An apparatus for improving the accuracy and sensitivity of impedance-based position tracking, comprising: an interface, configured to receive signals from a calibration probe inserted in an organ of a patient; and a processor, configured to: hold multiple electrode positions corresponding respectively to multiple electrodes attached externally to the patient; evaluate, based on the received signals, multiple data points, each data point comprising (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, wherein the data points are representable as a three-dimensional cluster; based on the multiple data points and on the electrode positions, select an electrode to be repositioned and determine an alternative position for the selected electrode that provides improved accuracy and sensitivity of impedance-based position tracking, by projecting the 3D cluster onto a selected two-dimensional coordinate system, the 2D coordinate system comprising first and second axes; evaluating a 2D cluster produced by projected data points relative to the first and second axes, the 2D cluster comprising an ellipse whose axes are defined by the first and second axes; calculating a ratio between lengths of the first and second axes within the 2D shape; and output an identification of the selected electrode and the determined alternative position. an interface, configured to receive signals from a calibration probe inserted in an organ of a patient; and a processor, configured to: hold multiple electrode positions corresponding respectively to multiple electrodes attached externally to the patient; evaluate, based on the received signals, multiple data points, each data point comprising (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, wherein the data points are representable as a three-dimensional cluster; based on the multiple data points and on the electrode positions, select an electrode to be repositioned and determine an alternative position for the selected electrode that provides improved accuracy and sensitivity of impedance-based position tracking, by projecting the 3D cluster onto a selected two-dimensional coordinate system, the 2D coordinate system comprising first and second axes; evaluating a 2D cluster produced by projected data points relative to the first and second axes, the 2D cluster comprising an ellipse whose axes are defined by the first and second axes; calculating a ratio between lengths of the first and second axes within the 2D shape; and output an identification of the selected electrode and the determined alternative position. hold multiple electrode positions corresponding respectively to multiple electrodes attached externally to the patient; evaluate, based on the received signals, multiple data points, each data point comprising (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, wherein the data points are representable as a three-dimensional cluster; based on the multiple data points and on the electrode positions, select an electrode to be repositioned and determine an alternative position for the selected electrode that provides improved accuracy and sensitivity of impedance-based position tracking, by projecting the 3D cluster onto a selected two-dimensional coordinate system, the 2D coordinate system comprising first and second axes; evaluating a 2D cluster produced by projected data points relative to the first and second axes, the 2D cluster comprising an ellipse whose axes are defined by the first and second axes; calculating a ratio between lengths of the first and second axes within the 2D shape; and output an identification of the selected electrode and the determined alternative position.