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Height above mean sea level is a measure of the vertical distance ( height, elevation or
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
) of a location in reference to a historic
mean sea level There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value ( magnitude and sign) of a given data set. For a data set, the ''arith ...
taken as a vertical datum. In geodesy, it is formalized as '' orthometric heights''. The combination of unit of measurement and the
physical quantity A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For examp ...
(height) is called " metres above mean sea level" in the metric system, while in United States customary and imperial units it would be called "
feet The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...
above mean sea level". Mean sea levels are affected by climate change and other factors and change over time. For this and other reasons, recorded measurements of elevation above sea level at a reference time in history might differ from the actual elevation of a given location over sea level at a given moment.


Uses

Metres above sea level is the standard measurement of the elevation or altitude of: *
Geographic location In geography, location or place are used to denote a region (point, line, or area) on Earth's surface or elsewhere. The term ''location'' generally implies a higher degree of certainty than ''place'', the latter often indicating an entity with an ...
s such as towns, mountains and other
landmark A landmark is a recognizable natural or artificial feature used for navigation, a feature that stands out from its near environment and is often visible from long distances. In modern use, the term can also be applied to smaller structures or f ...
s. * The top of buildings and other structures. * Flying objects such as
airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocket engine. Airplanes come in a variety of sizes, shapes, and wing configurations. The broad spectr ...
s or helicopters.


How it is determined

The elevation or altitude in metres above sea level of a location, object, or point can be determined in a number of ways. The most common include: * Global Navigation Satellite System (like GPS), where a receiver determines a location from pseudoranges to multiple satellites. A
geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extende ...
is needed to convert the 3D position to sea-level elevation. *
Altimeter An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
, that measures
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
, which decreases as altitude increases. As atmospheric pressure changes with the weather too, a recent local measure of the pressure at a known altitude is needed to calibrate the altimeter. *
Stereoscopy Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...
in
aerial photography Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other airborne platforms. When taking motion pictures, it is also known as aerial videography. Platforms for aerial photography include fixed-wing airc ...
. * Aerial
lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
and satellite laser altimetry. * Aerial or satellite radar altimetry. * Surveying, especially
levelling Levelling or leveling (American English; see spelling differences) is a branch of surveying, the object of which is to establish or verify or measure the height of specified points relative to a datum. It is widely used in geodesy and cartogra ...
. Accurate measurement of historical mean sea levels is complex. Land mass
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
(as occurs naturally in some regions) can give the appearance of
rising sea levels Rising may refer to: * Rising, a stage in baking - see Proofing (baking technique) *Elevation * Short for Uprising, a rebellion Film and TV * "Rising" (''Stargate Atlantis''), the series premiere of the science fiction television program ''Starg ...
. Conversely, markings on land masses that are uplifted due to geological processes can suggest a lowering of mean sea level.


Abbreviations

Often, just the abbreviation MSL is used, e.g. Mount Everest (8848 m MSL), or the reference height is omitted completely, e.g. Mount Everest (8848 m). Metres above mean sea level is commonly abbreviated mamsl or MAMSL, based on the abbreviation AMSL for "above mean sea level". Other abbreviations are m.a.s.l. and MASL for "metres above sea level".


See also

* Depth below seafloor *
Geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extende ...
* Height above average terrain *
Height above ground level In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL or HAGL) is a height measured with respect to the underlying ground surface. This is as opposed to height above mean sea level (AMSL or HAMSL), height above ell ...
*
List of places on land with elevations below sea level This is a list of places on land below mean sea level. Places artificially created such as tunnels, mines, basements, and dug holes, or places under water, or existing temporarily as a result of ebbing of sea tide etc., are not included. Places ...
* Vertical metre


References

{{reflist Geography terminology Geodesy Topography Altitudes in aviation Vertical position Zero-level elevation points