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Given a layer of polygons that is topologically perfect, i.e., has no overlaps or gaps between the polygons, is it possible to manually edit the edges of the polygons in QGIS in a way that preserves the topological correctness?

As illustrated In the image below, suppose I want to change:

  • the "border" between polygons A and B to the red dotted line
  • the "border" between polygons A and C to the yellow dotted line

enter image description here

This is the POSTGIS code to generate the polygons:

CREATE TABLE a_toy_polygons AS
SELECT      1 AS gid, 'A' as name, ST_MakePolygon(ST_GeomFromText('LINESTRING(0 0,1 0  ,1 1,0.5 1,0 1, 0 0)')) AS geom
UNION   SELECT  2 AS gid, 'B' as name, ST_MakePolygon(ST_GeomFromText('LINESTRING(0 1,0.5 1,1 1,1 2  ,0 2, 0 1)')) AS geom
UNION   SELECT  3 AS gid, 'C' as name, ST_MakePolygon(ST_GeomFromText('LINESTRING(1 0,2 0  ,2 1      ,1 1, 1 0)')) AS geom

Notice that in the common "border" between A and B there is already a vertex in the middle. There are actually two superimposed vertexes, one for each polygon. Is there a way to move them simultaneously? Or should I move one, then move the other close while having "snap" activated?

In the "border" of A and C, however, there are no such intermediate vertexes to be moved.

This is, of course, a toy example, my end use would be to fix manually some features of a Postgis topology. I am aware of the "Topology Checker" QGIS plug-in, but that is not capable of moving nodes yet. So I figured it could be possible to cast the topology as a geometry (of polygons) and edit in QGIS.

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If these are in the same layer this is pretty straigtforward.

Simply tick the "Topological Editing" box in your snapping options. Then if the two polygons share a node, this node will be moved in both polygons.

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