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 scanning instrument having a partly spherical scanning tip, providing a topographic model of an object or body part having a model surface, calculating from the model surface of said topographic model a virtual model surface that has a distance from the model surface, which equals a radius of the partly spherical scanning tip of the scanning instrument, scanning a surface of the object or body part by means of said scanning instrument, detecting position values that are related to the sphere center of the scanning tip during the scanning, and assigning detected position values to points of said virtual surface of the topographic model.