Snapping/moving points near polygon to inside polygon using QGIS

Question

I have a set of polygons (buildings) and points that I'd like to spatially join. Unfortunately, some of these points are positioned outside the polygon because all points are positioned on the centroid of each building and sometimes when a building is shaped like an L, for example, the point feature isn't placed inside the polygon.

Now I'd like to preferably use QGIS to move the points that are outside polygons to be situated inside the nearest polygon.

I've tried the Snap Geometries to Layer function but the points sometimes end up on a node which is a corner node of more than one building.

Any ideas?

Grey buildings are the ones where the point is positioned outside the building boundaries and therefore can't be joined.

Answer 1

You can achieve this join + displacement using a virtual layer.

Its underlying query would compute the distance between each point and each polygon, then will keep the closest polygon and will create a new geometry within the polygon.

go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query. Feel free to add any field of interest from the two layers.

SELECT point_on_surface(p.geometry) as geometry, p.id, b.bldID,
       MIN(ST_Distance(p.geometry,b.geometry)) AS distance
FROM p, bld as b
GROUP BY b.bldID
ORDER BY distance DESC;

The group by will ensure we get a single row for each row in the point layer (bld). Because we order by distance, the group by will retain the 1st entry, i.e. the nearest one.

The function point_on_surface guarantees that the output point will be within the specified polygon. Most of the time it will be at the centroid, but this is not guaranteed.

Let's note that you could filter building that are not already inside a polygon, or that you could completely remove the point displacement and simply keep the buildingID-polygonID relationship.