File:Illustration Multipolygon Touching Inner Rings.png

Summary

This illustrates that it is not beneficial per se to declare touching inner rings as valid multipolygons.

Usually you would either draw a ring around B and C (overlapping some parts of their boundaries) to have a single inner ring to define A. Another option is to map B and C as multipolygons as well, which allows parts of their boundaries to form the inner ring needed to define A. Both methods would be a valid multipolygon within osm, ogc and geojson realms.

In osm, some project members claim that geometries with touching inner rings are valid multipolygons, effectively breaking ogc compatibility (but not geojson compat). One of the reason given to allow touching inner rings is that it would solve mapper problems, when contributing data to osm. While this might be appealing for some very simple cases, it /creates/ lots of problems further down the road. The typical example given is a farmland adjoin to a lake within a larger wood - farmland and lake share a border, so it's claimed beneficial to add these as inner rings without using one of the methods above.

In the majority of cases, a finer granularity of data sampling the reality is mapped at a later point in time, very possibly leading to some setup shown in the picture. If B and C were the only areas, with D and E added, then if A has been mapped using touching inner rings, D and E need to be added to the multipolygon definition of A - which is extra effort compared to the first two methods considered above, that are valid in all three realms, osm+ogc+geojson.

Also, consider it is forgotten to update A after E has been mapped:

"A" will have a part as inside lying completely within an outside defined by touching "inner" rings, but _without_ that inside being defined as an "outer" ring in the multipolygon definition of "A". What an ogc converter will do in such a case is completely undefined (and it is to be tested if all osm tools handle such a case equally).

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