<P> Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to the average sea level datum . It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with flight level), and in the atmospheric sciences, and land surveying . An alternative is to base height measurements on an ellipsoid of the entire Earth, which is what systems such as GPS do . In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and mean tidal height . The alternative is to use a geoid - based vertical datum such as NAVD88 . </P> <P> When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines . The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both . </P> <P> In the rare case that a location is below sea level, the elevation AMSL is negative . For one such case, see Amsterdam Airport Schiphol . </P> <Ol> <Li> Ocean </Li> <Li> Reference ellipsoid </Li> <Li> Local plumb line </Li> <Li> Continent </Li> <Li> Geoid </Li> </Ol>

How much of earth's surface area is below sea level