Spatially join attributes from point to polygon side with numerous contiguous polygons and numerous adjacent points

Question

I have been trying for 2 days to join points representing addresses (in Paris region) to the closest polygon (parcel of land).

The problems:

• The parcels of land are contiguous, sometimes a point related to the next parcel could actually be closer to the next parcel's centerpoint/vertice midpoint than to its own
• There are sometimes several points (addresses) by land parcel
• Some points are located quite close to the vertice of "their" parcel, sometimes further away (depending also on the street width) so it is difficult to use a buffer strategy (plus the buffers would overlap over each other since land parcels are adjacent)
• I tried "NNJoin" but it doesn't move forward at all (I have a large database) and I am not sure how to specifiy how many points I want to select
• I tried "Join attributes by nearest" but some dots are equally distant to the polygon and then all included, which is ok but the distance problem means it sometimes include some dots it shouldn't include
• I tried a space matrix with vertices extracted from the polygons, but it froze...
• My guess is that ideally, I should establish a kind of perpendicular projection of the parcels on the street side only, that would stop at the median line of the street. This way I would be sure not to include the points of adjacent parcels or parcels on the other side of the street. But I have no idea if and how to do this... Otherwise, my less ideal solution would be to use "Join attributes by nearest", specify 3 or 4 as maximum possible addresses and a maximum distance of 1 or 2 m, and hope to get the majority of points without too many of the neighbouring points. Not so great as you see...

Below is a screenshot + what I envision with the median line of the streets and the perpendicular projection.

Now I have tried to use Voronoi (the SAGA one the other ones seem to take a lot of time) around the Pole of Inaccessibility of the land parcels, but the result is a kind of disaster.

I think I am onto something though, @Bruce Xiaolong Liu is already developing a plugin for Voronoi for polygons: Calculating Voronoi Diagrams for polygons I didn't manage to install it, but maybe I can find some workaround to achieve something a bit similar. I first thought about applying Voronoi to extracted vertices from the polygons, then combining each polygon from the same parcel into one larger polygon, but it seems a bit complicated. I will see if I find something else.

Another idea: replacing the polygons with points, then using the simple region growing tool by SAGA. But I am not an advanced user and unsure how to do it.

Answer 1

Not a pure solution, just couple of ideas how this task can be tackled

Idea #1

1. Geocode each point, i.e. convert address into a x,y-tuple and it as a layer into QGIS. As you said that "points representing addresses (in Paris region)" apparently means for me, that each point has a street and a building number. IMHO each address will be geocoded and perhaps placed inside a parcel or hopefully close to.
   For geocoding in QGIS, see these artciles:
   

   • How to Geocode Addresses Using QGIS
   • DSC Tutorials and Guides QGIS - Geocoding Addresses

2. Then migrate address values from new points into polygons

3. Spatially join attributes from points to polygons via an address field

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Idea #2

1. Convert polygons to lines

2. Delete intersections, so only the outer edges will remain

3. Generate points along those lines sections

4. Spatially join attributes from points to points on edges

   A pitfall: neighbour parcels will have a vertex in common, so the spatial join may not work properly in that case

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Idea #3

1. Create buffers on the outer parcels' borders OR enlarge those polygons on their open edges

   Check for implementation this thread Enlarge a Polygon without changing its shape or position

   

2. Spatially join attributes from points to extended polygons

Answer 2

There are two ways (at least) to solve this (and probably a third one as shown by @Taras) Both times the key point is to see things from the perspective of adresses (points), not parcels (polygons). Because while 1 polygon may have several addresses, addresses are only related to 1 specific polygon (parcel/plot/lot/piece of land).

Method 1 :

Use "Join attributes by nearest" and specify as first input layer the address points, and as second input layer the parcel polygons. Also specify that they can only have 1 nearest neighbour (as one address can only be linked to one parcel, while 1 parcel may have two or three addresses). No need to convert the polygon to polylines and the polylines to points, because while they might be further away or closer, near the vertex or near the midpoint of the parcel boundary, the address A related to parcel A will always be positioned closer to it than to parcel B (unless a mistake has been made when creating the point).

Test + attribute table showing correct match (same letter).

Method 2

I actually found this method before the first one although the first one is even easier. In this case, you can create a wide even buffer around each address point which will necessarily catch its related land polygon (for instance a 8m radius buffer is more than enough). Then I use "Join attributes by location". Like in method 1, it is critical to choose (unlike the usual choice) the buffered address points as "Base Layer", and the polygon layer as "Join Layer". You can then choose "intersects" and "overlaps" as geometric predicate (still unsure of the difference so I checked both boxes) and then (key point) specify "Take attributes of the feature with largest overlap only (one-to-one) as Join type, because addresses are only related to one piece of land (while parcels can be related to several addresses). Since the polygon which will be closer to an address point will be covered by the largest overlap from this address point buffer, it will be joined with this address.

Voilà !