[Imports] UN Mappers import of UNSOS waterways in Somalia

Christoph Hormann osm at imagico.de
Wed Apr 8 15:38:52 UTC 2020

Thanks for the additional info, that makes things a bit clearer.

Based on this it seems your suggested tagging is not right.  You base 
the classification into waterway types on the HYP attribute - which is 
not what this indicates apparently.  

More generally speaking i have doubts about the wisdom in importing 
large parts of the data.  Looking over it a large fraction of the 
waterways in the data set are not correct.  This is already hinted in 
the information you provided.  Most of the waterways (more than 40k, 
i.e. more than 90 percent) are indicated to be dry, that is without any 
evidence of present day water flow.  Looking over the data i could find 
a lot of cases where a drainage shaped landform was interpreted to be a 
stream and that stream was then continued downhill without any physical 
indication of even historic water flow - sometimes along tracks misread 
to be streambeds, sometimes also right across villages and other human 
built structures.

In subtropical Africa it is very common that due to climate change (both 
recent human made and natural changes over the last few thousand years) 
as well as immensely intensified groundwater use valleys created by 
water flow - with often indications of that visible in imagery - do not 
carry even sporadic water flow any more at present time.  This is 
called a fossile waterway.  According to OSMs verifiability principle 
mapping such structures as waterway however is clearly wrong.

The problem i see is that importing such data where a large portion of 
the features are factually incorrect will either result in

* a lot of incorrect data in serious need of cleanup imposing a serious 
debt on the local community.
* a lot of work to evaluate every single one of the >40k features to 
assess if it really represents a verifiable waterway.  My estimate 
would be that this work might be more efficiently invested in mapping 
those from scratch.

Now this of course varies a bit across the coverage area - in the 
western part a significant fraction of the HYP=4 waterways show 
indications they could be legitimate intermittent streams based on 
available images (though you often have to spend a lot of time looking 
for hints for that).  In the east this is much less so and i would 
probably consider the majority of features bogus.

At the same time the positional accuracy of many of the features is poor 
with positional errors in the order of often 50-100m.  This is another 
indication that importing this could be quite wasteful in terms of time 

Overall i would probably say that for mappers invested in improving the 
area this data could be useful to help identify where there are 
possibly waterways to map.  But as an actual import where the mapper 
just does a quick verification and tag adjustment before adding the 
feature more or less as is it seems less suitable.  And actual import 
would bear a high risk of well meaning mappers adding a lot of 
incorrect data because of the principal difficulty of proving a 
negative (it is hard to reliably prove a waterway from the data does 
not exist in reality and many likely will be inclined to trust the data 
being correct in such cases).

Existing waterway data in the area is - while being incomplete - fairly 
decent quality it seems and it would be unfortunate if that was diluted 
by low reliability data.

Christoph Hormann

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