Disclosed is a method for defining a common reference system in a record of volume data that represents an area of a patient's jaw and is captured by means of an X-ray imaging process and a record of surface data, at least some of which represents the same area of the patient's jaw and which is captured by means of a process for measuring visible surfaces. Volume data and surface data are unhidden on a screen. An object, especially a tooth, which is recognizable in both the volume data and the surface data, is superimposed on each other as congruently as possible in a preliminary positioning step. A volume structure characterizing the object is extracted from the volume data, particularly as a type of edge image, and is made to overlap as much as possible with a corresponding surface structure of the surface data by means of a transformation function, the overlap of the volume structure being adjusted to the surface structure in iterative steps by optimizing a predefined quality level.