Abstract: Method and apparatus embodiments can acquire and update a 3-D surface of a dentition in real time by replacing the corresponding portion of the 3-D surface of the dentition with the contents of newly acquired 3-D image. In certain embodiments, the position of the 3-D scanning device relative to the 3-D surface of the dentition can be determined in real time by comparing the size and the shape of the overlap to the cross-section of the field-of-view of the 3-D scanning device, where the size and the shape of the overlap of the newly acquired 3-D image is used to determine the distance and the angles from which the 3-D image was acquired relative to the 3-D surface of the dentition.