I am working in QGIS 3.0.2-Girona.

I am digitizing images in which there are rocks that can be encrusted, totally or partially, by algae. I want to have a polygon for the rock and one for the algae. In particular I can have 3 cases:

  1. the polygon of a rock and the one of the algae coincide
  2. the polygon of a rock could potentially contain more than one polygon of the algae
  3. the polygon of a rock could contain NO algae polygon(s) at all.

For this reason I thought that one effective way to go is to have 2 polygon vectors open in edit mode: 1 for rock and 1 for algae. When digitizing a rock with algae, I digitize the rock first and then copy paste the Geometry only (i.e. without attribute) to the algae vector layer. Repeat this process whenever I find a similar case while digitizing.

My problem is that selecting a feature and copy/paste it to a different layer doesn't seem to work: although the 2 layers are in edit mode, when I paste the geometry to the target layer I can see the pasted feature in its attribute table (with just the ID value, that's OK) but it doesn't appear on the map. If I select the feature in the target layer's attribute table, and click "zoom to selection", I am shown a white part of the map (I don't even know where) clearly not in the same position as the copied polygon.

Am I doing something wrong? Does anybody have a strategy to achieve this task?

  • Make sure both layers are using the same CRS. – Erik Oct 9 '18 at 14:51
  • They are. I really do not see a geometry at all in the target layer. It is not that I see the pasted geometry in the wrong position. – Filippo Oct 9 '18 at 14:56
  • I've found QGIS's feature display to be buggy at times. Could you save the layer (stop editing) and re-load it to see if your features were actually copied? It might just be a display issue. – Jon Oct 9 '18 at 18:34
  • @Jon it is possible... I managed to see them once only though, and clearly when I was doing a test. Now it is not showing them again. in the attribute table they appear. I saved layer, stop editing, save project, removed/added the layer.... still nothing. – Filippo Oct 9 '18 at 19:37

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