QGIS – Overlap Tool

By David Crowther

Question:

How can I calculate the Area Overlap between Layers within QGIS?

Answer:

The Overlap Analysis Tool allows you to calculate the Area Overlap and Percentage Area that a set of input features overlay a source feature.  

In this example we will be using two layers:

• A Hardstanding Layer with two input Polygons
• A Play Area Layer with one Polygon

The Overlap Analysis Tool DOES:

• Calculate the AREA that each Hardstanding Polygon overlaps into the Play Area.
• Calculates what that AREA value is as a percentage of each feature’s own total area.

The Overlap Tool DOES NOT:

• Calculate the AREA that the input features cover the Play Area – although this can be calculated later.
• It DOES NOT therefore tell you the percentage that the Play Area is being covered by the Hardstanding polygons – although again this can be calculated later.

In QGIS below we have our two layers – the Play Area shown in Green and the Hardstanding Areas shown in Grey.

Before we run the Overlay Analysis Tool, we have already extracted the AREA value for each of the three Polygons:

• Play Area = 10704.378sq metres
• Hard Standing 1 = 3166.418
• Hard Standing 2 = 3801.548

From the Processing menu choose > Toolbox > and open the Vector Table section.

Choose the Overlay Analysis menu option.

The Overlay Analysis tool will open.

We will use the following settings:

• In the Input Layer choose the Hard Standing Layer – as we are going to calculate the Area and % Area overlaps for these features.
• In the Overlay Layer choose the Play Area Layer – as we are going to calculate how the input layer features overlap this layer.

If you now Run the Overlap Analysis tool, it will create a Temporary Layer which is a copy of the Input Layer – Hardstanding.

If we browse the Overlap Layer, we can see that we have the original input attributes and now 2 added fields:

• PlayArea Area – the Area that this feature overlaps the Play Area Polygon
• PlayArea_PC – the % of the Total features Area that overlaps with the Play Area Polygon

Looking in the map and adding some Labels we can start to visualise these Overlap values.

Now that we have these extra values, if we want to, we can then calculate some additional statistics:

1 – Total Hard Standing Area overlapping the Play Area =

• HardStanding1 Overlap Area - 1461.083 +
• HardStanding2 Overlap Area - 1496.752 =
• 2957.835 square metres.

2 – Percentage of the Play Area covered by the Hard Standing Areas

• Total Play Area – 10704.378
• Total Overlapping HardStanding Area –2957.835
• % of Play Area covered by HardStanding = 27.63%

This looks a great tool to help you analyse how your layers overlap each other, which you can then use for statistical analysis and for calculating costs!