A technique for determining a transformation between a navigation reference coordinate system (302) for navigation of a surgical device (150) relative to patient image data and an image coordinate system (304) in which the patient image data define a shape of a patient surface is provided. A computer-implemented method implementation of that technique comprises receiving multiple data sets that have been taken from different perspectives of the patient surface. Feature coordinates of multiple features (170) identifiable in the picture data sets are determined from the picture data sets and in the navigation reference coordinate system (302). From the feature coordinates, a shape model of the patient surface in the navigation reference coordinate system (302) is determined. Then, surface matching between the shape model and the shape of the patient surface defined by the patient image data is applied to determine the transformation (T1) between the navigation reference coordinate system (302) and the image coordinate system (304).
