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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 ...
,
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 ...
,
hydrography Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary ...
and
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...
, vertical datum or altimetric datum is a reference coordinate surface used for
vertical position Vertical position or vertical location is a position (mathematics), position along a vertical direction (the plumb line direction) above or below a given vertical datum (a reference level surface, such as mean sea level). Vertical distance or vert ...
s, such as the
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 ...
s of Earth-bound features (
terrain Terrain (), alternatively relief or topographical relief, is the dimension and shape of a given surface of land. In physical geography, terrain is the lay of the land. This is usually expressed in terms of the elevation, slope, and orientati ...
,
bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), river floors, or lake floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of wate ...
,
water level Water level, also known as gauge height or stage, is the elevation of the free surface of a sea, stream, lake or reservoir relative to a specified vertical datum. Over long distances, neglecting external forcings (such as wind), water level ten ...
, and built structures) and
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 ...
s of satellite orbits and in
aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...
. In
planetary science Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of ...
, vertical datums are also known as zero-elevation surface or zero-level reference. Commonly adopted criteria for a vertical datum include the following approaches: *
Tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
, based on
sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
when specific conditions occur, such as
NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...
's
National Geodetic Survey The National Geodetic Survey (NGS) is a List of federal agencies in the United States, United States federal agency based in Washington, D.C. that defines and manages a national geographic coordinate system, coordinate system, providing the fou ...
-produced tidal datums; * Gravimetric, based on 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 ...
; or geometric, based on the same
Earth ellipsoid An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximation ...
s that are used in computing a horizontal datum, such as NOAA's planned gravimetric and Global Navigation Satellite Systems (GNSS)-based Datum of 2022 set to be released that year by the National Geodetic Survey. In the United States, prominent vertical datums for professional uses include the National Geodetic Vertical Datum of 1929 and the
North American Vertical Datum of 1988 The North American Vertical Datum of 1988 (NAVD 88) is the vertical datum for orthometric heights established for vertical control surveying in the United States based upon the General Adjustment of the North American Datum of 1988. It superse ...
.


Methods

In common usage, elevations are often cited in height above
sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
, although what "sea level" actually means is a more complex issue than might at first be thought: the height of the sea surface at any one place and time is a result of numerous effects, including waves, wind and currents, atmospheric pressure,
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
s, topography, and differences in the strength of gravity due to the presence of nearby ice sheets, mountains, and density variations in the crust and deep in the mantle. For the purpose of measuring the height of objects on land, the usual datum used is
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 ...
(MSL). This is a tidal datum which is described as the arithmetic mean of the hourly water elevation taken over a specific 19 years cycle. This definition averages out tidal highs and lows (caused by the gravitational effects of the sun and the moon) and short term variations. It will not remove the effects of local gravity strength, and so the height of MSL, relative to a geodetic datum, will vary around the world, and even around one country. Countries tend to choose the mean sea level at one specific point to be used as the standard "sea level" for all mapping and surveying in that country. (For example, in Great Britain, the national vertical datum, Ordnance Datum Newlyn, is based on what was mean sea level at
Newlyn Newlyn () is a seaside town and fishing port in south-west Cornwall, England, United Kingdom.Ordnance Survey: Landranger map sheet 203 ''Land's End'' It is the largest fishing port in England. Newlyn lies on the shore of Mount's Bay and for ...
in
Cornwall Cornwall (; or ) is a Ceremonial counties of England, ceremonial county in South West England. It is also one of the Celtic nations and the homeland of the Cornish people. The county is bordered by the Atlantic Ocean to the north and west, ...
between 1915 and 1921). However, zero elevation as defined by one country is not the same as zero elevation defined by another (because MSL is not the same everywhere), which is why locally defined vertical datums differ from one another. A different principle is used when choosing a datum for nautical charts. For safety reasons, a mariner must be able to know the minimum depth of water that could occur at any point. For this reason, depths and tides on a nautical chart are measured relative to chart datum, which is defined to be a level below which tide rarely falls. Exactly how this is chosen depends on the tidal regime in the area being charted and on the policy of the hydrographic office producing the chart in question; a typical definition is Lowest Astronomical Tide (the lowest tide predictable from the effects of gravity), or Mean Lower Low Water (the average lowest tide of each day), although MSL is sometimes used in waters with very low tidal ranges. Conversely, if a ship is to safely pass under a low bridge or overhead power cable, the mariner must know the minimum clearance between the masthead and the obstruction, which will occur at high tide. Consequently, bridge clearances etc. are given relative to a datum based on high tide, such as Highest Astronomical Tide or Mean High Water Springs. Sea level does not remain constant throughout
geological time The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronolo ...
, and so tidal datums are less useful when studying very long-term processes. In some situations sea level does not apply at all – for instance for mapping Mars' surface – forcing the use of a different "zero elevation", such as mean radius. A geodetic vertical datum takes some specific zero point, and computes elevations based on the geodetic model being used, without further reference to sea levels. Usually, the starting reference point is a tide gauge, so at that point the geodetic and tidal datums might match, but due to sea level variations, the two scales may not match elsewhere. An example of a gravity-based geodetic datum is NAVD88, used in North America, which is referenced to a point in
Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...
,
Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
. Ellipsoid-based datums such as
WGS 84 The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descri ...
, GRS80 or NAD83 use a theoretical surface that may differ significantly from the
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 ...
.


