1. Process for defining a common reference system in a set of volume data that represents an area of a patient's jaw and that is recorded with an x-ray imaging process, and a set of surface data, at least some of which represents the same area of the patient's jaw and which is recorded with a process for measuring visible surfaces, comprising: integrating the volume data and surface data on a screen, in a prepositioning, superimposing an object detected both in the volume data and in the surface data, as precisely as possible, extracting a volume structure that characterizes the object, in particular in the form of an edge image, from the volume data, and said edge image is superimposed as much as possible by means of a transformation function with a corresponding surface structure of the surface data, and matching the overlap of the volume structure under optimization of a specified quality criterion in iterative steps to the surface structure.