I have a small problem now. I have now drawn an even surface with Cad based on the point cloud. But in order to be able to use all the material that is stored above, I would have to raise a few points of the drawn surface.
How can I possibly raise some points by 20 cm so that all the material has space?

In your case, I would simply add more break-lines to define the bottom face.

Using these lines I got this bottom surface:

The volume calculation then looked like this:


This result depends a bit on how we define the bottom surface with break-lines. For best results, we would ideally take a first measurement of the terrain with no material, and then take a second measurement when the material is present. Of course, this is not always possible. In this case, we then have to define the underside as well, if possible with broken lines and points that are still visible.