An apparatus includes an interface and a processor. The interface is configured to receive signals from a calibration probe inserted in an organ of a patient. The processor is configured to hold multiple electrode positions corresponding respectively to multiple electrodes attached externally to the patient, to evaluate, based on the received signals, multiple data points, each data point including (i) a respective measured coordinate of the calibration probe, and (ii) a respective set of electrical values indicative of respective impedances between the calibration probe and the multiple electrodes. The processor is further configured to evaluate, based on the multiple data points and on the electrode positions, an alternative position for a selected electrode to be repositioned, and to output an identification of the selected electrode and the evaluated alternative position.
