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This page is only a proposition, it does'nt represent a consenus of what is or isn't valid, but only what could and couldn't be considered valid

General Rule for validity : A multipolygon relation in OSM is considered valid if it can be used, without discarding nodes or ways (or part of ways) to build a valid geometry as define by the OGC Simple Feature standard ( with the notable exception of touching inner rings on 2 or more consecutiv points.

Valid multipolygon relations

A - 2 touching rings on single points

B - Multipolygon in "8" shape with point at intersection

C - Multipolygon made of two ways touching in a middle point

D - A inner ring touching the outer ring on a single point

Maybe this one is the GIS banana case

E - touching outer in one point

Simple touching outer in one point

F - Polygon self touching on one node

Simple touching outer in one point

Invalid multipolygon relations

1 - Unclosed polygons

This is an example of an invalid multipolygon because unclosed.
invalid Unclosed polygon

2 - Overlapping, unclosed member ways belonging to the same role

This is an example of an invalid multipolygon as way #2 and way #3 endpoints share more than two ways.
invalid ambiguous multipolygon relation

3 - Self intersection without point

self intersection without point

4 - 2 Overlapping outer-ring

2 Overlapping outer-ring

5 - Polygon sharing a way with itself

See also