I have a global DEM and try to calculate contour polygons for 4 different heights in QGIS. In the first image below, you can see the colored DEM (white = lowest; yellow = higher; orange = highest) as well as the overlying contour lines. The white area inside the yellow area is a sink.

DEM with overlying contour lines. White area is a sink in the higher, yellow area

After this i did convert the contour lines into polygons with the "Lines to polygons" function. Unfortunately, the sinks are not cut out in the polygon and remain the same class like the surrounding as you can see in the image below. The white areas from above have now the same class as the yellow area (its all pink).
enter image description here

Can someone explain me how I can archive, that the white area would have another class after polygonizing?

enter image description here

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  • How are the different values coded (what are your classes)? Is your lowest class assigned a zero value? Could you add a sample screenshot of the attribute table of your polygons? – user3386170 Jan 9 '19 at 14:13
  • I did create the contour lines by SAGA "Contour lines" with a minimum contour value of 0 and a maximum value of 9000 with a 3000 equidistance. This creates 3 different classes and the lowest one has a value of 3000. Screenshot is edited to the question – Fabio Jan 9 '19 at 14:49

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