Counting points in polygon to analyze data using QGIS

Question

I am using QGIS, and I have a shapefile that has 23 different land classifications. I have another file that lists coyote observations as points. I would like to find out how many coyote observations are found within each of the 23 categories.

I can create a separate shapefile for each land type and then run "count points in polygon" for each one but does QGIS provide an alternative approach?

Answer 1

You can use the algorithm "Join attributes by location (Summary)".

For my example, I have two layers : points (your coyotes), polygons (your areas).

Run the algorithm Join attributes by location (Summary) with the following configuration :

As a result, you get a layer with multiple statistics (which you can filter when running the algorithm), the one you are interested in is the count.

Answer 2

There is a possibility of using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer...

Let's assume there are two layers, a point layer called 'points' and a polygon layer called 'polygons', see the image below.

With the following query, it is possible to find out how many coyote observations are found within each of the categories.

SELECT
    poly.*,
    COUNT(*) AS "numpois"
FROM
    "polygons" AS poly
JOIN
    "points" AS poi
    ON st_within(poi.geometry, poly.geometry)
GROUP BY
    poly."id"

The output point layer with its attribute table will look like

Mind, that if some fields have to be concatenated then apply the GROUP_CONCAT() function. So, the new query will look like

SELECT
    poly.*,
    COUNT(*) AS "numpois",
    GROUP_CONCAT(poi."Coyote") AS Coyotes
FROM
    "polygons" AS poly
JOIN
    "points" AS poi
    ON st_within(poi.geometry, poly.geometry)
GROUP BY
    poly."id"

And the output

Answer 3

I believe this question was answered but just thought I would provide an alternative - the "Point Sampling Tool" plugin in QGIS is helpful in doing exactly what you describe.

Recommend checking it out and then importing the file to R or Excel for further analysis.