device the measurements of the induced voltage amplitudes and the knowledge of the geometry and effect of the shield device provide sufficient information to compute the angular orientation and the positional coordinates of this sensing coil in the presence of the shield device definitions as used herein u201c sensing coil u201d refers to an electrically conductive magnetically sensitive element that is responsive to time dependent magnetic fields and generates an induced voltage as a function of and representative of the applied time dependent magnetic field the sensing coil is adaptable for secure engagement to the distal end of a catheter probe as used herein u201c tool coil u201d refers to an electrically conductive magnetically sensitive element that is responsive to time dependent magnetic fields and generates an induced voltage as a function of and representative of the applied time dependent magnetic field the tool coil is adaptable for secure engagement to an object with a ferromagnetic and conducting core as used herein u201c navigational domain u201d refers to a fully enclosed spatial region whose internal volume substantially encloses the complete prospective range of movement of the sensing coil the navigational domain may be defined by any geometrical space but preferably takes the form of a spherical volume under surgical operating conditions the navigational domain will correspond to an anatomical region of the recumbent patient where surgical viewing or investigation is desired e g a diseased area of tissue or an organ as used herein u201c peripheral domain u201d refers to the spatial region outside of the navigational domain under surgical operating conditions the peripheral domain may include the region that contains the operating table or the region that encompasses other equipment within the operating room as used herein u201c last navigational point u201d hereinafter u201c the.