Dissolving polygons contained within the same larger polygon in QGIS

Question

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.

Answer 1

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.

Answer 2

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:

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))

Answer 3

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.