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

[Karl Newman]

On 11/3/07, Frederik Ramm <frederik at remote.org> wrote:
> Hi,
>
> > > > In retrospect I guess this was short-sighted. The relation should
> > > > probably have included information about positive and negative areas,
> > > > or inner/outer rings. But it's a little late to fix now :)
>
> > > I don't see anything which stop us from adding in this additional
> > > information. It would take some time to update all the existing
> > > multipolygons but is something which can be automated.
>
> > We discussed this on IRC at one point, and I think we came to the same
> > conclusion.
>
> Yes. I volunteered to retrofit the "outer/inner" thingie to existing
> multipolys. It's a SMOP.
>
> Bye
> Frederik

Absolutely +1 here, too. Absent the "outer" role (and the fact that
relation members are unordered), renderers/exporters would have to
look at each way and determine if all the points for one way are
entirely contained in the other. 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?

Karl