An apparatus includes an interface and a processor. Theinterface is configured to receive signals from a calibrationprobe inserted in an organ of a patient. The processor isconfigured to hold multiple electrode positions correspondingrespectively to multiple electrodes attached externally to thepatient, to evaluate, based on the received signals, multipledata points, each data point including (i) a respectivemeasured coordinate of the calibration probe, and (ii) arespective set of electrical values indicative of respectiveimpedances between the calibration probe and the multipleelectrodes. The processor is further configured to evaluate,based on the multiple data points and on the electrodepositions, an alternative position for a selected electrode tobe repositioned, and to output an identification of theselected electrode and the evaluated alternative position.
