– Using GPS satellites triangulation. Slow and not accurate but it works even with no internet connection
– 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.
Related tags: download Accurate Altimeter Free, Accurate Altimeter Free for Android