A method and apparatus are disclosed. The apparatus includes a store of at least one historical sensor measurement, a store of potential cut-off frequencies, and a filter. Each potential cut-off frequency is associated with a respective potential difference between a sensor measurement and the stored historical sensor measurement. The filter has a cut-off frequency dynamically selected from the stored potential cut-off frequencies on the basis of a difference between the stored historical sensor measurement and a current sensor measurement. The method includes determining a difference between a current sensor measurement and a historical sensor measurement; and dynamically selecting a cut-off frequency for a filter for the current sensor measurement from the difference.