I have tried both GRASS v.drape and ThToolBox 2D->3D to attach raster z values to line vertices. I can't tell whether this worked or not since I don't seem to be able to extract the values using python. Perhaps I'm not doing it correctly in python.

layers = QgsProject.instance().mapLayersByName('Lines with Z')
layer = layers[0]

for f in layer.selectedFeatures():
    geom = f.geometry().asPolyline()   
    first_vertex = QgsPoint(geom[0])   




Either z values are not being assigned or I'm not retrieving them correctly. What I am trying to do, which I have not found a tool for, is generate a 2D profile of lines with z on the x-axis and distance on the y-axis with each line starting at 0 and also extract min and max z values for each line along the way.


I think the problem is how you are accessing your feature geometry. Try using the constGet() method to access the feature geometry like this:

geom = f.geometry().constGet()
first_vertex = QgsPoint(geom[0])

This should return a valid z value. By the way, I would recommend using the QGIS algorithm 'Drape (set z-value from raster)' under Vector geometry in the processing toolbox to set your z values.

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