I have two lines that overlap, so I clip the overlap, and I should get its length in kilometers.

enter image description here

The clip part is ok but I can't find any resources on how to get the length. It can be printed in the console or add a field in the clipped shapefile that contains the km length. I used Calculating line lengths using PyQGIS as a guide but it didn't work.

import processing
from PyQt5.QtCore import QVariant

overlay_layer = QgsProject.instance().mapLayersByName("route2")[0]
bottom_layer = QgsProject.instance().mapLayersByName("route1")[0]
overlap = r'C:\Users\imper\Documents\python\calculate_overlap_lines\overlap.shp'

# clip overlap - works
processing.run("native:clip", {'INPUT': bottom_layer, \
                               'OVERLAY': overlay_layer, \
                               'OUTPUT': overlap})
iface.addVectorLayer(overlap, '', 'ogr')

# get length - doesn't work
overlap_lyr = QgsProject.instance().mapLayersByName("overlap")[0]
for overlap_lyr in QgsMapLayerRegistry.instance().mapLayers().values():
    features = overlap_lyr.getFeatures()
    for f in features:
        geom = f.geometry()
        leng = geom.length()
        res = overlap_lyr.dataProvider().addAttributes([QgsField("Length", QVariant.Int)])
        fieldIndex = overlap_lyr.dataProvider().fieldNameIndex("Length")
        attrFeatMap = {}
        attrMap = { fieldIndex : leng }
        for feature in overlap_lyr.getFeatures():
            attrFeatMap[ feature.id() ] = attrMap

I'm getting this error:

raise QgsProcessingException(msg) _core.QgsProcessingException: There were errors executing the algorithm.

  • @BERA , is that what the for loop does? I'm not yet comfortable with PyQGIS so I just copied it from the GIS SE post I linked and replaced the variables.
    – BallpenMan
    Commented Nov 18, 2020 at 8:15
  • 1
    Hi Taras! Yes, I forgot about this one. Will do :D
    – BallpenMan
    Commented Sep 2, 2021 at 7:53

1 Answer 1


You are adding the same field for every feature in your layer. I think this should work:

for overlap_lyr in QgsProject.instance().mapLayers().values(): #For each layer in the layer tree
    #Add the field once
    res = overlap_lyr.dataProvider().addAttributes([QgsField("Length", QVariant.Int)])

    with edit(overlap_lyr):
        for f in overlap_lyr.getFeatures(): #For each feature/line in the layer calculate length
            f['Length'] = f.geometry().length()/1000

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