Simple pressure altimeter for Android
This app uses the built-in pressure meter on your Android device and computes your altitude.
This is a "sensitive altimeter" (https://en.wikipedia.org/wiki/Altimeter#Use_in_aircraft) which means that you calibrate it by entering the nominal sea level pressure. In aviation use, the sea level pressure is normally obtained via radio information broadcasts (ATIS) which provide the altimeter setting for the local airport.