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

[Martin Bain]

Ciao Carlo,
I’m a new QGIS user, I’m intrigued by your last paragraph:

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.

Can you elaborate on the meaning of  “network conscious database” or point me to some links to get me started, this sounds like a very useful capability.

Martin.


From: qgis-user-bounces at lists.osgeo.org [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Carlo A. Bertelli (Charta s.r.l.)
Sent: Wednesday, 15 July 2015 4:24 PM
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] rivers growing in size as they go downhill

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<mailto: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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