QGIS – Repeating $area Function on Multiple Layers for Area Calculations

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I, I have 15 layers on which I want to do exactly the same operation add columns and calculate the area (–> $area)
I looked online and found some hints with batch processing tools, BUT I've got errors messages with the Python advanced field calculator and errors with batch processing tools… can someone helps me?

Best Answer

You can use pyqgis like this:

newfieldname = 'area'
for layer in QgsProject.instance().mapLayers().values(): #For each layer added to the map
    prov = layer.dataProvider()
    prov.addAttributes([QgsField(name=newfieldname, type=QVariant.Double)]) #Add an area field of type Double
    layer.updateFields()
    attributemap = {} #A dictionary to store feature id, new fieldindex, area
    fieldindex = layer.fields().indexFromName(newfieldname)
    for f in layer.getFeatures(): #For each feature add values to the dictionary
        attributemap[f.id()]={fieldindex:f.geometry().area()}
    prov.changeAttributeValues(attributemap) #Update all features

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[GIS] Updating field with layer name on multiple layers with PyQGIS script

Just happens that I created a script for a very similar thing recently (my script merges multiple layers into a new layer, while adding a new attribute to each feature describing where it stems from; this would be more in line with what you apparently intend to do in the future). I slashed the write-to-new-merged-layer part so it solves your question.

The script iterates over multiple vector layers of your choice, then adds the new field with field_name, unless this field already exists. After making sure this field exists, it then iterates over all features, applying the layer.name() to it. This is done inside an edit session, so all the changes are only applied when layer.commitChanges() is reached. This is both a safety and a performance measure.

NOTE: This is on QGIS 2.18.5.

##input_layers=multiple vector
##field_name=string
##General Tools=group
##layername as field=name

from qgis.core import *
from PyQt4.QtCore import QVariant

input_layers = input_layers.split(';')
for i in input_layers:
    layer = processing.getObject(i)

    if layer.fieldNameIndex(field_name) == -1:
        layer.dataProvider().addAttributes([QgsField(field_name, QVariant.String)])
        layer.updateFields()
    name = layer.name()
    layer.startEditing()
    for f in layer.getFeatures():
        #f[field_name] = layer.name() # NOTE: This should work, but for some reason doesn't. To manipulate feature attributes, always use the below method! Maybe QGIS3 will fix this :) .
        layer.changeAttributeValue(f.id(), layer.fieldNameIndex(field_name), name)
    layer.commitChanges()

QGIS – Calculating Area of Polygon Intersection with Conditions

This expression appears to be working just fine:

area(
    intersection(
        $geometry,
         aggregate(
             'Layer_1',
             'collect',
             $geometry,
              "Value" = 10
            )
        )
    )

Best Answer

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[GIS] Updating field with layer name on multiple layers with PyQGIS script

QGIS – Calculating Area of Polygon Intersection with Conditions

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