QGIS: How to only get the intersection part of self-intersecting polygons?

Question

I am trying to find places where different features in one layer intersect. An example picture of the layer:

And the result I would want from it:

However, at best, using Intersection, Clip or SAGA Self-intersecting polygons I get this result, basically just the Union:

Is there any way to do this automatically?

See also these threads: https://gis.stackexchange.com/questions/210959/summing-up-values-of-overlapping-polygons-in-qgis, https://gis.stackexchange.com/questions/418407/merging-polygons-only-if-three-or-more-are-on-top-of-each-other-in-qgis, https://gis.stackexchange.com/questions/366296/intersect-overlapping-entities-and-their-attributes — Taras, Mar 10 '24 at 07:30
Do my answer work? What should I do when someone answers my question? — BERA, Mar 25 '24 at 07:26

BERA · Answer 1 · 2024-04-03T07:20:50.573

You can create a virtual layer.

The query joins the layer to itself where two polygons intersect and their ids are different. And then calculates their intersection geometries.

You need an id column with unique integers for each polygon. Mine is named id.

Change id to the name of your id column and Buffered to the name of you layer in the query below. (I also use a filter to exclude overlaps with an area <=1000, you can modify or remove it.)

SELECT a.id, st_intersection(a.geometry, b.geometry) AS geometry
FROM Buffered as a
JOIN Buffered as b
ON st_intersects(a.geometry, b.geometry)
AND st_area(st_intersection(a.geometry, b.geometry))>1000
AND a.id<b.id

The layer is temporary, you can save it to file by right-click and export.

score 4 · Answer 2 · answered Mar 08 '24 at 18:02

Using SAGA's polygon self intersection:

Run the self intersection processing tool
Run "extract by expression" and use for example strpos("ID",'|') > 0 as expression to extract only the overlapping portions

Background: SAGA's self intersection adds a new field with the chosen id's of the input layer and uses | as delimiter between the original id's. So if that sign is in the result string there was at least one self intersection. Of course you could also use a regular expression or something else to detect it. Just make sure the original id field does not contain | characters.

score 4 · Answer 3 · answered Mar 08 '24 at 18:05

Using ProcessX Plug-In:

Run "Conditional Intersection" with default settings and choose the same layer as source and overlay

This Algorithm builds an intersection if an optional condition is fullfilled. It can also be used as polygon-self-intersection.

Disclaimer: I am the author of this plug in

QGIS: How to only get the intersection part of self-intersecting polygons?

3 Answers3