[OSM-talk] multipolygon (lake) not rendering

Frederik Ramm frederik at remote.org
Mon Oct 26 09:24:42 GMT 2009


Rahkonen Jukka wrote:
> I meant "Do all the polygon parts get their correct names in
> rendering, 

There are multiple renderers and they generally don't agree on how to 
handle multipolygons.

I would assume that most can deal with a hole of a multipolygon having 
its own identity (e.g. hole in forest tagged as farm).

Most will probably not be able to derive the identity of something from 
the kind of polygon surrounding it - i.e. you will probably have to tag 
an island as such, and not count on the renderer knowing that everything 
that is a hole in a water polygon will automatically be an island.

There is also a problem with touching holes; if you have a forest and in 
the forest there is a farm and a lake next to each other, then some 
people draw a forest with two holes, whereas standard simple feature 
geometry does not allow touching holes (and anyway, as you correctly 
say, for export e.g. in shapefiles, extra polygons would have to be 
created for the holes).

A further problem arises when the hole boundary consists of multiple 
ways, something that is allowed for advanced multipolygons, but you 
would not be able to place the name of the island (or whatever) in this 


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