6

I have a road network 'TR_ROAD' with EPSG:4283 crs and 1200 features. I am trying to calculate length of each line segment in road by using "length" geometry in PyQGIS.

When I tried below code, the PyQGIS editor got hanged for long time and program terminates.

I followed the existing answers to Calculating line lengths using PyQGIS and Calculating elipsoidal length of line in PyQGIS, but they didn't work for me.

layer = QgsProject.instance().mapLayersByName('TR_ROAD')[0]
features = layer.getFeatures()

for f in features:
    geom = f.geometry()
    leng = geom.length()
    res = layer.dataProvider().addAttributes([QgsField("Length", QVariant.Int)])
    layer.updateFields()


with edit(layer):
    for f in layer.getFeatures(): #For each feature/line in the layer calculate length
        f['Length'] = f.geometry().length()/1000
        layer.updateFeature(f)
0

1 Answer 1

8

You are adding the field once for each feature, you only need to add the field once for the table.

When updating features like this:

with edit(layer):
    ....

I have found it often hangs with many features (thousands).

Try storing the new data in a dictionary, and update with changeAttributeValues.

Like this:

lyr = QgsProject.instance().mapLayersByName('jl_riks_wgs84')[0]
p = lyr.dataProvider()
p.addAttributes([QgsField('Length', QVariant.Double)])
lyr.updateFields()

distance = QgsDistanceArea()
distance.setEllipsoid(lyr.crs().ellipsoidAcronym())

fieldindex = lyr.fields().indexFromName('Length')
attrmap = {} #Dictionary of {feature id: {field index: field value}}

for f in lyr.getFeatures():
    leng = distance.measureLength(f.geometry())
    attrmap[f.id()] = {fieldindex:leng}
p.changeAttributeValues(attrmap)

enter image description here

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.