A position monitoring system comprises a fiducial reference, a tracker for obtaining image information of surgical site, and a computer system with processor, memory, a software program, and access to a database with scan data of the surgical site with the fiducial reference fixed to the site. Uniquely identifiable tracking markers are fixed to the fiducial reference and to a surgical implement. The computer determines the relative positions and orientations of the markers based on live image information from the tracker, relates and displays on one or more display systems the current position and orientation of the fiducial reference and the surgical implement. The computer renders on the one or more display systems a surgical guidance graphic user interface showing planned and actual locations for the surgical implement to impinge on the area of surgical patient, an allowed range of angles of incidence for the surgical implement, and an actual instantaneous angle of incidence of the surgical implement. When the implement is a drill, the graphic user interface may display the drilling depth.
