1. A method of determining a reference object coordinate transformation between a surgical operation navigation system and an X-ray system having a pivotable C-arm designed to detect an interior of an object, the method which comprises: arranging marking elements of a reference object in a region that is detected by the surgical operation navigation system and in at least two pivoting positions of the C-arm by the X-ray system; detecting a position of the marking elements with the surgical operation navigation system; detecting the marking elements with the X-ray system from at least two projections; wherein each projection is taken from a different pivoting position of the C-arm; determining a position of the reference object from at least two two-dimensional projection images; and calculating the reference object coordinate transformation from the position of the marking elements detected by the surgical operation navigation system and the position of the marking elements determined by the X-ray system.