Types

Common types of vertical datums include: * The surface of the datum ellipsoid, resulting in an ''
ellipsoidal height Geodetic coordinates are a type of curvilinear orthogonal coordinate system used in geodesy based on a '' reference ellipsoid''. They include geodetic latitude (north/south) , ''longitude'' (east/west) , and ellipsoidal height (also known as g ...
'', ''h''. * The mean sea level as described by the gravity
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 ...
, yielding the ''
orthometric height The orthometric height (symbol ''H'') is the vertical distance along the plumb line from a point of interest to a reference surface known as the ''geoid'', the vertical datum that approximates mean sea level. Orthometric height is one of the sci ...
'', ''H''.DMA Technical Report
Geodesy for the Layman, The Defense Mapping Agency, 1983
Along with the
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
and
longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...
, the ellipsoidal height provides the three-dimensional ''
geodetic coordinates Geodetic coordinates are a type of curvilinear orthogonal coordinate system used in geodesy based on a '' reference ellipsoid''. They include geodetic latitude (north/south) , ''longitude'' (east/west) , and ellipsoidal height (also known as g ...
'' (or ''geographic coordinates'') for a location. So, to completely specify a location of a topographical feature on, in, or above the Earth, one also has to specify the vertical position. The Earth is not a sphere, but an irregular shape approximating a biaxial ellipsoid. It is nearly spherical, but has an equatorial bulge making the radius at the Equator about 0.3% larger than the radius measured through the poles. The shorter axis approximately coincides with the axis of rotation. Though early navigators thought of the sea as a horizontal surface that could be used as a vertical datum, this is not actually the case. The Earth has a series of layers of equal
potential energy In physics, potential energy is the energy of an object or system due to the body's position relative to other objects, or the configuration of its particles. The energy is equal to the work done against any restoring forces, such as gravity ...
within its
gravitational field In physics, a gravitational field or gravitational acceleration field is a vector field used to explain the influences that a body extends into the space around itself. A gravitational field is used to explain gravitational phenomena, such as ...
. Height is a measurement at right angles to this surface, roughly toward the Earth's center, but local variations make the equipotential layers irregular (though roughly ellipsoidal). The choice of which layer to use for defining height is arbitrary.


Examples

*Australia:
Australian Height Datum The Australian Height Datum was introduced in 1971 as the official vertical datum for Australia, and thereby serves as the Benchmark (surveying), benchmark to which all height measurements are referred. The Australian Height Datum is an amalgama ...
*Austria, Albania and former
Yugoslavia , common_name = Yugoslavia , life_span = 1918–19921941–1945: World War II in Yugoslavia#Axis invasion and dismemberment of Yugoslavia, Axis occupation , p1 = Kingdom of SerbiaSerbia , flag_p ...
n republics: Metres above the Adriatic *France: General levelling of France *Germany: Normalhöhennull, preceded by
Normalnull ("standard zero") or (short N. N. or NN) is an outdated official vertical datum used in Germany. Elevations using this reference system were to be marked (“meters above standard zero”). has been replaced by (NHN). History In 187 ...
*Great Britain: Ordnance Datum Newlyn *Netherlands: Amsterdam Ordnance Datum, was also used by
Prussia Prussia (; ; Old Prussian: ''Prūsija'') was a Germans, German state centred on the North European Plain that originated from the 1525 secularization of the Prussia (region), Prussian part of the State of the Teutonic Order. For centuries, ...
*Switzerland: Metres above the Sea *United States: National Geodetic Vertical Datum of 1929,
North American Vertical Datum of 1988 The North American Vertical Datum of 1988 (NAVD 88) is the vertical datum for orthometric heights established for vertical control surveying in the United States based upon the General Adjustment of the North American Datum of 1988. It superse ...
, and the difference of the two in VERTCON *Global (geoid): Earth Gravitational Model (EGM), EIGEN-6C4 and others


Chart datums


See also

* Datum reference * Horizontal datum *
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 ...
*
Reduced level In surveying, reduced level (RL) refers to equating elevations of survey points with reference to a common assumed vertical datum. It is a vertical distance between survey point and adopted datum surface. Thus, it is considered as the base level ...
* Reference datum height * Reference water levels * Vertical Offshore Reference Frames *
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 ...
* Height above sea level *
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 ...


References

{{reflist


External links


Tidal Datums
produced for
NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...
by its
National Geodetic Survey The National Geodetic Survey (NGS) is a List of federal agencies in the United States, United States federal agency based in Washington, D.C. that defines and manages a national geographic coordinate system, coordinate system, providing the fou ...