Representing direction on line from two Z-values in QGIS

Question

I have a stream layer. Each stream segment contains a "z_ini" and a "z_fin", where "z_fin" < "z_ini".

How can I graphically display the direction of the flow of the courses by positioning an arrow on the watercourses?

Answer 1

You can modify the line itself with "geometry by expression" from processing toolbox. Use this expression:

case
when z_at(@geometry,0) < z_at(@geometry,-1) then reverse(@geometry)
when z_at(@geometry,0) = z_at(@geometry,-1) then @geometry
when z_at(@geometry,0) > z_at(@geometry,-1) then @geometry
end 

In your case you can replace z_at(@geometry,0) with "z_ini" and z_at(@geometry,-1) with "z_fin".

If you run it, your lines will follow z direction and you can use an arrow style line with standard settings as you like.

Answer 2

You can use a marker style with a pyramid/triangle as arrow. Then you can set the rotation of the markers to follow the z direction. Use

case
when z_at(@geometry,0) < z_at(@geometry,-1) then -90
when z_at(@geometry,0) = z_at(@geometry,-1) then 0
when z_at(@geometry,0) > z_at(@geometry,-1) then 90
end

as data defined override on the marker rotation. In your case you can replace z_at(@geometry,0) with "z_ini" and z_at(@geometry,-1) with "z_fin".

Rotation of -90 and 90 is valid for the ones marked in red. For the ones in blue you need to use another value for the rotation.

Answer 3

For arrow style you can use a data defined expression:

case
when z_at(@geometry,0) < z_at(@geometry,-1) then 'reversed'
when z_at(@geometry,0) = z_at(@geometry,-1) then 'double'
when z_at(@geometry,0) > z_at(@geometry,-1) then 'single'
end

In your case you can replace z_at(@geometry,0) with "z_ini" and z_at(@geometry,-1) with "z_fin".