To generate accurate digital 3D models by CT or CBCT scanning of an impression and to provide a method which can be used so that acquisition of another optical operation device does not become necessary to be able to create an accurate digital 3D surface model.

In order create a digital surface model of an intra-oral anatomy, a recess or a hole having a predetermined shape and size is scanned in an impression material. The scanning results and information on the predetermined shape and size are used for determining a threshold value representing an interface between the impression material and another material. The threshold value is then used in generating the digital surface model from CT scanning result of an impression of an intra-oral anatomy on the impression material.