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I'm trying to get a clean vector layer from an elevation raster layer. I'm using Raster > Extraction > Contour... to get lines from the elevation raster here:

enter image description here

This tool produces this layer (I kept only one elevation level to keep it simple here):

The lines produced by Contour

I'm then using Vector > Geometry tools > Lines to polygons to transform these lines into polygons and it produces this result:

enter image description here

The problem is: All the parts which are below the main elevation contour are filled. But those should be holes in the main polygon.

enter image description here

How can I get the polygon with the holes?

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I found a solution by using the Polygonize tool instead of the Contour one. This process is quiet slow though.

First, I use the Raster Calculator to isolate each level I want to show. Then I use the Polygonize (raster to Vector) and filter the level I want (in my case, 1). Then Vector geometry tools > Simplify geometry and finally the Smooth geometry tool.

It gives me this nice vector: enter image description here

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