Vertical datum

Source: Wikipedia, the free encyclopedia.
Vertical datums in Europe

In

aviation
. In planetary science, vertical datums are also known as zero-elevation surface[1] or zero-level reference.[2]

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

  • tidal datums
    ;
  • 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.

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

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

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

NAD83 use a theoretical surface that may differ significantly from the geoid
.

Types

Common 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 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

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. PMID 10348732
    .
  2. 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. "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. ^ a b Australian Bureau of Meteorology National Tide Centre Glossary (retrieved 30 April 2013)
  9. ^ Sjofartsverket: Mean Sea Level

External links

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