Difference between two features in same polygon in QGIS

Question

I have a polygon and some features are below another features. If I would have two different polygons I would use the "Difference" tool so it would take the overlapping feauters to the "surface".

Does anybody knows a method (if there´s any) on how to do a kind of difference tool but with different features in the same polygon?

Answer 1

Is creating the magenta polygons where the polygon layer intersects itself what you are trying to do? If so you can use pyqgis and intersect all combinations of polygons:

from itertools import combinations

layer = QgsProject.instance().mapLayersByName('Overlaps')[0] #Change to match your data

intersections = []
for poly1, poly2 in combinations(layer.getFeatures(), 2):
    if poly1.geometry().intersects(poly2.geometry()):
        intersections.append(poly1.geometry().intersection(poly2.geometry()))

vl = QgsVectorLayer("MultiPolygon?crs={}&index=yes".format(layer.crs().authid()), "intersectionsLayer", "memory")
provider = vl.dataProvider()
for poly in intersections: 
    f = QgsFeature()
    f.setGeometry(poly)
    provider.addFeature(f)

QgsProject.instance().addMapLayer(vl)

Answer 2

If I understood the question correctly, you want to achieve two layers:

• First that contains only overlapping features
• Second that includes features with no overlaps

Let's assume there is a polygon layer "grid" with its attribute table accordingly, see image below.

Step 1. Apply the SAGA's module "Polygon self-intersection"

Step 2. Apply the following expression if("ID" LIKE '%|%', 0, 1) in the Field Calculator.

Step 3. Continue with a "Split vector layer" using the "key"-field created at the Step 2. Application of "Fix geometries" on this step may be necessary. And get the output, see image below.

Note: Mind that the final output(s) is(are) in .gpkg-format.