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 . 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 topo map with the AMSL height shown in either metres or feet or both. In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case see Amsterdam Airport Schiphol . To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or datum , called the geoid . In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field. In reality, due to currents, air pressure variations, temperature and salinity variations, etc., this does not occur, not even as a long term average. The location-dependent, but persistent