Vertical datum

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Vertical datums in Europe Vertical references in Europe.svg
Vertical datums in Europe

In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built structures) and altitudes of satellite orbits and in aviation. In planetary science, vertical datums are also known as zero-elevation surface [1] or zero-level reference. [2]

Contents

Commonly adopted criteria for a vertical datum include the following approaches:

In the United States, prominent vertical datums in use by professionals include the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988.

Methods

In common usage, elevations are often cited in height above sea level, 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, tides, topography, and even differences in the strength of gravity due to the presence of mountains etc.

For the purpose of measuring the height of objects on land, the usual datum used is mean sea level (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 in Cornwall between 1915 and 1921). [3] 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, 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, Canada. Ellipsoid-based datums such as WGS 84, GRS80 or NAD83 use a theoretical surface that may differ significantly from the geoid.

Types

Common types of vertical datums include: [4]

Along with the latitude φ and longitude λ, the height h provides the three-dimensional geodetic coordinates or geographic coordinates for a location. [6]

To completely specify a location of a topographical feature on, in, or above the Earth, one also has to specify the vertical distance from the Earth's center or surface. 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 within its gravitational field. 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

Chart datums

This section is an excerpt from Chart datum.[ edit ]
U.S. civil and maritime uses of tidal data Tide legal use.gif
U.S. civil and maritime uses of tidal data

A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [8] Common chart datums are lowest astronomical tide (LAT) [8] and mean lower low water (MLLW). In non-tidal areas, e.g. the Baltic Sea, mean sea level (MSL) is used. [9]

A chart datum is a type of vertical datum and must not be confused with the horizontal datum for the chart.

See also

Related Research Articles

<span class="mw-page-title-main">Geodesy</span> Science of measuring the shape, orientation, and gravity of the Earth and other astronomical bodies

Geodesy is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems.

<span class="mw-page-title-main">Sea level</span> Geographical reference point from which various heights are measured

Mean sea level is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location.

<span class="mw-page-title-main">Geoid</span> Ocean shape without winds and tides

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 extended through the continents. According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of Earth. It can be known only through extensive gravitational measurements and calculations. Despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.

<span class="mw-page-title-main">Physical geodesy</span> Study of the physical properties of the Earths gravity field

Physical geodesy is the study of the physical properties of Earth's gravity and its potential field, with a view to their application in geodesy.

<span class="mw-page-title-main">World Geodetic System</span> Geodetic reference system

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 describes the associated Earth Gravitational Model (EGM) and World Magnetic Model (WMM). The standard is published and maintained by the United States National Geospatial-Intelligence Agency.

<span class="mw-page-title-main">Height</span> Measure of vertical distance

Height is measure of vertical distance, either vertical extent or vertical position . For example, "The height of that building is 50 m" or "The height of an airplane in-flight is about 10,000 m". For example, "Shaq O’Neal is 7 foot 1 inches in vertical height."

<span class="mw-page-title-main">U.S. National Geodetic Survey</span> U.S. federal surveying and mapping agency

The National Geodetic Survey (NGS) is a United States federal agency based in Washington, D.C. that defines and manages a national coordinate system, providing the foundation for transportation and communication, mapping and charting, and a large number of science and engineering applications. Since its founding in 1970, it has been part of the National Oceanic and Atmospheric Administration (NOAA), a division within the United States Department of Commerce.

<span class="mw-page-title-main">Geodetic datum</span> Reference frame for measuring location

A geodetic datum or geodetic system is a global datum reference or reference frame for precisely representing the position of locations on Earth or other planetary bodies by means of geodetic coordinates. Datums are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. A horizontal datum is used to measure a location across the Earth's surface, in latitude and longitude or another coordinate system; a vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL). Since the rise of the global positioning system (GPS), the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications. The WGS 84 is intended for global use, unlike most earlier datums.

<span class="mw-page-title-main">Elevation</span> Height of a geographic location above a fixed reference point

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 . The term elevation is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and depth is used for points below the surface.

<span class="mw-page-title-main">Benchmark (surveying)</span> Point with known height used in surveying when levelling

The term benchmark, bench mark, or survey benchmark originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiseled arrow below the horizontal line. A benchmark is a type of survey marker.

<span class="mw-page-title-main">Chart datum</span> Level of water from which depths displayed on a nautical chart are measured

A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are lowest astronomical tide (LAT) and mean lower low water (MLLW). In non-tidal areas, e.g. the Baltic Sea, mean sea level (MSL) is used. A chart datum is a type of vertical datum and must not be confused with the horizontal datum for the chart.

<span class="mw-page-title-main">Ordnance datum</span> Vertical datum used as the basis for deriving altitudes on maps

An ordnance datum (OD) is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as above ordnance datum (AOD). Usually mean sea level (MSL) at a particular place is used for the datum.

