This altimeter works online and offline. It estimates your elevation with three different methods:- Using GPS satellites triangulation. No internet connection required, quite accurate with clean sky but works only outdoors and it's slow.- Using the ground elevation at your current position. Accurate but require internet connection and works only if you are at ground level.- Using the pressure sensor of your device (if available). If it's available internet connection it calibrates itself on the nearest weather station pressure to improve accuracy.
The best choice for your excursion it's the pressure sensor method. But we recommend to launch the App just before leaving so it calibrates itself taking pressure from the nearest airport weather station. So measure will be reliable for few hours even without an internet connection.
You can also calibrate the sensor to eliminate systematic error.