[Imports] UN Mappers import of UNSOS waterways in Somalia

Christoph Hormann osm at imagico.de
Thu Apr 9 16:33:07 UTC 2020


On Thursday 09 April 2020, Rafael Avila Coya wrote:
>
> If a river gets dry for any reason (let say for some decades ) but u
> could still see the river course (which is only possible in very rare
> occasions) u could still call it a river just to maintain the
> landmark notation and geography."

I have not made a statement on what can be called a river and what not.  
That is a speciality problem of English language semantics.  What i 
said is that a feature that shows no verifiable water flow in present 
times is not to be tagged as waterway=*.

> I am not an expert, and I would do what Arne suggest if I was mapping
> without discussing this with anybody, but still, like Arne, I see the
> lack of a tag for these kind of rivers.

Feel free to introduce a new tag for something verifiable for which no 
tagging exists yet.  But you should keep in mind that there are already 
a lot of tagging ideas in that wider domain that have either failed or 
are used without a well defined meaning:

https://wiki.openstreetmap.org/wiki/Tag%3Awaterway%3Dwadi
https://wiki.openstreetmap.org/wiki/Tag:waterway%3Ddrystream
https://wiki.openstreetmap.org/wiki/Tag:natural%3Dvalley

> As for the Lagh Dara, this colleague has sent me two images of the
> rivers passing by Afmadow town.
>
> One shows the river as we can see it normally:
> https://flic.kr/p/2iNyA5a
>
> But the second shows how it was during the 2018 devastating
> floodings: https://flic.kr/p/2iNyA7z
>
> He has been on UN missions in Somalia many times, so he has too on
> the ground knowledge.
>
> By the way: the Lagh Dera river geometry is right now of very poor
> geometry in OSM, while the UNSOS one is very good in comparison:
> https://flic.kr/p/2iNyQ6G

Please keep in mind that anecdotal examples are not representative for 
the whole.  I could easily provide a dozen examples for the existing 
mapping being fairly accurate and the import data being off by at least 
50-100m.

Flooding is a completely different subject.  We do not map 2/3 of 
Bangladesh as water just because it becomes flooded on a regular basis.  
waterway=river/stream is for mapping localized, verifiable surface 
water flow, it might be seasonal, it might be ephemeral - but it needs 
to be there.  What your example image shows is specifically the lack of 
water flow over longer distances.  Water from heavy local rain does not 
drain on the surface along the supposed river but accumulates in local 
depressions as temporary lakes.

Overall i think we are departing from the matter of import review here.

-- 
Christoph Hormann
http://www.imagico.de/



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