OK, a couple of things to consider: - What happens if you modify your scenario so that the island is not a forest, but a building instead? - OSM multipolygon machine-processing requires that you determine the actual roles of rings based on geometry, not on the membership roles. So in fact you then end up with a tree-hierarchy of rings. In that case the island is a ring inside the lake ring. And lake ring is inside of "big forest" ring.. - This then becomes important in terms of tagging. Wiki states that a ring becomes a hole (= inner ring) if it is not tagged or is tagged the same as its outer ring. One could then apply that same rule down the tree hierarchy.
One example: 3 ways 1. way: the outer way, tagged as forest. 2. way: inner ring, not tagged 3. way: inner ring, geometrically inside 2nd way, not tagged. How would you interpret the 2nd and the 3rd way? Igor On Wed, Oct 17, 2012 at 9:31 AM, Jochen Topf <[email protected]> wrote: > On Wed, Oct 17, 2012 at 08:15:55AM +0200, Igor Brejc wrote: > > Here's another complex multipolygon that I'm not sure the Wiki properly > > addresses: http://www.openstreetmap.org/browse/relation/11104 > > > > Situation: there are several holes within holes ( > > http://www.openstreetmap.org/browse/way/143604319 is one). The geometric > > situation is clear (and wiki covers that in > > > http://wiki.openstreetmap.org/wiki/Relation:multipolygon#Island_within_a_hole > > ). > > > > But the problem is in how to handle tagging in this case. The existing > wiki > > "rules" describe only the tagging of top outer rings, but they don't > > mention how to handle various cases/combinations of tagging holes within > > holes. > > Why would there a difference between "top outer rings" and "holes within > holes"? There is no difference between multiple outer rings next to each > other > and multiple outer rings that are inside each other in some way. There is > no > "hole in hole" hierarchy. > > You can think about it this way: Make a forest thats as big as the whole > planet. Make a small hole in it with a lake. Make an island in the lake > with a > forest. Now grow the lake and the island with it. Make it bigger and > bigger. > You'll end up again with a forest on an island in a lake in a forest. But > the > original huge forest is now on the small island. There is no difference > between > outer rings. > > Another way to think about multipolygons: A multipolygon divides all points > on the planet into two classes. This inside and those outside. Thats it. > There > isn't more than this. > > Jochen > -- > Jochen Topf [email protected] http://www.remote.org/jochen/ > +49-721-388298 >
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