A method, consisting of generating, using aplurality of magnetic transmitters, a magnetic field in aregion and introducing a field perturbing element intothe region. The method includes characterizing multipleimages of each magnetic transmitter in the fieldperturbing element, and calculating a reaction magneticfield in the region based on the characterized images.The method further includes positioning a probe in theregion and measuring a perturbed magnetic field at theprobe, and determining a location of the probe inresponse to the measured perturbed magnetic field and thecalculated reaction magnetic field.
