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 predetermined 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 predetermined height change using the determined change in the air pressure and the determined pressure change threshold.