QGIS – Creating Field with Area Differences Between Two Layers

areadifferencesexpressionpolygonqgis

In QGIS how can I get the difference between the areas of two layers through field calculator? And is there a way to automatize it so every time a new feature is added the difference updates?

Layer 1 = Big polygon
Layer 2 = small polygons inside polygon in layer 1

The "difference" field in this case would be an attribute of the Layer 1.
The area attribute in both layers are virtual fields ($area)

I thought putting a code in the Default Value of the field in Attribute Form and check 'Apply default Value on update' but I just can't write the code, maybe something with aggregate?

The image bellow shows a schematic representation of what I'm trying to achieve:

Best Answer

Use the following expression in the Field Calculator of layer 1:

$area - array_sum(overlay_contains('layer_2', "area"))

Explanation:

With overlay_contains(), you get all the polygons from layer 2 that are inside the current feature of layer 1. With "area", you refer to the attribute with this name. The result is an array, thus to add all the areas of all polygons inside the current feature, use array_sum(), then subtract this from the area of the current feature.

Related Solutions

Vector Difference – Finding Vector Difference Between Two Polygon Layers by Attribute

Assuming that you have a polygon layer named 'polygonsA', with a field named 'idA'; and you want to make the difference between any feature of that layer and the feature of the polygon layer 'polygonsB' that have the same (idA) value in the field 'idB'.

It is possible in a simple way as long as there is a single feature in polygonsB for each value of the attribute. If not, Dissolve first the polygonsB layer by the idB field.

Open the Geometry by expression tool.
Select polygonsA as input layer.
Select Polygon as output geometry type.
Write the following geometry expression:

difference( $geometry, geometry( get_feature( 'polygonsB', 'idB', attribute( $currentfeature, 'idA'))))

Run.

The output is a polygon layer named 'Modified geometry' with the difference by attribute between polygonsA and polygonsB.

QGIS Virtual Layer – Creating Difference of Two Polygon Layers Using Virtual Layer in QGIS

Another solution is to use the ST_Union() inside of the query (it was also mentioned in
@geozelot's comment), because features from a "cut" layer need to become one geometry (of type MULTIPOLYGON). This idea was taken from this thread : More on cutting polygons with polygons, in PostGIS.

Let's assume there are two polygon layers called 'grid_test' (green) with nine features and 'grid_test2' (orange) with five features in it, see image below.

So, your query may look like this:

WITH union_gd2 AS (
    SELECT
        ST_Union(gt2.geometry) as geometry
    FROM
        "grid_test2" AS gt2
    )

SELECT
    gd1.id,
    st_difference(gd1.geometry, ugd2.geometry) AS geometry
FROM
    "grid_test" AS gd1,
    "union_gd2" AS ugd2
WHERE
    st_isvalid(st_difference(gd1.geometry, ugd2.geometry))

or like this:

SELECT
    gd1.id,
    st_difference(gd1.geometry, union_gd2.geometry) AS geometry
FROM
    "grid_test" AS gd1,
    (SELECT
        ST_Union(gt2.geometry) as geometry
    FROM
        "grid_test2" AS gt2
    ) AS union_gd2
WHERE
    st_isvalid(st_difference(gd1.geometry, union_gd2.geometry))

and then get the final output

References:

Select empty Postgis (Postgresql) geometry column

Best Answer

Related Solutions

Vector Difference – Finding Vector Difference Between Two Polygon Layers by Attribute

QGIS Virtual Layer – Creating Difference of Two Polygon Layers Using Virtual Layer in QGIS

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