I have vector layer of Polylines. I want to build from them a 3D surface model, so then, I would be able to move some of the Polylines to better position them, or alternatively - create (draw) new Polylines.
The surface model here is important since the modified/new vertices should be taken from this model.
For the sake of simplicity we can assume linear interpolation.
(Working env. Qgis, The solution may involve programming - Python)