Counting points per category in polygons using QGIS

Question

I am looking for a QGIS function to count specific point values inside polygons. I have a number of points, all sorted in specific categories "HOOFDGROEP" and a rectangular grid (see image).

I would like to know per grid cell, how many points per category "HOOFDGROEP" are in every grid cell.

Does anyone have any idea how to do this?

Answer 1

One thing you can do to get the data, is to create a Virtual Layer, using SQL count(), st_contains() and group_by

Starting from a Grid and a source_layer with a 'category' field:

Create a Virtual Layer with the following definition:

select Grid.id as grid_id, source_layer.category, count(*) as count, Grid.geometry
 from Grid,source_layer
 where st_contains(Grid.geometry,source_layer.geometry)
 group by Grid.id,source_layer.category

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Resulting virtual layer with geometries, attributes and count:

The resulting Attribute Table is the following:

with the grid_id, category, count along with the grid.geometry There are as many lines per grid item than there are unique categories present in that grid item.

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Using the virtual layer to display Labels in the original Grid

If you want to use this data in your Grid layer for instance to label it with the count per category, you can do the following:

Create an expression based label with this expression:

array_to_string(overlay_contains(layer:='count_layer',expression:='\n' || category || ' : ' || count ))

Answer 2

Open the field calculator in the grid layer and calculate a new text field with a considerable length (lets say 64 characters) with this expression.

array_to_string(
    with_variable(
        'classes',
        overlay_contains(
            'Points',  -- set here point layer name / id
            "HOOFDGROEP" -- set here name inside double quotes of the category field 
        ),
        array_foreach(
            array_distinct(@classes),
            format(
                '%1: %2',
                @element,
                array_count(@classes, @element)
            )
        )
    ),
    ','
)

What this expression does, is to get all the values of the "HOOFDGROEP" field from the points layer that are contained by the current cell that's being calculated and count how many times each different value appears. If you want to display labels you can set in the Formatting tab the Wrap on character option to , (comma).

Answer 3

I have just added the processing algorithm "Count Features in Features by Category" to v1.4 of ProcessX-PlugIn.

You can find it in your processing toolbox in ProcessX -> Vector - Conditional -> Count Features in Features by Category.

You can use all geometry types as Source and Overlay, so you can e.g. also count polygons intersecting lines if you wish. Then select a field or an expression that contain your categories. You may choose how the desired output shall look like and can choose between three options:

1. Create a new Source-Feature for each category
2. Create one Source-Feature with a new field for each category (be warned: if you have too many categorys (lets say be careful if there are more than 250; but there is no fixed limit), this may crash QGIS when you open the resulting attribute table due to an overflow)
3. Create one Source-Feature with one new field for all categorys and store the result as a string in form of a Python dictionary (be warned: if the string exceeds the limit of maximum characters (lets say be careful if its longer than 1000 chars, but there is no fixed limit), this may crash QGIS when you open the resulting attribute table due to an overflow)

Then simply choose the desired geometric predicates, whether at least one or all of them must match, and if a feature should be counted more than once.

An example result:

Disclaimer: I am the author of this PlugIn