[Imports] UN Mappers import of UNSOS waterways in Somalia

[Arne Kimmig]

Hi Christoph,

Thank you very much for the examples. I think Lagh Dera is a good example of a fossil river at least inside Somalia. I wouldn’t map it as a river because I don’t see any erosion features nor any tributaries like gullies or creeks.

If and how these fossil rivers should be mapped is an other discussion and might be beyond OSM philosophy (I actually don’t know very well) and for the planned project I would recommend not to map them or add an annotation to the already mapped ones.

Regards

Arne


> Am 09.04.2020 um 00:33 schrieb Christoph Hormann <osm at imagico.de>:
> 
> On Wednesday 08 April 2020, Arne Kimmig via Imports wrote:
>> 
>> - how can we distinguish a 'drainage shaped landscape' from a stream?
> 
> Reliably only through local knowledge or through imagery depicting 
> actual waterflow.  There are occasionally good indications of recent 
> waterflow even in dry situation images - like a recently newly built 
> river crossing of a road next to an old and destroyed one.  Bridges or 
> culverts are also a clear indicator.
> 
>> Actually, what do you mean with this term? - same question for
>> 'fossil river'. Is that something like a dry valley?
> 
> As mentioned throughout subtropical Africa - both in the north and in 
> the south - there are a lot of valleys evidently shaped by water that 
> have not seen any more recent surface water flow.  
> 
> On images you can sometimes see that for example from visible artificial 
> or natural structures of significant age that would not withstand 
> significant waterflow:
> 
> https://mc.bbbike.org/mc/?lon=0.130422&lat=17.486138&zoom=15&num=2&mt0=bing-satellite&mt1=mapnik
> 
> or dunes across the riverbed:
> 
> https://mc.bbbike.org/mc/?lon=0.340901&lat=17.834483&zoom=14&num=2&mt0=bing-satellite&mt1=mapnik
> https://mc.bbbike.org/mc/?lon=31.690906&lat=17.633554&zoom=16&num=2&mt0=bing-satellite&mt1=mapnik
> 
> In most cases however this is very hard to reliably see (the problem of 
> proving a negative - what i mentioned in my reply to Rafael) - 
> especially in relatively flat areas like much of Somalia where 
> scrubland covers much of what could be useful hints.
> 
> Famous example of a fossile river is the Molopo River in South Africa 
> where it is generally said that it has not carried water over its whole 
> length for at least a hundred years:
> 
> https://www.openstreetmap.org/relation/7370966
> 
> I could imagine that this similarly applies today to the Lagh Dera:
> 
> https://www.openstreetmap.org/relation/5967248
> 
> -- 
> Christoph Hormann
> http://www.imagico.de/
> 
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