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For example: I created the small polygon (red), now I want to create the bigger polygon (green) surrounding the small (common boundary). How to do this?

For example: I created the small polygon (red), now I want to create the bigger polygon (green) surrounding the small (common boundary).

How to do this using QGIS 2.18.4 ?

  • Do you already have a polygon that represents the red+green area? If you are trying to digitize the outer boundary then what steps have you tried when editing so far? – PolyGeo May 3 '17 at 11:49
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If I understand your question correctly you basically need to create a buffer around your red polygon, and then if you want a polygon of only that buffered area you need to erase the hole in the middle of the buffered area that is your original polygon.

In QGIS you use the vector > spatial analysis > buffer tool to create the buffer. You need to set the distance of the buffer to suit you (in the units of the projection you're working in). Then use the vector > geoprocessing > difference tool to find the difference between your buffered polygon and your original polygon. The output will be the buffered area only.

  • I want to create 2 polygons with different attribution. For example, the red polygon is "house" and the green polygon is "grass". – Việt Hoài May 3 '17 at 16:15
  • You then need to add an attribute field for each polygon and populate it with the correct values. You can do this easily for such a small example by opening the attribute table for each polygon and adding a new field. – scabecks May 3 '17 at 21:34
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@scabecks describes a perfectly valid approach that will work. However, here is a one-step alternative. Using the Multiple-ring Buffer plugin, set the number of rings to 1. It will then produce the doughnut without the need for the differencing step.

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