A calibration system is provided for use in generating calibration data for calibrating an X-Y digitiser. The calibration system uses an alignment head having an array of magnetic field generators which are for alignment with the X-Y digitiser. Each of the magnetic field generators is individually drivable so that calibration data can be obtained for different positions on the X-Y digitiser corresponding to the positions of the magnetic field generators in the array. The resulting calibration data is then used to generate a correction map which is stored in the X-Y digitiser