relationship to the localizing emitters 137 computer models of the features of the tracking adapter 129 and im rod relative to coordinate frame a and graphic representations of features of the im rod 130 relative to coordinate frame a are stored in the computer 39 s long term memory a second coordinate frame z is defined relative to the localizing emitters 137 of the tracking device 129 and is preferably located on the im rod 130 halfway between the transverse holes 131 it is oriented with the z axis coincident to the long axis of the im rod 130 and the x axis parallel to the bore of the transverse holes 131 another coordinate frame g is selected relative the localizing emitters 137 such that its x axis and y axis define a picture plane 138 upon which instrument representations may be projected to form an image for display the z axis of coordinate frame g is preferably oriented such that it passes through the centers of both distal transverse holes 131 thus causing representations of both distal transverse holes 131 to project to the same location on the picture plane 138 alternatively the picture plane 138 may be selected in any pose that is near orthogonal e g within 20 degrees to the long axis of the im rod without departing from the instant invention further separate picture planes may be selected for each transverse hole through which the system is to assist the surgeon in inserting a screw with the tracking device 129 attached to the im rod 130 and its pose being read by the localizing device a graphic representation of the im rod 130 is projected onto the picture plane 138 defined by coordinate frame g because the picture plane 138.