Counting touching and near points for every polygon in QGIS

Question

I have a polygon and a point layer. The polygons contains buildings, the points should be the edges of the buildings. A building is "good" as much as points are touching or in the near (buffer 0.2 meter) of the building. There are no other references between the layers.

I need the number of touching or intersecting points and points within a near of 20cm. With that result I'm able to say, which building have got the best quality...

The result could be written into a virtual field of the building-polygon.

I know, SQL could do the thing easily. But I need a QGIS (aggregate?) way because the input data will be provided as shapefile or GeoJSON.

Answer 1

In QGIS there is a possibility using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer....

Let's assume there are three features in 'buildings' and 12 in 'point_layer' accordingly, see image below.

With the following query, it is possible to get the number of touching or intersecting points and points within a near of 20cm.

SELECT
    b.*,
    SUM(ST_Touches(b.geometry, p.geometry)) AS touch,
    SUM(PtDistWithin(b.geometry, p.geometry, 20)) AS near
FROM
    point_layer AS p,
    buildings AS b
GROUP BY
    b.id

The output Virtual Layer will maintain initial attributes and geometries and additional fields 'touch' and 'near'.

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References:

• SQL functions for distance relationships | PtDistWithin

Answer 2

Also an expression example:

aggregate(layer:='point_layer', aggregate:='count', expression:="fid",  filter:=intersects(buffer($geometry, 0.2), geometry(@parent)))

• 'point_layer' is your points
• "fid" can be any field (e.g. "id")

Answer 3

Thanks to @Kazuhito and @Taras I found a aggregate way to check the intersection with the edges (nodes) of the buildings and the (control-) points, while the points will be buffered with 1m (green):

aggregate( 'point', 'count',"fid",intersects(buffer($geometry,1),nodes_to_points(geometry(@parent))))

As you can see: The building in the center have got 6 nodes (edges) and 7 control points around - 5 of them intersecting the edges of the building within 1m. The other ones are not in the near of an edge.

Answer 4

Note that this is more an enhanced comment, but will likely be edited to a qualified answer. I tried drawing a scetch of what is desired, with some resulting questions related to it (cp. my comment above):

For me, the question is what points are taken into account to gain a qualified statement on the "building quality".

• point A is surely within the 20cm buffer, but for me it does contribute nothing to a statement on the buiding quality (I think only the point "near the vertices" should be taken into account)
• points B1 and B2 are within the buffer, but should be counted only once, because they mean the same building edge
• using a buffer with sharp edges, point B1 is inside the buffer but obviuosly more than the buffer distance away from its related building edge, so the result depens on what kind of buffer is generated (buffer with round eged should be preferred).
• I recommend counting only the points in a 20 cm neighborhood of the buildings vertices, and furthermore only 1 point per neighborhood.

So with all these considerations, the point count lies somewhat between 5 (without A, either B1 or B2) and 7 (all points within buffer).

So if we can get clarified on all this, one could provide a QGIS process/model (or SQL?)