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I want to manually trace over an existing map image to replicate the boundaries using QGIS. If this was an existing polygon and I need to split this into 3 separate polygons, I would take the existing outer boundary, and then I would have to manually trace the boundaries of the inner areas - as a polyline or region? How would I join the inner boundaries to the outer boundary, and then create the 3 separate polygons within the base map table? I hope I have explained this clearly?

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  • If I understood you correctly, my answer is what you are after. If so, the heading of your question does not reflect what you want to do. Please, edit it to help others to find similar questions
    – Marco
    Commented Apr 27, 2018 at 12:03

2 Answers 2

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You are after the Polygonize tool:

This algorithm takes a lines layer and creates a polygon layer, with polygons generated from the lines in the input layer.

You first need to convert you polygon to lines:

enter image description here

Then, drawing the new lines dividing the boundaries: enter image description here

And here comes the Polygonize in the Processing Toolbox: enter image description here

Voilà!

enter image description here

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  • This is a lot more steps than using the Split tool. Is there any advantage to this method?
    – csk
    Commented Apr 27, 2018 at 16:17
  • Please, go ahead with the explanation of the method, I do not know how to use it for this task
    – Marco
    Commented Apr 27, 2018 at 21:38
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Boundaries are usually polygons that only borders are visible. Duplicate the layer file with your big polygon, split one into multiple polygons and then edit the style to only show the borders:

Outer boundary Split, inner boundaries Style

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