Height above mean sea level is a measure of the
vertical distance (
height,
elevation or
altitude) of a location in reference to a historic
mean sea level taken as a
vertical datum. In
geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...
, 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 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 locations 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
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.
* Flying objects such as
airplanes 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 is needed to convert the 3D position to sea-level elevation.
*
Altimeter, that measures
atmospheric pressure, 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
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
Satellite geodesy is geodesy by means of artificial satellites—the measurement of the form and dimensions of Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques. ...
.
* Aerial or satellite
radar altimetry.
*
Surveying
Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...
, especially
levelling.
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. 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
*
Height above average terrain
*
Height above ground level
*
List of places on land with elevations below sea level
*
Vertical metre
References
{{reflist
Geography terminology
Geodesy
Topography
Altitudes in aviation
Vertical position
Zero-level elevation points