Altitude is different from the qualities of tall and short. Mount Everest has the highest altitude of all mountains, because its highest point is higher above sea level than any other mountain. However, it is not the tallest mountain, which is measured from base to top.
Sea level variations occur with gravity, temperature, weather and currents, but the sea level throughout the world is generally consistent. The elevation above sea level is based on the average of these measurements, which are taken over 19 years, and the height of a place or landmass above it. The period of 19 years is a Metonic cycle.Learn more about Maps & Cartography