[mkgmap-dev] [PATCH v1] Multipolygon: Allow nested inner polygons

Fri Mar 5 20:45:43 GMT 2010

> I have found that some multipolygons use nested inner polygons.
> Example:
> * A forest (outer) with a lake (inner) that has an island (inner) which
> is not a forest. (example:
> http://www.openstreetmap.org/browse/relation/331402)
>
> By definition of the multipolygon relation
> (http://wiki.openstreetmap.org/wiki/Relation:multipolygon) one has to
> create two mps. But I see that it is a nice simplification lots of
> mappers are using.
>
> Attached patch handles this situation.
> The inner and outer roles are now used as follows:
> * the outmost polygon must have role=outer (or role=<empty>)
> * all outmost polygons with role=inner are not used (a warning is issued)
> * all inside polygons with role=inner use their own tagging
> * all inside polygons with role=outer use the mp tagging if available or
> their own tagging if the mp is not tagged
> * all inside polygons with role=<empty> are categorized automatically as
> outer/inner depending on the nest level
> * all inside polygons with role=outer without a direct ambient inner
> polygon are dropped with a warning message (outer in outer polygon)
>
> Most of the renderers do not support inner in inner polygons but I think
> it is a very reasonable extension of the multipolygon handling.
>
> This patch also improves the error messages. I haven't seen any more
> "are not processed due to an unknown reason" messages and I look forward
> that the community will also not be able to generate any more ... ;-)
>
> WanMil
>

There have been different opinions about this patch.

There is one controversal point:
Should mkgmap allow nested inner polygons?

 From my point of view: YES. Because it does not harm in any way. The 
nested inner polygons I analysed were all reasonable. Ok, they are not 
100% compliant to the specification but this is common to lots of others 
things in (anarchic) OSM.

At the moment I see two possible changes:
1. Issue a warning for any nested inner polygon but process it anyhow.
2. Reject nested inner polygons (and issue a warning). What a pity but 
they are not 100% compliant.

Please let me know what's the opinion of mkgmap community and committers 
because I want the improvements of error messages to be committed.

WanMil