A system for determining the disposition of a probe within the body of a patient includes a probe having probe field transducers and a plurality of reference field transducers. The reference field transducers are independently movable with respect to one another to desired, customizable positions close to the body of the patient. Calibration transducers determine the relative positions of the field transducers with respect to one another after they are located in their desired positions. Non-ionizing fields are transmitted and detected between the probe and reference field transducers. From the detected fields, the relative disposition of the probe with respect to the reference field transducers is determined.