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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 landuse=basin. To avoid the rendering of the whole area as a lake/basin, I immediately removed the landuse=basin tag and replaced it with a fixme note.

  • By the way I was wondering if there was really a way to add watershed basin limits to OpenStreetMap? (This proposed relation type takes in waterways (main stream + tributaries) instead of limits)
  • What do I do with this polygon: should I let it there as it is or delete it?


asked 20 Dec '17, 05:19

Privatemajory's gravatar image

accept rate: 23%

edited 20 Dec '17, 05:20

It's a good question (and not one that has been asked directly before, although people have asked about watersheds )

(20 Dec '17, 10:24) SomeoneElse ♦

A web search of "watershed site:" finds mailing list discussions in the US about it, but surprisingly few.

(20 Dec '17, 10:26) SomeoneElse ♦

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.

(20 Dec '17, 22:26) SK53 ♦

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 ( 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.

(21 Dec '17, 08:51) Privatemajory

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question asked: 20 Dec '17, 05:19

question was seen: 1,272 times

last updated: 21 Dec '17, 08:51

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum