In the example below, I have Urban Areas (UA1) and NUTS (NUTS1, NUTS2, NUTS3) regions in Europe.

Using the population of NUTS1, NUTS2 and NUTS3, I want to estimate the population of UA1.

I can join the population attribute of the NUTS region, but how does QGIS calculate the mean ? Is it a "basic" mean (suming the 3 populations and dividing by 3), or is it a weighted mean according to the surface of the urban area on each region ( in the example, it would be roughly 80% NUTS3 + 10% NUTS1 + 10% NUTS2)?

I could find this information on the QGIS documentation. enter image description here

  • Can you perform the calculation manually and compare the answers? – firefly-orange Sep 25 '17 at 12:27
  • I could, but I don't know how to get from QGIS the overlapping proportion of 1 polygon over others polygons. In this example, I don't know how to calculate how much of UA1 is covering NUTS1, NUTS2 and NUTS3. edit - just found how to do it here : gis.stackexchange.com/questions/214893/… – Franperr Sep 25 '17 at 16:53

So I did the calculation on my example manually, first doing a cut of one layer with the other, then calculating the simple average and the weighted average by proportional area.

In QGIS 2.14, if you select "mean" in the Join attributes by location operation, you will have a simple mean of the values of the polygons covered, it is not weighted according to the proportion of coverage.

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