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If a building has a larger basement, how should it be mapped? There may be grass or streets on top of the basement. |
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Your problem can be considered a special case of providing outlines for a building's levels. As of now, there is no established approach for this. While some proposal drafts for mapping building levels exist on the wiki (such as 1 and 2), not all of these account for levels' outline polygons. Moreover, these drafts are incomplete, haven't been tested in practice, are not supported by any tools and will likely change a lot in the future. You can either invent your own tagging, carry the discussion over to the "Tagging" mailing list or wait until someone else defines a solution. |
For many complex tagging questions, there is no one true answer - there may be many ways to do it, none of them perfect. This platform, help.openstreetmap.org, is about finding the best answer for clear-cut questions; it is not a brainstorming platform nor one where you can hold polls or who everyone tells you how he solved a particular problem. A question like yours - and indeed quite a bunch of other, similar questions here - is perhaps better suited for the tagging mailing list.
There may in fact be many ways, but I can't think of even one, and the wiki is no help.