[OSM-dev] Applying outer way attributes to multi-polygons

[Sebastian Klein]

Ben Supnik wrote:
> Hi Y'all,
> 
> Does anyone have a good set of heuristics for the cases where 
> OSM-reading software should treat outer-way attributes as applying to a 
> multipolygon?
> 
>  From what I can tell there are two possible cases I should look at:
> 
> - If the outer multipolygon has _no_ tags (other than type=multipolygon) 
> then I should probably go get some attributes from the outer ways, or 
> else the whole multipolygon is a no-op.
> 
> - If the outer multipolygon only has one way, then __maybe_- that way 
> might contain all of the attributes I care about?  This second one makes 
> me nervous because I could have a state park way where the boundary of 
> the park is a water way with an island in it...the island (inner way) 
> might be meant to cut a hole in some attributes (like water) but not 
> others (like park).

You can see it the other way around: If the multipolygon _has_ tags 
(other than type=multipolygon), then these tags fully describe the 
multipolygon.

If there are other tags on the single closed outer way (like 
highway=service or leisure=park), then this is an completely independent 
object. In the case of leisure=park it would be equivalent to a second 
multipolygon with these tags and only this single closed way as outer 
member.

The situation you describe (multipolygon: natural=water; outer way: 
leisure=park & natural=water), is a little ambiguous, though: In strict 
interpretation, there would be 2 water areas. This is probably not the 
intention, so I would consider natural=water on the outer way a legacy 
artefact and ignore it.


Sebastian