[OSM-talk] waterway - "routable network" and reservoirs/lakes

[Christoph Hormann]

On Tuesday 28 July 2015, Colin Smale wrote:
> If we can separate the flow direction discussion from the routing,
> the latter becomes a more generic "routing through areas"  problem
> which has been discussed before in the context of pedestrian routing.

Water flow structure is not only about flow direction of individual 
segments, it is also about connectivity - hence the routable network.

And yes, you can try to take polygons into account to determine 
waterflow - you have to at the moment since missing line mapping is 
just too widespread.  But for non-trivial polygons (i.e. ones with 
holes or areas represented by multiple polygons) this is generally 
ambiguous and it is hard to analyze as well.

The analogy between water flow analysis and traffic routing is 
misleading here since traffic routes are mostly bidirectional.  If you 
imagine a road network exclusively built from oneway roads you can 
immediately see that having parts of that network represented as 
polygons will make routing difficult.

And - this is even more important - it also makes it difficult to spot 
errors in mapping.  If you have a full line mapping of a river network 
it is very easy to identify problems locally for QA tools or validators 
in editors - just like in case of the coastline.  If you rely on 
polygon features creating connectivity in the waterflow network you 
need to analyze it in full before you are able to spot where mapping is 
broken.

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