1. A method for assigning position values, detected by means of a position detection system, to a topographic image of an object or body part, the method comprising the following steps: providing a topographic model of an object or body part, scanning a surface of the object or body part by means of a scanning instrument, which has a scanning tip with a partly spherical external surface, detecting position values during the scanning and assigning detected position values to corresponding positions in the topographic model, the detected position values being assigned to points of a virtual surface of the topographic model, which has a distance from a model surface which equals a radius of a partly spherical scanning tip of a scanning instrument, by means of which the surface of the object or body part is scanned in each case.