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With QGIS 3.8 a new Tool has been introduced: Overlap Analysis. It says that it "calculates the area and percentage cover by which features from an input layer are overlapped by features from a selection of overlay layers".

Now I have a feature layer including spatial data of operating areas of my company (which unfortunately are not congruent with administrative regions), which I would like to intersect with a layer of administrative areas of Germany (landkreise) and some other information which I only have for these administrative regions. As a result I need a new layer which shows me to what degree my company's operating areas are overlapped by Germany's administrative regions (landkreise).

So I chose one operating area and the overlapping administrative regions on the map. As Input Layer I chose my companys operating areas and as overlay I chose the administrative regions. Once I run the tool, the result is not as I imagined it. Most of the time I only get one feature in the resulting layer, but it doesn't show me to what degree the administrative regions overlap the operating areas.

Anyone got any experience with the new tool overlap analysis?

  • The description for Overlap Analysis says it should create new attributes for overlap area and overlap percentage. Does it do that? – csk Sep 25 '19 at 18:14
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I used the new Overlap Analysis tool today for the first time (QGIS version 3.8.3). Here's how it worked for me.

Input parameters:

  • Input layer: operating areas [EPSG:4326] (shown in yellow on the map)
  • Overlay layers: (one selected from multi-select list) administrative areas [EPSG:4326] (shown in gray on the map)

enter image description here

Output:

A layer called output layer, which is a duplicate of the input layer (operating areas), with two new attributes:

  • "administrative areas_area" - this is the total area of overlap between each feature in the input layer and any feature in the overlay layers. It's calculated in square meters, which is not the measurement unit of the input or overlay layers, not the measurement unit of the project CRS, and not the measurement unit that I have set in project properties.
  • "administrative areas_pc" - this is the percent of overlap between each feature in the input layer and any feature in the overlay layers

Unfortunately, the overlap is calculated for any feature in the overlay layers. It's not broken down by each feature in the overlay layers. So in my example, operating area 1 overlaps area2, area5, area3 and area6. It would be nice to know the relative proportion that overlaps each of those areas. I suppose I would need to reverse the input and overlay layers.

enter image description here


Here's what it looks like if I reverse the input layers. Input parameters:

  • Input layer: administrative areas [EPSG:4326]
  • Overlay layers: operating areas [EPSG:4326]

Output:

The output layer is a duplicate of the input layer, but this time it doesn't have any additional attribute table columns.

enter image description here

Summary: This tool does not seem to work as intended (yet).

You may need to revert to the old standard method of calculating area of overlap:

  1. Calculate feature area.
  2. Intersect the two layers.
  3. Calculate area for the intersected layer.
  4. Divide the original feature area by the intersected feature area.
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    Can you share your test files? – ndawson Sep 25 '19 at 21:02
  • I can't. They were temporary scratch layers. Sorry, I didn't realize anyone would want them. – csk Sep 26 '19 at 15:14

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