Editing Sea level (section)

=== {{Anchor|Above mean sea level|AMSL}} Height above mean sea level === <!-- Other articles link here. -->
{{Main|Height above mean sea level}}

''Height above mean sea level'' (''AMSL'') is the elevation (on the ground) or altitude (in the air) of an object, relative to a reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with [[flight level]]), and in the [[atmospheric sciences]], and in [[land surveying]]. An alternative is to base height measurements on a [[reference ellipsoid]] approximating the entire Earth, which is what systems such as [[GPS]] do. In aviation, the reference ellipsoid known as [[WGS84]] is increasingly used to define heights; however, differences up to {{convert|100|m|0|abbr=off}} exist between this ellipsoid height and local mean sea level.<ref name="Geoid"/> Another alternative is to use a [[geoid]]-based vertical [[datum (geodesy)|datum]] such as [[North American Vertical Datum of 1988|NAVD88]] and the global [[EGM96]] (part of WGS84). Details vary in different countries.

When referring to [[geographic]] features such as mountains, on a [[topographic map]] variations in elevation are shown by [[contour line]]s. A mountain's highest point or summit is typically illustrated with the AMSL height in metres, feet or both. In unusual cases where a land location is below sea level, such as [[Death Valley, California]], the elevation AMSL is negative.

==== Difficulties in use ====
[[File:Geoida.svg|thumb|{{ordered list |Ocean |[[Reference ellipsoid]] |Local [[Plumb-bob|plumb line]] |[[Continent]] |[[Geoid]]}}]]

It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the [[geoid]]. In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's [[gravitation]]al field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits [[gravity anomalies]] such as those measured by NASA's [[GRACE satellites]]. In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and the geoid is referred to as (mean) [[ocean surface topography]]. It varies globally in a typical range of ±{{convert|1|m|ft|0|abbr=on}}.<ref>{{cite web |url=https://science.nasa.gov/earth/climate-change/sea-level-101-what-determines-the-level-of-the-sea/#h-waves-in-the-bathtub |title=Sea Level 101: What Determines the Level of the Sea? |publisher=NASA |date=3 June 2020 |access-date=17 April 2024}}</ref>