A data processing method for determining rotation by a marker device 1 of a medical tracking system 10 about its mount 3, comprising the steps of: acquiring a first marker device position dataset which represents the position of the marker device 1 before a movement of the marker device 1； acquiring a second marker device position dataset which represents the position of the marker device 1 after a movement of the marker device 1； calculating a marker device displacement dataset, which represents the displacement of the marker device 1, from the first and second marker device position datasets； calculating at least one axis of rotation 6, 9 of the marker device displacement dataset； and determining whether or not the marker device 1 is rotated about its mount 3 from the position of the at least one axis of rotation 6, 9 relative to the marker device 1.
