[OSM-talk] area topology

Frederik Ramm frederik at remote.org
Tue May 13 14:36:46 BST 2008


Hi,

> Am I right in assuming that OSM ways always belong
> to one single area?

Yes and no.

Normally we don't map the borders between areas - we map tha areas  
themselves. So you would have one way that encloses the forest, and  
one way that encloses the adjacent farmland, and they would "meet"  
somewhere.

There are exceptions to this rule when it comes to very large  
entities like counties or countries; in these cases we tend to  
actually map the border line and tag it with something like "left:  
France, right: Germany", and there will be no single polygon named  
"France" or "Germany". Coastline is another exception.

> If this is so, do I have to duplicate ways along the
> common border? JOSM informs about double ways when it validates  
> data so
> I had the slight impression that they are not really wanted.

There is no 100% consensus on how to deal with these things but most  
people, including me, suggest to duplicate ways (not nodes) along the  
common border, i.e. you will have two ways sharing the same nodes.

The issue is especially contended when it comes to linear features  
straddling areas, like a road that forms the forest boundary for a  
bit. I would re-use the same nodes here, but there are people who say  
that this would indicate the forest stretching up to the road  
centreline which of course isn't true, and they would rather have the  
road and the forest boundary run in parallel and use their own nodes.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"







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