A data processing method for determining the positional information of characteristic points of a leg, the method comprising the following steps performed by a computer: a) acquiring, by detecting via a hand-held device a stationary reference (R3) and at least one further information, at least four different positions of the femur (F), wherein the pelvis within which the femur (F) can turn is stationary with respect to the stationary reference (R3) and the femur (F) is in a different position each time a positional information value of the femur (F) is acquired； b) determining from the at least four different acquired positional information values of the femur (F) the position of the center of rotation (COR) of the femoral head in relation to a femur reference (R1, R4)； c) acquiring a femur information by detecting via a hand-held device a femur reference (R1), and at least one further information； d) determining from the femur information and the at least one further information acquired in step c) the distal end point of the femur axis and the proximal end point of the tibia axis at least in relation to the femur reference (R1)； and e) determining the distal end point of the tibia axis by acquiring via a hand-held device the positional information of an ankle reference (R2) being at the distal end point of the tibia axis.
