How to insert some holes in a polygon?

Question

I have a problem with QGIS digitizing advanced tool plugin.

I have a landcover-like shapefile. I have to perform some corrections on it, and I have to cut or split some polygons.

I want to insert some holes (that will be filled) in a polygon. I found these instructions (http://docs.qgis.org/2.0/it/docs/training_manual/create_vector_data/topo_editing.html): "It allows you to take a hole out of a feature, as long as the hole is bounded on all side by the feature. For example, if you’ve digitized the outer boundaries of South Africa and you need to add a hole for Lesotho, you’d use this tool." The problem is that I need to digitize a hole that is not completely bounded by a single polygon. (following the above example, I had to digitize Egypt from Africa shapefile. It is not completely included in Africa, but one of its side confines with the ocean).

How can I do that?

Answer 1

There's a couple of ways of going about this. In both cases ensure you have set up snapping.

1. Cut your hole (Egypt) as per the tutorial and you will get your original polygon with a hole where you want the new feature to be but note how, in creating the ring, it leaves the original outside edge alone (you need to digitize along the edge in this case because it is a ring in reality). Then, in the standard digitizing tools use Fill Ring With New Feature (interactive mode) - click on one of the vertices to tell QGIS which ring you mean. It then fills your hole with a new feature.
2. Digitize your lines as a line feature set in a separate layer and then use Split Selected Features with selected line(s) from another layer. For this to work well, extend your lines over the outside edge of your polygon.