The topology of a filling material surface is first determined by sampling the surface of the filling material in order to determine the fill level. When calculating the surface topology of the filling material, the measurement signal, which has been reflected at the filling material surface and has been picked up by the antenna unit of the fill level measurement device, is evaluated, taking into account the distance between the source of the measurement signal and a center of rotation of the main emission axis of the antenna. This makes it possible to accurately determine the fill level of bulk materials, even if the source of the measurement signal and the center of rotation of the main emission axis do not coincide.