undefined (the "holes"
> might be separate, not concentric). I would think that true concentric
> multipolygons would be rare, limited to things like a lake on an
> island in the middle of a larger lake. But in that case, does the
> island lake even need to be part of a multipolygon relation? Isn't the
> exclusion of the island from the larger lake sufficient? Because when
> the island lake is rendered it will be drawn over the unfilled "hole"
> (which is the island), right? What am I missing?

If we follow the normal GIS definition of a polygon then you can only
have an outer ring plus a number of inner rings which are
non-intersecting and not contained within one another. If you want
something inside one of the holes then it should be a different
multipolygon.

In practice you probably want a park with no holes and a lake with holes
for each of the islands. That matches the current Bettersea Park
http://www.openstreetmap.org/?lat=51.47919&lon=-0.15541&zoom=16&layers=B0F

The island appears with the park rendering because it is a hole in the
lake. It does not require any individual tagging.

A different example where someone has tried to tag the islands instead
of using holes is the park next to Schiphol Airport
http://www.openstreetmap.org/?lat=52.3074&lon=4.8336&zoom=14&layers=B0F 
This has several rendering problems both in the Mapnik and Osmarender
layer. 

	Jon