In some aspects, a method includes (i) securing multiple sets of current injecting electrodes to an organ in a patient's body, (ii) causing current to flow among the multiple sets of current injecting electrodes to generate a field in the organ, (iii) in response to current flow caused by the multiple sets of current injecting electrodes, measuring the field at each of one or more additional electrodes, (iv) determining expected signal measurements of the field inside the organ using a pre-determined model of the field, and (v) determining a position of each of the one or more additional electrodes in the organ based on the measurements made by the additional electrodes and the determined expected signal measurements of the field.