[Qgis-developer] calculate distance from two points along line in pyqgis

[gene]

I don't have the code posted by Giovanni but this is very easy to do with 
Shapely <http://toblerity.org/shapely/manual.html>   (a Python wrapper of
GEOS).

With PyQGIS, I do it in this way:

1) by definition a polyline is composed of  straight  segments between
points (nodes)
2) after computing the intersection between the polylines , the intersecting
point lies on a particular segment, easy to determine.
3) Then a simple loop through the segments (length) allows to compute the
distance between the first point of the line and the intersection point (=
sum of segments length + the length of the segment between the first point
of the segment where the intersecting point lies and the intersection
point).



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