A method for operating a navigation device. The navigation device carries out a position determination on the basis of received signals. In parallel, another position determination is carried out on the basis of signals from inertial sensors. These two determined positions are compared. If these two determined positions differ by more than a predetermined amount, which has been defined by a predetermined limit value, an error value is recognized. If an impermissible deviation is recognized, then especially the direction of movement, the speed differences and the position changes are taken into account. When the error value is recognized, shaded areas in the surroundings of the vehicle are determined and associated with the determined momentary position, as a result of which an association with a lane level is carried out.