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My question relates to a work-around that I assumed might work, instead of trying this: Count points per polygon, classify point count in original layer in QGIS as all counting attempts have either failed or returned a wrong count number.

I have a layer with polygons (buffers), each polygon has been assigned a unique ID (IDBuffer). I have a second point-layer with the column "neue_bez", in which there are three classes (Kultur, BildungHochschule, NaherholungFreizeit) and a column that assigns each point a unique ID (IDBaupoint), too.

I want to know how many points of each class are within each polygon and I need this information added to the original polygon-layer in three columns (one for each class of "neue_bez"). I know I can get the raw information from a statistic by category, selecting IDBuffer and neue_bez like so:

example of my statistic by cat

But as mentioned, I would like a column added for each of the three classes, containing the count for that class per polygon.

This far, I have already intersected the two mentioned layers, so that I now have a layer that contains both IDs (IDBuffer, IDBaupoint), as well as the column "neue_bez" and thus the three classes.

enter image description here

I have since tried the following functions in the attribut table, to create a count-column for the class "Kultur" from the column "neue_bez"- for each polygon

sum(count("neue_bez", group_by:="IDBuffer", filter:="Kultur"))

This just returns 0 for every row.

I tried

count("neue_bez", group_by:="IDBuffer")

But this just returns the overall count of points per polygon and doesn't classify them.

Lastly, another idea I had was:

sum("neue_bez",group_by:="IDBuffer",filter:="Kultur")

Which doesn't work/is invalid, because right now I'm asking it to sum up "neue_bez", although the content is not numerical. If I created a column that just held the value "1" in each field to represent the amount of entries, could this then work, including the two expressions of group_by and filter?

Are there more elegant ways of doing this than my last idea?

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  • 1
    Have you tried using overlay_intersects() with the count()-part as expression on the unprocessed polygon layer?
    – Erik
    Commented Aug 30, 2022 at 11:43
  • I have tried something similar with the overlay-function: array_length(overlay_intersects('pointlayername',$id,filter:="fieldcontainingcategory"='nameofcategory')) But the count was never right when I checked for overlapping polygons. Could you specify your approach please, if it differs from this one?
    – gruenbet
    Commented Aug 30, 2022 at 11:46
  • 1
    If the count is incorrect with this function check if both layers are in the same CRS
    – MrXsquared
    Commented Sep 3, 2022 at 11:28

1 Answer 1

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Use QGIS expressions with overlay_contains() with the following pattern on your polygon layer to get the number of points from layer points with attribute Kultur that has a value of 14:

array_length(
    overlay_contains(
        'points',  -- replace with name of your point layer
        $id,
        filter:=Kultur=14  -- replace with attribute name/value
    )
)

Expression inserted as dynamic label: the large number counts how many points with attribute Kultur=14 each polygon contains: enter image description here

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