Calculating average values in point layer with respect to polygon layer in QGIS

Question

In QGIS I have two layers:

• a point layer with a column that contains numbers (pollution index from specific measurement point);
• a polygon layer with different regions.

I want to find the average pollution for each region so I need compute the mean for all points in each region.

How can I do that in QGIS?

Answer 1

One way to do this is using a Spatial Join. You would do a spatial join with the point to the polygon layer. The point layer will need to have codes (ie: unique IDs, or a grouping ID system). When you do the spatial join you will have the polygon IDs, which will allow you to do calculations. With the data you can query out features based on the polygon IDs to the min, max, mean, etc. You can also look at using the Group Stats plugin for QGIS.

Answer 2

Here's how you can do averages, etc using the spatial database SpatiaLite. First import both of your layers into a spatialite DB. Let's call them "sensors" (the point layer) with a "pollution" column and "regions" (the polygons) with a "name" column. You can do this easily with the spatialite-gui, Import shapefile button.

Now this query will give you various statistics from the sensors for each polygon:

SELECT AVG(s.pollution) AS "Average Pollution", 
   MAX(s.pollution) AS "Maximum Pollution",
   COUNT(s.*) AS "Number of Sensors",
   r.name AS "Region Name" 
FROM sensors AS s JOIN regions AS r 
ON ST_Contains(r.geometry, s.geometry)
GROUP BY r.name

(Side note: If you have tens of thousands of points, you'll want to add a spatial index on the polygon table, and implement it in the query.)