QGIS – Create Singlepart Polygon from Multiple Polygons

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I would like to create one polygon including others polygons that has an ID < 16 .

enter image description here

The yellow polygons have ID < 16 and I would like to create a polygon like this:
enter image description here

EDIT : I do not want to get a multipart polygon, but a singlepart polygon and fill the spaces in between.

Probably not a good idea, but polygons being always alligned by ascendant numbering ( 1 – 2 – 3 – 4 – …) , a logics could be to join polygon ID 1 to the polygon ID 2, then the polygon ID 2 to the polygon ID 3, etc. by joinning the closest corners to the next ID polygon.

I've no idea how to accomplish this task, is there a way to automatize it?

EDIT :
the aim is to have only one polygon at the end

Best Answer

  1. Select by expression with the expression ID < 16

  2. Merge Selected Features

enter image description here

To further proceed to fill the space between the parts of the merged polygon to get a contiguous single-part polygon, you can use concave hull:

  1. Extract vertices (you might use this with Selected features only, then you can skip step 2 from above).

  2. Run Menu Processing > Toolbox > Concave Hull (alpha shapes) on the vertices layer. Set an appropriate value for Threshold - in my case, 0.14 produced the result shown below.

The pink polygon represents the output of Concave Hull (alpha shapes): enter image description here

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