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Mac 10.9.5; QGIS 2.8.4-Wien; history and unioned data, in GDA94, zone 55 here.

I have a polygon of fire history with a number of overlapping features. I want to create a new polygon for fire frequency, where each feature has a count of the overlapping features in the history polygon.

The attached data is a small subset of my full data set, so I need a method that will be manageable with a large data set.

I unioned the history polygon to itself to get the individual features that should contain the frequencies. I previously followed Donovan Cameron's method in ArcGIS (http://forums.esri.com/Thread.asp?c=93&f=987&t=282764) with success. However, there is no 'summarize' option in QGIS, so I get stuck at point #6 in that method. I no longer have access to ArcGIS, so I am looking for a QGIS solution.

Here are some other things I've tried:

  • In another ArcGIS method (gis.stackexchange.com/questions/51929/how-to-count-and-rasterize-polygon-overlaps-in-arcgis-desktop), I get stuck at the 'dissolve' step (step 2) because there is no option to count in the dissolve tool for QGIS.

  • In the spaghetti and meatball (blogs.esri.com/esri/arcgis/2012/11/13/spaghetti_and_meatballs/) method, I substituted 'Feature to polygon' with union, and used Vector>Geometry tools>Polygon centroids (instead of the suggested Feature to point tool). At the intersect step I get an empty polygon so I tried to use the spatial join then join steps. This produced a range of the counts I was expecting (0-4) but not all of the original unioned polygon features were retained and the spatial configuration of counts were incorrect.

  • Finally, I've tried raster solutions (e.g. Creating a raster surface of overlaps from vector polygon shapefiles?), but I can't work out how to split my polygon features into separate polygons (multipart to single part returns an indentical polygon).

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I think you could use the Join attributes by location tool which allows you to extract some statistical data from two layers such as sum of values, feature count etc. This can be accessed from:

Processing Toolbox > QGIS geoalgorithms, Vector general tools > Join attributes by location

Join attributes by location interface

Note that the we are taking the summary of intersecting features. You can choose to keep the sum, mean etc parameters or remove them entirely. The output will always contain a count field which counts the number of intersecting features. Here was the result:

Result

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    I did try that too; the counts should be in the range 0-8, not what is returned using join attributes by location. I had a chance to test this on ArcGIS yesterday and realised that the union polygon (attached), generated in QGIS is not correct either. So I need to sort that out before I make further progress with summarising frequencies. I'm using the Arc solution for now, but would still like a QGIS method for future use. – asmith Feb 24 '16 at 22:24
  • @asmith - Interesting, I wonder where the Union function went wrong. Atleast you have more correct results from Arc as this method for QGIS has worked for me in the past :) – Joseph Feb 25 '16 at 11:44

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