1. A medical system for treating a patient comprising: a plurality of transducers operable to determine the location of a medical device in a patient, the transducers are electromagnetic and include one or more device transducers and one or more external transducers; a medical device for insertion into a patient, the medical device having the one or more device transducers; an external locator operable outside and adjacent to the patient, the external locator having the one or more external transducers; a driver connected to sequentially energize one or more of the device transducers and/or the external transducers; a processor operably connected to receive sensor signals from one or more of the transducers, each sensor signal on a given one of the transducers depending on the energizing of another of the transducers, the processor configured to iteratively determine at least two location parameters of the medical device in a patient using a plurality of equations that remain valid for zero separation distance from the energized transducer to the transducer generating the sensor signal for validity.