of tool 24 are determined relative to field sensors such as location pads 34 which are fixed to the patient 39 s body and sense the magnetic fields generated by the position transmitters in the example shown in fig1 the pads are placed on the patient 39 s calf and thigh in proximity to implants 26 the location pads comprise sensing antennas such as coils alternatively or additionally the field sensors may be fixed to the operating table or to another structure in the vicinity of patient 23 the position indicative magnetic field generated by the position transmitter in tool 24 may induce eddy currents therein the eddy currents in turn generate a parasitic magnetic field that distorts the position indicative field location pads 34 detect the distorted field and transmit a corresponding position signal to a control unit such as signal processing console 38 console 38 derives the true position of the tool by determining the parasitic field present in the distorted field and subtracting the parasitic field from the distorted field to provide a more accurate representation of the position indicative field determination of the parasitic field is facilitated by a parasitic field map obtained in a calibration process described hereinbelow fig4 a tracking system computer 41.