[Qgis-user] rivers growing in size as they go downhill

[Carlo A. Bertelli (Charta s.r.l.)]

Ciao Luca,
if you mean cutting the stream and manually assigning a width to every arc,
well, it's not the right procedure anyway, but if you deal with a network
as a network you should allow waterflow grow for other reasons besides
going downhill. Every node brings affluents so more water becomes a bigger
stream. So classifying the rank of the network is the proper thing to do.
You don't have to do it manually, it's a directed planar graph so there are
tools to deal with it (plugings and processing).
If you look at old cartography, you may argue streams grow gradually and
that is true, but there are tiny gutters and invisible kennels feeding
them...
Yes you may desire to kill all hydrologists, but this is a graph and the
easiest thing is assuming the size of the stream depends on the rank of the
starting node.
You may also assume some "intelligence" in the tools you use may provide
som help. It's more convenient to keep your streams in a network conscious
database like spatialite instead of storing into shapefiles.
c

On Tue, 14 Jul 2015 at 22:08:45, <luca.galuppini at gmail.com> wrote:

> Dear all
> i have this idea of rendering a river network so that each river starts as
> a very thin blue line, and the line width increases little by little as the
> river goes.
> i wouldn't like to modify the original file (eg cutting rivers in pieces
> and give a different width to each piece) but all rivers are digitized with
> the right direction so i was wondering if there is an expression to teach
> qgis what is the start end of a river, and assign a wider line for any
> given distance, for example:
> from start to 200m - line width 0.1
> from 200m to 400m - line width 0.2
> from 400m and over - line width 0.3
> i think this is an interesting problem because i cannot find a "from - to"
> command in qgis expression language, neither i know how to properly exploit
> the line direction with code, which would come useful in many other ways...
> any python expert out there can help?
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