[OSM-talk] Using multipolygons as boundaries

Frederik Ramm frederik at remote.org
Sun Jan 4 21:08:36 GMT 2009


    those of us who use relations to tag administrative boundaries 
usually apply the schema described in


which suggests to use a type=boundary relation with "enclaves" and 
"exclaves". At the time of conception, that was ok because 
administrative areas (e.g. countries) often required border lines taht 
consisted of many ways and exclaves, something that plain multipolygons 
did not support.

Since we now have "advanced multipolygons" as described here:


(which, being true to their name, support any number of disjunct areas 
which may have zero or more holes each, and even islands in holes and so 
on), there is an equivalence between the two: each administrative area 
corresponds to exactly one multipolygon.

I am thus suggesting that we drop using the special "type=boundary" 
relation and instead use a simple "type=multipolygon" for administrative 
areas. Everything else would stay the same (boundary=administrative, 
admin_level=x, name=y, ...). Members would not carry the roles "exclave" 
and "enclave" (which seem to have been difficult to understand for 
some), but instead simply "outer" and "inner" just like with plain 

I have described the suggested change in detail here:


The main advantage of this is that any piece of software that works with 
our data would just have to understand multipolygons - wheter they are 
additionally tagged as representing a boundary, a forest, a lake or 
whatever - instead of having to carry a list of relation types that form 
one or the other kind of area.


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

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