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Working with QGIS modeler, I need to do some arithmetic operations with some polygon areas.

Let's say there are 2 polygons in layer A and layer B. And I'm inputting them into the model using Vector Layer input. And I need to calculate the ratio of area of these 2 polygons. Then I'll use that number in another algorithm.

enter image description here

This is what I thought I could do, but apparently it's not working.

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Can anyone help me get around this problem?

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Expression:

aggregate( layer:=@poly_a, aggregate:='sum', expression:=$area) / aggregate( layer:=@poly_b, aggregate:='sum', expression:=$area)

Aggregate will sum all polygon areas of each input layer.

I use the output to calculate a column in layer B, just to verify that Calculate Expression is working. enter image description here

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    Yeah, I managed to run it without the Field Calculator. My mistake was thinking there's only 1 polygon in an input layer. Using aggregate makes sense and works. But is there another way with single polygon per layer assumption?
    – Tolga Evcimen
    Commented May 30 at 18:20

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