[OSM-dev] Tagging of multipolygons with holes within holes
Igor Brejc
igor.brejc at gmail.com
Wed Oct 17 08:56:23 BST 2012
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 <jochen at remote.org> 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 jochen at remote.org http://www.remote.org/jochen/
> +49-721-388298
>
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