[OSM-talk] Mapping rivers that flow into/through lakes?

[Christoph Hormann]

On Friday 23 February 2018, Rory McCann wrote:
>
> But then how far do you go? Should every stream be connected to the
> central river? e.g. what about here (
> http://tools.geofabrik.de/osmi/?view=water&lon=28.57869&lat=-16.75136
>&zoom=11 )?
>
> If some rivers/streams shouldn't be connected, then some data
> consumers will have to do an automatic connection anyway. When
> measuring water run off and pollution, you probably want to know that
> "stuff going into stream X will eventually get to point Y downstream"
> (right?).

For simple formal connectivity that is essentially a routing problem.  
Not having connectivity on the waterway level will require you to use 
the water area data in analysis and this will more than double the data 
volume to process and if you need more than formal connectivity, i.e. 
quantitative information like path length or route geometry things 
become very expensive computationally.

> Connecting all means that large lakes will be full of a "skeleton" of
> joining rivers/streams, and a small 1km stream could get a lot
> longer.

You could detect the 'virtual' waterways as those within a non-river 
water area.  This is hampered by the problem that we mostly have no 
consistent distinction between river and lake areas in OSM (i.e. 
standing and flowing water areas).

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