[mkgmap-dev] Merging adjacent multipolygons

Thu Feb 25 08:09:09 GMT 2010

Hi WanMil,

Have you looked at merging adjacent multipolygons?

There was some controversy on the Finnish forum <http://  
forum.openstreetmap.org/viewforum.php?id=15> but I was not alone with  
the opinion that lakes should be defined as multipolygons, no matter  
how complex they are, and that large map objects can or should be split  
to avoid problems with tools. Ways can have at most 2000 points, for  
instance, and rendering tools can merge them as needed.

Currently, Osmarender is flooding Kuopio, even though I did split Lake  
Saimaa into rather manageable chunks. I guess I should split it  
further, if the Osmarender bug cannot be fixed easily. But I do not  
dare to do it now given the strong opposition from one mapper.

I think I devised a reasonable means of identifying artificial cuts  
between multipolygons, which I also applied to the large multipolygons  
that I defined, in http://www.openstreetmap.org/browse/changeset/3926233

My artificial cuts are untagged lines that are shared between adjacent  
multipolygons in role=outer, such as this one between Heposelkä (a  
section of Lake Saimaa) and the rest of the lake:
http://www.openstreetmap.org/browse/way/50841967
Note that the browse window currently shows the main lake as inverted,  
even though the data appears to be OK in JOSM. Osmarender is having  
trouble with this small multipolygon too, rendering square-shaped  
blocks of water where the coastline should be.

Best regards,

	Marko