[OSM-talk] waterway - "routable network" and reservoirs/lakes
chris_hormann at gmx.de
Tue Jul 28 09:17:00 UTC 2015
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
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