A data processing method, performed by a computer, for determining the relative orientation of an object co-ordinate system of an anatomical object in a global co-ordinate system, comprising the steps of: - acquiring a reference direction dataset representing a first reference direction of a line between a first anatomical landmark of a reference object and a second anatomical landmark of the reference object, and a second reference direction of a line between a third anatomical landmark of the reference object and a fourth anatomical landmark of the reference object, wherein the first and second reference directions are given in a reference co-ordinate system and the reference object corresponds to the anatomical object; - acquiring an object direction dataset representing a first object direction of a line between the first anatomical landmark of the anatomical object and the second anatomical landmark of the anatomical object, and a second object direction of a line between the third anatomical landmark of the anatomical object and the fourth anatomical landmark of the anatomical object, wherein the first and second object directions are given in the global co-ordinate system and the landmarks (LM1, LM2, LM3) of the anatomical object correspond to the landmarks (LM1, LM2, LM3) of the reference object; - calculating the orientation of the reference co-ordinate system relative to the global co-ordinate system which results in the best match between each of the reference directions and the respectively corresponding object direction; and - using the orientation of the reference co-ordinate system in the global co-ordinate system as the relative orientation of the object co-ordinate system; characterized in that the object directions are determined from landmark directions, wherein a landmark direction is the direction of a line between a landmark (LM1; LM2; LM3) and a common reference point (P R) which is none of the landmarks (LM1; LM2; LM3) and which is static in the global co-ordinate system. - acquiring a reference direction dataset representing a first reference direction of a line between a first anatomical landmark of a reference object and a second anatomical landmark of the reference object, and a second reference direction of a line between a third anatomical landmark of the reference object and a fourth anatomical landmark of the reference object, wherein the first and second reference directions are given in a reference co-ordinate system and the reference object corresponds to the anatomical object; - acquiring an object direction dataset representing a first object direction of a line between the first anatomical landmark of the anatomical object and the second anatomical landmark of the anatomical object, and a second object direction of a line between the third anatomical landmark of the anatomical object and the fourth anatomical landmark of the anatomical object, wherein the first and second object directions are given in the global co-ordinate system and the landmarks (LM1, LM2, LM3) of the anatomical object correspond to the landmarks (LM1, LM2, LM3) of the reference object; - calculating the orientation of the reference co-ordinate system relative to the global co-ordinate system which results in the best match between each of the reference directions and the respectively corresponding object direction; and - using the orientation of the reference co-ordinate system in the global co-ordinate system as the relative orientation of the object co-ordinate system;