MP "island within a hole"

For an MP with the topology of “island within a hole”, there is the warning that a “dual determination of the area mapped by way 3 will be created”: Relation:multipolygon - OpenStreetMap Wiki
Carto renders the “island” with an overlay of the surrounding “hole”, e.g. as water AND swamp at once (fortunately, Osmand does not).

Is there any option to determine the inner area only by its own tags? I am inexperienced with MP and I exclusively use the ID editor. So many thanks for all hints and proposals,

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Could you please post a permalink or a way-id to the object?

Of course, apologies for forgetting.

Way: 220265522 | OpenStreetMap

The overlay is happening because both the multipolygon and the outer way are tagged with natural=wetland. And the warning about dual determination is probably because that area is covered by two wetland objects. It is very common to forget to remove tags from the original way when converting a way to a multipolygon.

The tagging you want is

  • Inner way tagged as water
  • Outer way with no tags
  • Multipolygon of inner and outer ways tagged as wetland.

By the way, this is the much simpler “One outer and one inner ring” case, though you don’t need to worry about the xml in that section. Just follow the iD instructions earlier on the page. It says “If any of the inner elements, whether they are Lines or Areas, also represent another feature that can be mapped; you are free to select the inner element and change it’s feature.” This is the case here, the inner way should be a water feature.


Graptemys, among all graptemes pseudogeographicae, you are the fastest ever. Thank you so much for your quick and clear explanations. And moreover for the exculpation that “it is very common to forget”… Yes, removing the left over tag cleared the water surface. A good example to keep in mind for future cases.

Now, under the false swamp appeared a false wood, obviously from the wood MP into which I had inserted my wetland MP as “inner” - but seemingly in vain. Do I, instead, need to make the outer line of the wetland MP an inner member in the wood MP? Thanks again, this time equally for your patience.

Yes, add the outer boundary of the wetland as an inner boundary of the wood. Fortunately it looks like that wood is already a multipolygon so it should be pretty easy.

Yes it works, and rendering is working hard. Many thanks again for perfect guidance.
Best, Yves

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I totally love threads that “solve” like this!