Creating list of adjacent polygons in QGIS

adjacencypolygonproximityqgis

The answer to Using join in Mapviewer describes an effective way to count the number of adjacent polygons. However, I need to create a list of the adjacent polys.

I envision a process that generates an output table containing the source and adjacent unique IDs, something like this for the image below:

Source poly Adjacent poly

1                  2
1                  3
1                  4
1                  5
2                  1
2                  3
3                  1
3                  2
3                  4
4                  1
4                  3
5                  1

The beauty of the aforementioned post is that the adjacent polygons do not have to touch; see polygon #2 in my example. Indeed, the "gap" distance can be set by the user.

I've searched the internet, as well as the Plugin repository, without success. There is an old SE-GIS answer involving Spatialite, but I fear it is out of date.

How do I create a list of adjacent polygons?

After @Spacedman's comments, in my example, I assumed that a user-defined threshold had been applied that was >= the gap around polygon #2, which would include it. Had the threshold been < the gap distance, polygon #2 would be excluded.

Best Answer

You can use SQL and join the layer to itself based on distance and where the ids are different.

Change mylayer to the name of your layer, and 30 to your distance limit:

select row_number() over() as newid,
        a.id as aid, b.id as bid, 
        a.geometry
from mylayer a
left join mylayer b
on st_distance(a.geometry, b.geometry)<30 and a.id<>b.id
order by a.id, b.id

Related Solutions

PostGIS – Simplifying Adjacent Polygons with PostGIS

A topological vector model will provide what you need. In a non-topological storage (such as a shapefile), a single edge between geometries is stored twice. In a topological vector, the areas are stored separately from the lines, so adjustments can be made without effecting topology. I couldn't find a good diagram, so I created this simple example, where the areas A, B and C are computed from the intersections of the lines (connecting 1-4) which separate them. example of a topological vector

This model is used by ArcInfo as coverages, in GRASS as its default vector model, and can be used in PostGIS with the experimental PostGIS Topology tool. Perhaps a simpler solution is converting your data into linework, removing the redundant segements, and then recreating your polygons after simplification.

[GIS] Unique SpatialPolygons object with multiple polygons slots in R

You have a SpatialPolygonsDataFrame with 24 features. Note carefully the use of upper case Polygons and lower case polygons in the following, as well as the use of what are slots and what are classes:

The geometries are stored in the @polygons slot, which is a list of length 24, one for each feature, each containing one object of class Polygons.

An object of class Polygons has a slot, @Polygons that stores a list of the rings and islands and holes that make up the geometry.

> str(countries@polygons[[1]]@Polygons,2)
List of 9
 $ :Formal class 'Polygon' [package "sp"] with 5 slots
 [8 more]

Your first feature has 9 Polygon objects in its @Polygons slot because it is made up of 9 rings. There's no way you can make this into 1 ring without losing geometry, and if your feature is made up of two similar sized land masses, you are going to lose one of them. You can't have two disconnected rings in a single Polygon object, which means if your feature has two disconnected land masses then you have to have 2 Polygon objects in your @Polygons list for that feature.

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