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Vertical datum

May 17, 2023

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]

Vertical datums in Europe

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

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

A chart datum is the water level surface serving as origin of depths displayed on a nautical chart. 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

Main category: Vertical datums

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. American Association for the Advancement of Science (AAAS). 284 (5419): 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. American Geophysical Union (AGU). 25 (24): 4397–4400. Bibcode:1998GeoRL..25.4397S. doi:10.1029/1998gl900085. ISSN 0094-8276. S2CID 28320895.
  3. ^ a b A guide to coordinate systems in Great Britain (PDF), D00659 v2.3, Ordnance Survey, Mar 2015, retrieved 2015-06-22
  4. ^ Taylor, Chuck. . 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. Retrieved 4 March 2014.
  7. ^ Reißland, Franz Barthelmes, Elmas Sinem Ince, Sven. "ICGEM International Center for Global Gravity Field Models". icgem.gfz-potsdam.de.
  8. ^ a b Australian Bureau of Meteorology (retrieved 30 April 2013)
  9. ^ Sjofartsverket: Mean Sea Level

External links

May 17, 2023
vertical, datum, geodesy, surveying, hydrography, navigation, vertical, datum, altimetric, datum, reference, coordinate, surface, used, vertical, positions, such, elevations, earth, bound, features, terrain, bathymetry, water, level, built, structures, altitud. 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 Vertical datums in Europe Commonly adopted criteria for a vertical datum include the following approaches Tides based on sea level when specific conditions occur such as NOAA s National Geodetic Survey produced tidal datums Gravimetric based on a geoid or geometric based on the same Earth ellipsoids 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 Prominent vertical datums in use by professionals include the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988 Contents 1 Methods 2 Types 3 Examples 4 Chart datums 5 See also 6 References 7 External linksMethods EditIn 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 EditCommon types of vertical datums include 4 The surface of the datum ellipsoid resulting in an ellipsoidal height The mean sea level as described by the gravity geoid yielding the orthometric height 3 5 Along with the latitude f and longitude l 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 EditAustralia Australian Height Datum Austria Albania and former Yugoslavian republics Metres above the Adriatic France General levelling of France Germany Normalhohennull preceded by Normalnull Great Britain Ordnance Datum Newlyn Netherlands Amsterdam Ordnance Datum was also used by Prussia Switzerland Metres above the Sea United States National Geodetic Vertical Datum of 1929 North American Vertical Datum of 1988 and the difference of the two in VERTCON Global geoid Earth Gravitational Model EGM EIGEN 6C4 and others 7 Chart datums EditThis section is an excerpt from Chart datum edit 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 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 EditMain category Vertical datums Datum reference Horizontal datum Levelling Reference datum height Vertical Offshore Reference Frames Height above ground level Height above sea level Height above average terrainReferences Edit 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 American Association for the Advancement of Science AAAS 284 5419 1495 1503 Bibcode 1999Sci 284 1495S doi 10 1126 science 284 5419 1495 ISSN 0036 8075 PMID 10348732 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 American Geophysical Union AGU 25 24 4397 4400 Bibcode 1998GeoRL 25 4397S doi 10 1029 1998gl900085 ISSN 0094 8276 S2CID 28320895 a b A guide to coordinate systems in Great Britain PDF D00659 v2 3 Ordnance Survey Mar 2015 retrieved 2015 06 22 Taylor Chuck Locating a Point On the Earth Archived from the original on 3 March 2016 Retrieved 4 March 2014 DMA Technical Report Geodesy for the Layman The Defense Mapping Agency 1983 Kwok Geodetic Survey Section Lands Department Hong Kong Geodetic Datum Transformation p 24 PDF Geodetic Survey Section Lands Department Hong Kong Retrieved 4 March 2014 Reissland Franz Barthelmes Elmas Sinem Ince Sven ICGEM International Center for Global Gravity Field Models icgem gfz potsdam de a b Australian Bureau of Meteorology National Tide Centre Glossary retrieved 30 April 2013 Sjofartsverket Mean Sea LevelExternal links EditTidal Datums produced for NOAA by its National Geodetic Survey Retrieved from https en wikipedia org w index php title Vertical datum amp oldid 1140201609, wikipedia, wiki, book, books, library,

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