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There is a layer in QGIS that contains multiple polygons, and I want to dissolve the polygons which are inside another polygon.

I don't want to dissolve the entire layer of polygons. For example, in the following map, is there a way in the QGIS to make it two polygons only by combining any inner polygon with the bigger one containing it?

I want to select the polygons automatically. It is very hard to select polygons manually since there are more than 1.2K polygons.

enter image description here

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  • 1
    What have you tried?
    – Taras
    Nov 27 '21 at 20:25
  • @Taras Run Vector -> Geoprocessing Tools -> Dissolve Nov 28 '21 at 11:54
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To merge all the features of layer 'polygon2' inside the same same polygon from layer 'polygon1' automatically (without manual selection), use Menu Processing / Toolbox / Aggregate (see documentation) with 'polygon2' as input layer and this expression as Group by expression:

array_first(overlay_within('polygon1', $id))

2 purple polygons = layer 'polygon1'; 6 yellow polygons = layer 'polygon2'; aggregated result consists of two features: A) 4 part MultiPolgon with blue outline, B) 2 part MultiPolygon with red outline:

enter image description here

If the polygons are all on the same layer, you can use the following trick:

Buffer the input layer with a (very small) negative buffer of like 0.1 meters. Then set the initial polygon layer as input layer in the Aggregate tool and the buffered layer in the expression:

array_first(overlay_within('buffer', $id))
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  • Thank you. But unfortunately, all polygons are on the same layer. Nov 28 '21 at 13:27
  • Then simply replace the layer's name in the expression by your polygon layer's name - in my case: polygon2 instead of polygon1 (or use the variable @layer to get the current layer). Or even easier: dissolve the whole layer and run multipart to single parts.
    – Babel
    Nov 28 '21 at 13:30
  • Added a solution in case all polygons are on the same layer, see edited answer at the bottom.
    – Babel
    Nov 28 '21 at 14:09
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To get as a different layer from the original one, use the following steps.

  1. Select the polygons you want to dissolve

  2. Run "Vector Geometry" -> "Dissolve tool". (Check "Selected features only")

  3. Then run "Vector geometry" -> "Multipart to singleparts" tool for the dissolved layer.

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If you want to dissolve in-place:

  1. Start edit mode for the layer.

  2. Select the bounding and interior polygons

  3. Click "Merge selected features" button () in the Advanced digitizing Toolbar.

  4. Repeat 2-3 for all bounding and interior polygons separately.

enter image description here

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  • Thank you. is there a way to select polygons automatically? Nov 28 '21 at 11:54

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