This altimeter either uses GPS or the built in barometer (available since iPhone 6) to determine your current elevation on earth. While GPS provides the absolute altitude as long as you see the sky it is not very accurate. With the help of the barometer we can calculate the altitude indoors and outdoors with a much higher accuracy. But we need a reference altitude to calibrate the barometer altitude. This calibration will degrade over time as the air pressure changes with the weather. Features: - GPS and barometer based altitude - set a base altitude as starting point and get the altitude difference (e.g for hiking or biking) - calibrate the barometer altitude (connect the current air pressure to your current altitude) - calibrate with the help of GPS, the standard sea level air pressure or to any altitude you want - time difference to the last calibration - warning when it is time to calibrate - switch between metric and imperial units The Apple Watch app: - GPS and barometer based altitude - set a base altitude as starting point and get the altitude difference (e.g for hiking or biking) - switch between metric and imperial units