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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.

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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.

  1. 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).
  2. 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).
  3. Now select both the external portions and stitch them back together (Result: One whole 'outside' polygon and one whole 'inside' polygon)

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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.

  • Yes this two ways i've also discovered but i've hoped that i'd missed smth more simole. Thanks – laechoppe Jan 10 '14 at 18:00

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