There is provided a method of monitoring a change in height of a device, the device comprising a sensor for measuring the air pressure at the device, the method comprising determining a pressure change threshold from a pre-determined height change and an estimate or measurement of air pressure obtaining a plurality of measurements of the air pressure at the device determining a change in the air pressure from two or more of the measurements and determining if the height of the device has changed by more than the pre-determined height change using the determined change in the air pressure and the determined pressure change threshold.
