[OSM-dev] multipolygon relation and non-closed ways

[sly (sylvain letuffe)]

> I don't think that the "touching inner ring" issue was ever limited to
> "two or more consecutive points". Your page is the first time I read this.

As far as I understand the OGC validity of multipolygon, the answer is yes. 
Because in the first place, OGC standard does not consider touching inner ring 
by one isolated node to be invalid.
Therefore, the deviance to the standard as expressed in the sentence "with the 
notable exception of touching inner rings" is only true for "two or more 
consecutive points" (Like the example shown here : 
http://wiki.openstreetmap.org/wiki/File:Multipolygon_Illustration_8.png )


> Personally I don't consider the "8" shapes valid 

And :
http://wiki.openstreetmap.org/wiki/File:Multipolygon_Illustration_touching_on_one_point.svg
?

Your input is valuable ;-) But for a clarity's sake I think we have to answer 
those questions :
a) Do we need touching outer polygons in OSM
b) If yes, can it be allowed as single way in 8 shape or modeled as 2 ways 
touching at a single point
c) If no, the long standing sentence "the multipolygon relation can be used to 
build multipolygons in compliance with the OGC Simple Feature standard" should 
be amended with other exceptions added

My proposal for a) is yes we need them (I can show boundary examples on 
requests) and I don't care if b) is or isn't considered invalid as long as it 
is written on the wiki


> - and I don't think
> there's a difference between the one with a node in the middle and the
> one without. 

There isn't if you focus on the OGC MULTIPOLYGON geometry built from this 
relation (it will be the same in the end), but it might be a computing 
overhead that we might want to forbid in the first place : at mappers side.

> I don't consider the "inner ring touching outer ring in
> single point" case to be valid either.
I also have boundary examples on request to express it is needed, but we can 
either build those cases with that :
http://wiki.openstreetmap.org/wiki/File:Multipolygon_Illustration_self_touching_on_one_point.svg


-- 
sly (sylvain letuffe)