Altimeter

Most of the modern smartphones now contain barometric sensors (to help with GPS) which is how this Altimeter works. If you’re in a situation where you need to know how high up you are, this is the app to use. It actually uses either the barometric or GPS sensor, so there’s dual fallback in case you need to land that plane in an emergency. We couldn’t find a handy mountain to test, but it definitely worked up two flights of stairs.