QGIS 2.18.1 Predominant line attribute within polygon

Question

I've got a polygon shape (Catchments) and a lines shape (Rivers)

Each river body has an ID. What I need is to assign the ID of the predominant river within a polygon. For example, in 1 polygon there might be many rivers with different IDs ("MSPF_EM_CD" is the ID of the rivers):

As can be seen in the picture, in the attribute table of the rivers shape there is also a column with the length of each river line. What I need is to identify the longest line within each polygon, and add the ID of this longest line to the polygon shape, so I can get a new column in the polygon attribute table with the ID of the longest river within each polygon.

I am using QGIS 2.18.1 Las Palmas, in Windows 10.

Answer 1

NOTE I edited the code (and the explanation of how it works) because there were wrong results in some specific cases.

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I propose a solution with PyQGIS: starting from a linestring layer and a polygon layer, it looks for the line features within each polygon feature and then creates a new polygon memory layer (called "catchments_new") which stores the same attributes of the starting polygon layer plus one additional field (called "PREDOMIN_ID") containing the ID of the predominant river within it.

This is the code:

##Rivers=vector line
##Catchments=vector polygon

from qgis.core import *
from qgis.PyQt.QtCore import QVariant

layer1 = processing.getObject(Rivers)
crs = layer1.crs().toWkt()
layer2 = processing.getObject(Catchments)

# Create the output layer
outLayer = QgsVectorLayer('Polygon?crs='+ crs, 'catchments_new' , 'memory')
prov = outLayer.dataProvider()
fields = layer2.pendingFields() # Fields from the input layer
fields.append(QgsField('PREDOMIN_ID', QVariant.String, '', 50, 0)) # Name for the new field in the output layer
prov.addAttributes(fields) # Add input layer fields to the outLayer
outLayer.updateFields()

index = QgsSpatialIndex() # Spatial index
for ft in layer1.getFeatures():
    index.insertFeature(ft)

for feat in layer2.getFeatures():
    attrs = feat.attributes()
    inGeom = feat.geometry()
    idsList = index.intersects(inGeom.boundingBox())
    if len(idsList) > 0:
        req = QgsFeatureRequest().setFilterFids(idsList)
        temp_river_lengths = {} # Store the ids and lengths that are within the current polygon feature
        for elem in layer1.getFeatures(req):
            temp_geometry = elem.geometry()
            if temp_geometry.intersects(inGeom):
                itx = temp_geometry.intersection(inGeom)
                if elem[0] not in temp_river_lengths.keys():
                    temp_river_lengths[elem[0]] = itx.length()
                else:
                    temp_river_lengths[elem[0]]  += itx.length()
        k=list(temp_river_lengths.keys())        
        v=list(temp_river_lengths.values())
        if len(k) > 0:
            predom_id = k[v.index(max(v))]
            attrs.append(predom_id)
    outGeom = QgsFeature()
    outGeom.setAttributes(attrs) # Output attributes
    outGeom.setGeometry(inGeom) # Output geometry
    prov.addFeatures([outGeom]) # Output feature

# Add the layer to the Layers panel
QgsMapLayerRegistry.instance().addMapLayer(outLayer)