[Tagging] Overlapping lakes

[Richard Fairhurst]

In Finland, Puruvesi (a large lake) is part of Saimaa (a really large lake).

Wikipedia says "Puruvesi is… part of the Saimaa lake system": https://en.wikipedia.org/wiki/Puruvesi

Currently we have a Puruvesi multipolygon with 194 member ways:
https://www.openstreetmap.org/relation/899111

but we also have a Saimaa multipolygon with 7223 member ways:
https://www.openstreetmap.org/relation/7379046
(warning - opening this will boggle your browser)

All the member ways of Puruvesi are repeated in Saimaa.

Duplicating the geometries like this seems wrong to me. In "One feature, one OSM element" (https://wiki.openstreetmap.org/wiki/One_feature,_one_OSM_element) we say "A feature that consists of several smaller features is usually best described using a relation".

How should this be mapped?

The Great Lakes seems to provide a good precedent:
https://www.openstreetmap.org/relation/1124369

The Great Lakes is currently represented as a 'master' relation of natural=water, type=site, water=lake, which contains all the individual lake multipolygons. The tagging appears to be a recent change from type=group (see https://help.openstreetmap.org/questions/72769/is-there-a-relationsite-variant-to-use-for-natural-objects).

Richard
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