Using the field calculator to calculate intersections

Question

I'm comparing several maps of one area (made by different people) to each other, and used intersection to find how much of specific selected features (polygons) overlapped. I soon realised that the intersection did not calculate the overlapping area. I have been told that the easiest way to find the area of the overlap is to do it using the "Field calculator" after the intersection, but I can't figure out how.

If anyone knows an easier way, I'm open to that as well.

• Nil: Are you referring QGIS?
• Erik: $area calculates the area of the current geometry.

Answer 1

As an addition to the solution above (@babel): you can do it also only with the field calculator, without doing the intersection with the processing tools. The expression is something like this one to get the area for the blue ones:

area(intersection($geometry,aggregate('orange','collect', $geometry, intersects($geometry, geometry(@parent)))))

Answer 2

Use Menu Vector / Geoprocessing tools / Intersection, set the two polygon layers you use (named orange and blue in my case) as input and you get the intersecting area (outlined in red on the screenshot) as a output in a new polygon layer intersection. On this layer, open field calculator to calculate the area using either area ($geometry) or $area.

enter image description here

• J.fu: Thanks! That was way easier than I made it out to be.