QGIS Topological Editing stops working after adding Google Maps or OSM underlay

Question

When I work on a polygonal layer, I turn on 'Topological Editing' from the snapping toolbar, which allows moving the nodes from adjacent polygons together.

Everything works fine until I add any underlay from the QuickMapServices plugin - Google maps, OSM etc.

After any such layer is added, only individual vertices are selected. So, in order to move a node I have to move individual vertices from neighbouring polygons. The 'topological editing' button is still 'pressed' - no changes in the snapping toolbar.

The problem persists even when I remove the underlay.

How can I keep Topological Editing functional with the google/osm map enabled?

Answer 1

When you add a basemap, sometimes the project CRS switches automatically to match the CRS of the basemap. I don't see any reason it would cause the issue you describe, but that's the main thing I can think of that can change when you add a basemap.

I was able to reproduce your issue by changing the project CRS. The issue resolves when I change the project CRS back to the same CRS as the layer I'm trying to edit with topology.

After adding a base map, change the project CRS to match the CRS of the layer you're editing.

Based on my testing, it seems like the topological editing feature only works on a layer when project CRS matches the layer CRS. I couldn't find any documentation to support this assertion, but the documentation for the topological editing tool is not very detailed.

UPDATE:

To prevent this (unwanted) behaviour there's also a checkmark in QuickMapServices Settings:

"Enable OTF EPSG:3857 on tiled layer add:" which should be disabled.