I can split polygon if I put the points of start and end outside the boundaries. How can I do the same if I have a part that I want to split completely inside a polygon. In Arcgis I can do it if the splitting line is self-intersecting but it doesn't work in QGIS. I'm interesting in the existenceof a simple and quick way.
Sadly the self-intersecting cut-line method doesn't work in QGIS as you've discovered. However there is a slightly messy work-around that has helped me in the past.
- Cut from the edge of your polygon in such a way as to create one edge of your internal polygon. (Result: two polygons where part of the 'internal' edge matches part of the 'insdie' polygon boundary).
- With snapping turned on, draw a cut-line tracing the remainder of the boundary of your internal polygon (Result: Three polygons where one exactly equals the whole 'inside' polygon and the other two are halves of the 'outside' polygon).
- Now select both the external portions and stitch them back together (Result: One whole 'outside' polygon and one whole 'inside' polygon)
Alternatively you could try adding a ring that describes your 'inside' polygon and then (ensuring you have snapping on for vertices) digitize a new feature by tracing the interior boundary of your doughnut.