[mkgmap-dev] Merging adjacent multipolygons

Thu Feb 25 08:36:22 GMT 2010

On 25 Feb 2010, at 24:09 , Marko Mäkelä wrote:

> 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.
> 

Osmarender has problems with mp on the tile boundary of a rendered area. you can check the tile size at zoom level 12 to see if the lake crosses it.
It happens even for very small mp but it doesn't happen all time. 

> 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
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