QGIS – How to Perform Overlap Analysis

By David Crowther

Question:

Is it possible to perform Overlap Analysis between two separate layers in QGIS?

 Answer:

Yes – in QGIS 3.8 (Zanzibar) there is a new Overlap Analysis tool.

Using QGIS 3.8 I have a Planning Applications layer with a large polygon representing one application:

The development of this site is shared between two Contractors and we wish to calculate the percentage share for the total Planning Application area.

The Overlap Analysis tool is available in the Vector Analysis section of the Processing Toolbox:

In the Overlap Analysis tool you can then choose the Input Layer – which will be the main planning apps layer – and the Overlay Layer e.g. Contractor 1

If you wish to calculate the % overlap for only the main polygon, you should first select the polygon from the input layer (planning apps)…

and then ensure that you tick the box to use only the selected features from the Input Layer.

Once Ran an output (scratch) layer is created which contains one record which is a complete copy of the selected source polygon.

If we view the attributes of this new layer we have two new fields, with the last field detailing the exact % that the Contractor polygon covers the selected Planning App – in this case 74.37%

Tip – you can undertake Overlap Analysis using Multiple Overlay layers. So this time when we choose the Overlay Layers tick both Contractor 1 and Contractor 2.

Having run the Overlap Analysis we now have extra fields which detail the percentage area for both Contractor 1 (74.37%) and Contractor (25.63%)