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

Colin Smale colin.smale at xs4all.nl
Tue Jul 28 10:48:11 UTC 2015

Hi Christoph, my suggestion was to clearly separate the subject of water flow from the subject of routing. Whether roads are mostly bidirectional or not is irrelevant I think, as routers have to be able to handle one-way roads anyway. If I understand it right, edges in routing graphs are often one-way anyway as the characteristics from A to B may be different from B to A.
If you are interested in hydrodynamics on a large scale, like predicting the route an unpowered floating object would take, that is entirely different to the use of a routing engine to suggest a route to the skipper of a boat.
Simply adding a way from one side of a lake to the other to stop some QA program complaining is bordering on tagging for the renderer...

On 28 July 2015 11:17:00 CEST, Christoph Hormann <chris_hormann at gmx.de> wrote:
>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
>Christoph Hormann
>talk mailing list
>talk at openstreetmap.org
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