[mkgmap-dev] Weird MultiPolygon intersection warning

Tue Feb 2 08:44:59 GMT 2010

Hi WanMil,

On Mon, Feb 01, 2010 at 10:59:47PM +0100, WanMil wrote:
> If you have intersecting lines the mp code cannot decide which polygon 
> is contained by another polygon. From my point of view this cannot be 
> fixed without major drawbacks.
> 
> Overlapping lines are not allowed for multipolygons but are sometimes 
> used in OSM data. One could debate if they should be allowed or not but 
> in the end they exist. To detect overlapping lines the mp code must be 
> changed. At the moment I don't have a good and performant algorithm to 
> do that, so I cannot say for sure if that's possible or not.

As there seem to be fewer than 10 such problematic multipolygons in Finland,
I think that it will be easier to split the problematic multipolygons by
hand.  For instance, the Japanese garden
http://www.openstreetmap.org/browse/relation/48542
does not need to be a multipolygon at all.

I split the building multipolygon to a multipolygon and a separate polygon
at the NE edge.  According to municipal aerial imagery and data on property
boundaries, the building mass consists of several separate buildings, so my
resolution of the problem should better reflect the reality.

	Marko