Prevent polygons from overlapping after splitting them in QGIS

qgis

I have a FileGDB with several polygons. When I split them using the Split Features-Tool, they overlap with other polygons within the same layer I did not split.

Here are some screenshots:

Original:

Split: the two blue circulated polygons were split. Red are the errors topology checker shows:

Only merging some polygons using the merge selected features tool doesn’t create an error/overlap:

Doing the same thing with the same data but in a .gpkg also creates overlaps after splitting the features.

I'm using QGIS 3.36.2 (but the same problem already appeared in previous versions). The CRS is EPSG:2056 - CH1903+ / LV95.
Topology editing is off. It does not make any difference whether I select Allow Overlap or Avoid Overlap on Active Layer.

Does anyone know, how I can prevent polygons from overlapping after splitting (and/or merging) them?
Some sample Data can be downloaded here google drive link.

Best Answer

You need to enable the "Topological editing" feature available in the Snapping toolbar https://docs.qgis.org/3.34/en/docs/user_manual/working_with_vector/editing_geometry_attributes.html#topological-editing

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[GIS] PostGIS: Is it possible to identify overlapping polygons in the same layer using their geomteries

select *
  from b.mypolygons as t1, b.mypolygons as t2
  where ST_Overlaps(t1.geom, t2.geom) = true
    and t1.my_id < t2.my_id
;

Assuming my_id is integer or other type where '<' makes sense. If not use '<>', but the you'll get 2 records out for each overlap.

QGIS – Correcting Geometry of Overlapping Polygons in the Same Layer

First make sure your Buildings have a unique "id" (which does not contain a | character, you will see why below). You can add it for example via field calculator by adding a new field with the expression $id if they do not have one already.

Then run the "Polygon Self-Intersection" from SAGA Processing tools and choose this "id" as Identifier:

Which will create a new layer, where the overlapping Parts become their own polygons.

This new layer will contain a new "id" field, containing the "id"s of their original buildings split by a | sign.

Now open the field calculator of this new layer and calculate a new field with this expression:

if(regexp_match("ID",'\\|') > 0, regexp_substr("ID",'[^|]*'), "ID")

to get the second "ID" as "mergeID" or

if(regexp_match("ID",'\\|') > 0, regexp_substr("ID",'(?<=\\|).*'), "ID")

to get the first "ID" as "mergeID":

This expression looks up in your "ID" field, whether it contains a | character. If so, it chooses the ID before or after the | character. If it does not contain this character, it will just copy over the ID to the "mergeID".

Then run the "Dissolve" from processing toolbox and choose this "mergeID" as dissolve field:

Your final result:

Best Answer

Related Solutions

[GIS] PostGIS: Is it possible to identify overlapping polygons in the same layer using their geomteries

QGIS – Correcting Geometry of Overlapping Polygons in the Same Layer

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