In some aspects, a method includes (i) causing current to flow among multiple sets of current injecting electrodes to generate a field in an organ, (ii) obtaining the positions of one or more measuring electrodes used for measuring the field generated by the current injecting electrodes, (iii) in response to the current flow, measuring the field at multiple locations in the organ using the one or more measuring electrodes, (iv) modeling the field using the measurements of the field from the one or more measuring electrodes and the positions of the one or more measuring electrodes, and (v) determining expected signal measurements of the field at additional locations within the organ using the model of the field.