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    By David Crowther

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    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.

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

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

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

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    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

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

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

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

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    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%

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    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.

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    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%)

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