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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

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    Welcome HodaR! As a new user, please take the tour to learn more about our focused Q&A format. Please edit your question to include what software you are using, as that will help us give you more specific answers. If you are using ArcMap for example, include your version and license level. – smiller Nov 7 '18 at 18:10
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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.

Take the 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

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