[OSM-dev] Tagging of multipolygons with holes within holes

[Peter Wendorff]

Am 17.10.2012 10:02, schrieb Igor Brejc:
>
>
> On Wed, Oct 17, 2012 at 9:40 AM, Frederik Ramm <frederik at remote.org 
> <mailto:frederik at remote.org>> wrote:
>
>
>     The area of the "hole within the hole" does not require special
>     tagging, as it is covered by the multipolygon itself. If you have
>     a forest with a hole in a hole, then that hole in a hole is forest
>     as well.
>
>
> True, but you need to determine that geometrically (see my previous 
> answer) and then apply the tagging rules down the "holes hierarchy".
Most current applications don't need to distinct between a hole in a 
hole (or an "inner outer way") and a second outer way beneath the first 
one, as most current applications don't care about the topology, but on 
the geometry - and there it's only of interest which parts of the canvas 
to fill and which not.

For the topology interpretation again the interpretation of a way as 
"hole in a hole" isn't helpful either, as more often you will have 
different objects inside that you have to consider to be that hole in 
the hole, without a multipolygon being in place.
Therefore it's necessay to implement the logic Frederic described above 
nevertheless.
>
>
>     If you are, on the other hand, thinking of a "lake in the middle
>     of a meadow in the middle of a forest" situation, then this is not
>     "hole in a hole" - the forest has one hole which is the meadow,
>     and the meadow has one hole which is the lake. You would have
>     three ways:
>
>     F - tagged as forest
>     M - tagged as meadow
>     L - tagged as lake
>
>     and two multipolygons
>
>     MP1: outer=F, inner=M
>     MP2: outer=M, inner=L
>
>
> Yes, if you split it in two separate multipolygons, then it's clear. 
> But the problem is that Wiki does not explicitly forbids (just 
> recommends not to) doing it all in a single multipolygon, quote:
>
>     Such cascading is still recommended when the "island" in the
>     middle is something else than the area on the outside, but where
>     the "island" is the same stuff it can just be made a hole in the hole.
>
And where's the problem?
The island is part of the area described by the multipolygon, as every 
other outer way is, too.
It can be handled exactly the same as long as you don't want to do 
special stuff according to topology, but then again topology is missing 
for most osm model elements, yet (or: you have to interpret it from the 
geometry yourself).

regards
Peter
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