Above mean sea level Definition
It is not possible to determine the average sea level for the entire planet. The sea level also changes over time when measured at just one location. This is because the sea is in constant motion, affected by the high and low pressure zones above it, the tides, local gravitational differences, and other reasons. People can only pick a spot and calculate the mean sea level at that point and use it as a datum. An alternative is to base height measurements on an ellipsoid of the entire earth. GPS and other satellite systems do this. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define mean sea level. Another alternative is to use a geoid based datum such as NAVD88.