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

[sly (sylvain letuffe)]

hi,

> Perhaps a useless idea from my side. I would also like to hire you for an
> hour or two for quick reading some other OGC specifications for me.

I'm far from an expert for that ;-)
Perhaps you mean that the OSM wiki page I've created should be made more 
explicit for developers (or another page more technically oriented) about 
this OGC simple feature standard ?
 
> I tried ST_MakeValid and it seems to be able to convert also the two
> touching inner ring case into a valid simple feature
(...)
> "GEOMETRYCOLLECTION(POLYGON((-139 420,71 418,59 273,-156 272,-139
> 420),(-46 312,-5 314,-4 371,-46 370,-89 370,-92 313,-46
> 312)),LINESTRING(-46 312,-46 370))"

The fact that you end with a GEOMETRYCOLLECTION (with a LINESTRING) and not a 
MULTIPOLYGON after ST_MakeValid express the fact that the touching inner case 
isn't valid for OGC. But it has always been accepted as the OSM exception to 
the standard, because it would be more complex for mappers to do it in 
compliance with OGC standard.

> Perhaps examples B and F could also contain at least two ways? If there is
> only one way, why to make a multipolygon relation at all? 

You are right, both B and F (and 3 and 5) could and should be tagged as a 
simple closed way without relation at all. But my "polygon(s) validity" page 
could also be extended to non-relation polygons as well.
3 and 5, even if tagged without relation should (that's my opinion) still be 
considered invalid geometries.

-- 
sly
qui suis-je : http://sly.letuffe.org
email perso : sylvain chez letuffe un point org