Height above mean sea level is a measure of a location's
vertical distance (
height
Height is measure of vertical distance, either vertical extent (how "tall" something or someone is) or vertical position (how "high" a point is). For an example of vertical extent, "This basketball player is 7 foot 1 inches in height." For an e ...
,
elevation
The elevation of a geographic location (geography), ''location'' is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational equipotenti ...
or
altitude
Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
) in reference to a
vertical datum based on a historic
mean sea level
A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...
. In
geodesy
Geodesy or geodetics is the science of measuring and representing the Figure of the Earth, geometry, Gravity of Earth, gravity, and Earth's rotation, spatial orientation of the Earth in Relative change, temporally varying Three-dimensional spac ...
, it is formalized as
orthometric height. The zero level varies in different countries due to different reference points and historic measurement periods.
Climate change
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
and
other forces can cause sea levels and elevations to vary over time.
Uses
Elevation or altitude above sea level is a standard measurement for:
*
Geographic locations such as towns, mountains and other
landmarks.
* The top of
building
A building or edifice is an enclosed Structure#Load-bearing, structure with a roof, walls and window, windows, usually standing permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shapes, and functions, a ...
s and other
structure
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
s.
*
Mining
Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...
infrastructure, particularly underground.
* Flying objects such as
airplane
An airplane (American English), or aeroplane (Commonwealth English), informally plane, is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, Propeller (aircraft), propeller, or rocket engine. Airplanes come in a vari ...
s or
helicopters below a
Transition Altitude defined by local regulations.
Units and abbreviations
Elevation or altitude is generally expressed as "
metre
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...
s above mean sea level" in the
metric system
The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
, or "
feet above mean sea level" in
United States customary and
imperial units
The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thr ...
. Common abbreviations in English are:
* AMSL – above mean sea level
* ASL – above sea level
* FAMSL – feet above mean sea level
* FASL – feet above sea level
* MAMSL – metres above mean sea level
[
* MASL – metres above sea level][
* MSL – mean sea level][
For elevations or altitudes, often just the abbreviation MSL is used, e.g., ]Mount Everest
Mount Everest (), known locally as Sagarmatha in Nepal and Qomolangma in Tibet, is Earth's highest mountain above sea level. It lies in the Mahalangur Himal sub-range of the Himalayas and marks part of the China–Nepal border at it ...
(8849 m MSL), or the reference to sea level is omitted completely, e.g., Mount Everest (8849 m).
Methods of measurement
Altimetry is the measurement of altitude or elevation above sea level. Common techniques are:
* Surveying
Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...
, especially levelling.
* Global Navigation Satellite System
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are op ...
(such as GPS), where a receiver determines a location from pseudoranges to multiple satellite
A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
s. 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 exte ...
is needed to convert the 3D position to sea-level elevation.
* Pressure altimeter
Altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude, the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is a type of altim ...
measuring atmospheric pressure
Atmospheric pressure, also known as air pressure or 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 1,013. ...
, which decreases as altitude increases. Since atmospheric pressure varies 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 depth perception, illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any ster ...
in aerial photography
Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other flight, airborne platforms. When taking motion pictures, it is also known as aerial videography.
Platforms for aerial photography include fixed-wi ...
.
* Aerial lidar
Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...
and satellite laser altimetry.
* Aerial or satellite radar altimetry.
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 mov ...
(as occurs naturally in some regions) can give the appearance of rising sea levels. Conversely, markings on land masses that are uplifted (due to geological processes) can suggest a relative lowering of mean sea level.
See also
* Depth below seafloor
* Height above average terrain
Height above average terrain (HAAT), or (less popularly) effective height above average terrain (EHAAT), is the vertical position of an antenna site above the surrounding landscape. HAAT is used extensively in FM radio and television, as it is ...
* 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 ...
References
{{Authority control
Geography terminology
Geodesy
Topography
Altitudes in aviation
Vertical position
Vertical datums