A method includes, in a calibration phase, positioninga calibration-tool, including a mapping-electrode and asensor of a location-measuring system, in an organ. Thecalibration-tool is tracked at different positions in theorgan using the location-measuring system. A set ofcalibration data points is generated at the respectivedifferent positions, each calibration data point includingsignal-values obtained using the mapping-electrode and acorresponding position measurement of the sensor by thelocation-measuring system. The method further includes, inan investigation phase that is subsequent to the calibrationphase, positioning an investigation-tool at a location inthe organ. The signal-values at the location are measuredusing a mapping-electrode of the investigation-tool. Using asubset of the calibration data points, coordinates of thelocation are determined by deriving a respective nonlinearrelation between the signal-values obtained using themapping-electrode of the investigation-tool, and thecoordinates of the location and solving the nonlinearrelation.
