A method includes, in a calibration phase, positioning a calibration-tool, including a mapping-electrode and a sensor of a location-measuring system, in an organ. The calibration-tool is tracked at different positions in the organ using the location-measuring system. A set of calibration data points is generated at the respective different positions, each calibration data point including signal-values obtained using the mapping-electrode and a corresponding position measurement of the sensor by the location-measuring system. The method further includes, in an investigation phase that is subsequent to the calibration phase, positioning an investigation-tool at a location in the organ. The signal-values at the location are measured using a mapping-electrode of the investigation-tool. Using a subset of the calibration data points, coordinates of the location are determined by deriving a respective nonlinear relation between the signal-values obtained using the mapping-electrode of the investigation-tool, and the coordinates of the location and solving the nonlinear relation.
