[Qgis-user] Correcting river flow

[Nyall Dawson]

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