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Hello everyone. Yesterday I noticed a strange very big lake in the center of Madagascar [this polygon]. When inspecting the data, I found that it was supposed to be a watershed basin but was wrongly tagged as
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question asked: 20 Dec '17, 05:19
question was seen: 1,276 times
last updated: 21 Dec '17, 08:51
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It's a good question (and not one that has been asked directly before, although people have asked about watersheds https://help.openstreetmap.org/search/?q=watershed&Submit=search&t=question )
A web search of "watershed site:https://lists.openstreetmap.org/pipermail/" finds mailing list discussions in the US about it, but surprisingly few.
I think in general we should not try to map them. It is possible to find all waterways draining a given basin (either through explicit tags or finding discrete graphs of waterways) and draw a concave hull to create a polygon which will approximate the drainage basin. It should also be possible to post-process these to tessellate the drainage basins over the area.
But well, natural landscape elements and natural boundaries could also be part of OSM, just like other boundaries, right? Then watersheds can't be separate from hydrography, waterways + springs + watershed limits would be a more complete hydrographic data.
Using the search suggested by @SomeoneElse above, I found a 2009 topic on OSM-Talk-fr (https://lists.openstreetmap.org/pipermail/talk-fr/2009-September/014241.html) about adding ridge and thalweg lines to OSM but no one showed interest. This could match here as watersheds follow ridge lines. But I guess it's not interesting enough amongst the OSM community.