Finding corners where multiple polygons meet in QGIS

Question

I have a shapefile of polygons representing sectors of a city. I would like to identify "corners" where 3 or more of these polygons meet (that is, I don't want to identify the boundaries between two polygons).

In the figure above, I'd like to identify the red circles so that I can buffer and remove them from the layer before calculating transects. Does anyone have suggestions on how to do this so I can avoid doing it manually?

Answer 1

You can achieve this using a Virtual layer that identify the intersection point between 3 polygons of the same layer.

Go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the query.

select a.id, b.id, c.id, 
       st_intersection(st_intersection(a.geometry, b.geometry),c.geometry)
from city a
join city b 
  on st_intersects(a.geometry,b.geometry) 
    and a.id < b.id
join city c 
 on st_intersects(a.geometry,c.geometry) 
   and st_intersects(b.geometry,c.geometry) 
   and a.id < c.id 
   and b.id < c.id

Answer 2

There is a solution that can be produced with QGIS tools:

1. Make use of the "Extract vertices"

2. Proceed with the "Delete duplicate geometries"

3. Apply the "Extract by expression" with the following expression:

   array_length(
       overlay_intersects(
           layer:='polygon', -- change this name accordingly
           expression:=1
           )
       ) > 2 -- adjust this number
   

And get the following result:

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References:

• Identifying duplicate attributes in field using QGIS