The orthometric height is the vertical distance H 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 scientific formalizations of a laypersons' "height above sea level", along with other types of heights in Geodesy.

<span class="mw-page-title-main">National Geodetic Vertical Datum of 1929</span> Vertical datum in the United States

The National Geodetic Vertical Datum of 1929 is the official name since 1973 of the vertical datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. Originally known as Sea Level Datum of 1929, NGVD 29 was determined and published by the United States Coast and Geodetic Survey and used to measure the elevation of a point above and depression below mean sea level (MSL).

<span class="mw-page-title-main">North American Vertical Datum of 1988</span> Vertical datum for orthometric heights

The North American Vertical Datum of 1988 is the vertical datum for orthometric heights established for vertical control surveying in the United States of America based upon the General Adjustment of the North American Datum of 1988.

<i>Normalhöhennull</i> Vertical datum used in Germany

Normalhöhennull or NHN is a vertical datum used in Germany.

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 which is used as reference to reckon heights or depths of other places or structures in that area, region or country. The word "Reduced" here means "equating" and the word "level" means "elevation". Datum may be a real or imaginary location with a nominated elevation.

Vertical position or vertical location is a position along a vertical direction above or below a given vertical datum . Vertical distance or vertical separation is the distance between two vertical positions. Many vertical coordinates exist for expressing vertical position: depth, height, altitude, elevation, etc. Points lying on an equigeopotential surface are said to be on the same vertical level, as in a water level.

<span class="mw-page-title-main">Vertical Offshore Reference Frames</span> UK and Irish hydrographic vertical datum

Vertical Offshore Reference Frames (VORF) is a set of high resolution surface models, published and maintained by the UK Hydrographic Office, which together define a vertical datum for hydrographic surveying and charting in the United Kingdom and Ireland.

<span class="mw-page-title-main">Newlyn Tidal Observatory</span> British tidal observatory

Newlyn Tidal Observatory is a grade II listed tide gauge hut on the South Pier in Newlyn, Cornwall in England. Measurements of sea level taken at the observatory between 1915 and 1921 were used to define the reference level, Ordnance Datum Newlyn, for height measurement on the British mainland. The tide gauge has collected over 100 years of observations which has significantly contributed to studies in sea level science.

References

  1. ↑ Smith, David E.; Zuber, Maria T.; Solomon, Sean C.; Phillips, Roger J.; Head, James W.; Garvin, James B.; Banerdt, W. Bruce; Muhleman, Duane O.; Pettengill, Gordon H.; Neumann, Gregory A.; Lemoine, Frank G.; Abshire, James B.; Aharonson, Oded; David, C.; Hauck, Steven A.; Ivanov, Anton B.; McGovern, Patrick J.; Zwally, H. Jay; Duxbury, Thomas C. (1999-05-28). "The Global Topography of Mars and Implications for Surface Evolution". Science. 284 (5419). American Association for the Advancement of Science (AAAS): 1495–1503. Bibcode:1999Sci...284.1495S. doi:10.1126/science.284.5419.1495. ISSN   0036-8075. PMID   10348732.
  2. ↑ Smith, David E.; Zuber, Maria T. (1998-12-15). "The relationship between MOLA northern hemisphere topography and the 6.1-Mbar atmospheric pressure surface of Mars". Geophysical Research Letters. 25 (24). American Geophysical Union (AGU): 4397–4400. Bibcode:1998GeoRL..25.4397S. doi:10.1029/1998gl900085. ISSN   0094-8276. S2CID   28320895.
  3. 1 2 A guide to coordinate systems in Great Britain (PDF), D00659 v2.3, Ordnance Survey, Mar 2015, retrieved 2015-06-22
  4. ↑ Taylor, Chuck. "Locating a Point On the Earth". Archived from the original on 3 March 2016. Retrieved 4 March 2014.
  5. ↑ DMA Technical Report Geodesy for the Layman, The Defense Mapping Agency, 1983
  6. ↑ Kwok, Geodetic Survey Section Lands Department Hong Kong. "Geodetic Datum Transformation, p.24" (PDF). Geodetic Survey Section Lands Department Hong Kong. Archived from the original (PDF) on 1 February 2016. Retrieved 4 March 2014.
  7. ↑ Reißland, Franz Barthelmes, Elmas Sinem Ince, Sven. "ICGEM International Center for Global Gravity Field Models". icgem.gfz-potsdam.de.{{cite web}}: CS1 maint: multiple names: authors list (link)
  8. 1 2 Australian Bureau of Meteorology National Tide Centre Glossary (retrieved 30 April 2013)
  9. ↑ Sjofartsverket: Mean Sea Level
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