GPS uses a simplified smooth Earth surface called ellipsoid for positioning, including altitude. However, the planet is not smooth and therefore the difference between the actual user altitude and the GPS altitude can make many tens of meters!

We use Earth Gravitational Model 2008 (EGM2008) NGA data to calculate the geoid deviation from the ellipsoid surface for any location on the Earth's surface. All database is stored directly in the application and so the whole system works without the need for an Internet connection.