I have two separate layers and I want to identify polygons that overlap more than 85%. When I try "Extract by location" tool it gives me the polygons that overlap even in small areas. I am using QGIS Desktop 3.2.2
Add a unique identifier field to each of the parent layers, if they don't already have one.
Add a field called "old_area1" to the first layer, and add one called "old_area2" to the second layer. Calculate area into each of these fields using the
$area expression in the Field Calculator.
Union of the two layers.
Add two fields to the Union layer, "pct_overlap1" and "pct_overlap2". Use the Field calculator to calculate the percent of overlap between the polygons in this layer with each of their parent polygons using the expression
$area / "old_area1" *100.
Join the Union layer to each of the parent layers using the unique identifier field.
Select all features in the parent layers where
"pct_overlap1" > 85 or "pct_overlap2" > 85