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 Ad supported freeware