geometry - Adding Z value from a Point field to a LineString using pyqgis

I Have two layer in Shapefile format:

• a line layer made from a set of point which is a polyline 3D with z value to 0.


• a point layer which contains elevation in a field (field_4)

Each point is located on a vertex of the line. See below:

What I want to do is to had the Z value containing in the field_4 of the point layer to the coresponding vertex.

I found the setZAt method from QgsLineStringV2 which seems to exactly fit my need. So I wrote this code snippet :

line_layer  = QgsMapLayerRegistry.instance().mapLayersByName('line_truck')[0]
point = QgsMapLayerRegistry.instance().mapLayersByName('RT03')[0]
line_layer.startEditing()

for feat in line_layer.getFeatures(): 
    for i, elem in enumerate(point.getFeatures()): 
        #get the point lying on the vertex
        if str(feat.geometry().vertexAt(i).x())+str(feat.geometry().vertexAt(i).y()) == str(elem.geometry().asPoint().x())+str(elem.geometry().asPoint().y()):
            print i , elem['field_4']
            #update z value but nothing happen...

            #feat.geometry().geometry is a QgsLineStringV2
            feat.geometry().geometry().setZAt(i, elem['field_4'])

When I run it, the print statement show me the right values but the z value isn't updated in my feature...

Do I miss some steps or make a mistake in my logic??

Answer

I figured out the solution by my own

Actually, the geometry generate by the for loop was right

feat.geometry().geometry().setZAt(i, elem['field_4'])

but the feature geometry also need to be updated with this new geometry

so adding

line_layer.changeGeometry(feat.id(), feat.geometry())

at the end of the fisrt loop solve the issue!

The whole code become:

line_layer  = QgsMapLayerRegistry.instance().mapLayersByName('line_truck')[0]
point = QgsMapLayerRegistry.instance().mapLayersByName('RT03')[0]
line_layer.startEditing()

for feat in line_layer.getFeatures(): 

    for i, elem in enumerate(point.getFeatures()): 
        #get the point lying on the vertex
        if str(feat.geometry().vertexAt(i).x())+str(feat.geometry().vertexAt(i).y()) == str(elem.geometry().asPoint().x())+str(elem.geometry().asPoint().y()):
            #update z value but nothing happen...
            #feat.geometry().geometry is a QgsLineStringV2
            feat.geometry().geometry().setZAt(i, elem['field_4'])
    print feat.geometry().exportToWkt() #show the right geom with z values
    line_layer.changeGeometry(feat.id(), feat.geometry())