1. A system for determining a transformation between a navigation reference coordinate system for navigation of a surgical device relative to patient image data and an image coordinate system in which the patient image data define a shape of a patient surface of a patient, the system comprising: a first camera movable relative to the patient upon taking picture data sets from different perspectives; an interface adapted to receive the picture data sets from the first camera, wherein the first camera is a video camera and the picture data sets are received from the video camera in the form of a video data stream; and a processor adapted to determine, from the picture data sets and in the navigation reference coordinate system, feature coordinates of multiple features identifiable in the picture data sets, to determine, from the feature coordinates, a shape model of the patient surface in the navigation reference coordinate system, and to determine a transformation between the navigation reference coordinate system and the image coordinate system using surface matching between the shape model and the shape of the patient surface defined by the patient image data.