[Qgis-user] Correcting river flow

[Nicolas Cadieux]

Thank Nyall, 

I will play with this tomorrow.  I knew for the dissolve tool that is a bit extreme as it basically make one single huge object (there goes the database!) but not the split with line tool.  I think I used multipart to single part to achieve a similar result and had so far extract the first and last nodes.  Your method will be faster and simpler I think.

I will definitely try the QGIS drape tool and your algorithm.  I will adapt it to also flip the line direction. That will force me to get to use python in QGIS rather than using python alone.

I will have more questions tomorrow concerning networks.

Thanks all of you.
Nicolas



> Le 14 avr. 2019 à 20:50, Nyall Dawson <nyall.dawson at gmail.com> a écrit :
> 
> On Sun, 14 Apr 2019 at 21:12, Nicolas Cadieux
> <nicolas.cadieux at archeotec.ca> wrote:
>> 
>> Hi Micha,
>> 
>> Great ideas, thanks.  I was trying to get the v.drape to work in Qgis but I was getting errors.  I figure it’s bad topology.  I will fix it and try again directly in Grass.
> 
> Try using the native QGIS "drape" algorithm, no need for GRASS here.
> 
> Then you could use field calculator with an expression like:
> 
> z(end_point($geometry)) < z(start_point($geometry))
> 
> This will be true for any lines digitized in an overall "downstream"
> direction, or false for lines digitized "upstream"
> 
>> That has not easy as the vectorizing was done in very small sections.  I was surprised to see so little merging tools that could deal with this without manually selecting the lines.  I tried with an old Qgis 2.18 plugin but it was not working properly.
> 
> Use the inbuilt Processing tools "dissolve", followed by "split with
> lines". This will join all your linestrings into sections which are
> split whenever two or more lines intersect (e.g. a fork in the river).
> 
> Nyall