Height above mean sea level is a measure of the
vertical distance (
height,
elevation
The elevation of a geographic 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 surface (see Geodetic datum § ...
or
altitude) 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 '' ...
taken as a
vertical datum. In
geodesy, it is formalized as ''
orthometric heights''.
The combination of
unit of measurement and the
physical quantity (height) is called "
metres above mean sea level" in the
metric system
The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Intern ...
, 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
landmarks.
* The top of
building
A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and func ...
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 a ...
s.
* 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 ...
s or
helicopters
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribu ...
.
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
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
s. 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 in
aerial photography.
* Aerial
lidar 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 technique ...
.
* Aerial or satellite
radar altimetry.
*
Surveying, especially
levelling.
Accurate measurement of historical mean sea levels is complex. Land mass
subsidence (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
*
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
*
Vertical metre
Vertical position or vertical location, also known as vertical level or simply level, is a position along a vertical direction above or below a given vertical datum (reference level).
Vertical distance or vertical separation is the distance betw ...
References
{{reflist
Geography terminology
Geodesy
Topography
Altitudes in aviation
Vertical position
Zero-level elevation points