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Encyclopedia > Datum
It has been suggested that this article or section be merged with Geodetic system. (Discuss)

In surveying and geodesy, a datum is a reference point or surface against which position measurements are made, and an associated model of the shape of the earth for computing positions. Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude or another coordinate system. Vertical datums are used to measure elevations or underwater depths. Wikipedia does not have an article with this exact name. ... Geodetic systems or map datums are used in geodesy, navigation, surveying and the Global Positioning System to indicate the datum or reference point to which a measured position is relative. ... Surveyor at work Surveying is the art and science of accurately determining the position of points and the distances between them. ... It has been suggested that Geodetic datum be merged into this article or section. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third planet outward from the Sun. ... Latitude, denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Map of Earth showing curved lines of longitude Longitude, sometimes denoted Î», describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ...

The horizontal datum is the model used to measure positions on the earth. A specific point on the earth can have substantially different coordinates, depending on the datum used to make the measurement. There are hundreds of locally-developed horizontal datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of the earth, are intended to cover larger areas. The WGS84 datum, which is almost identical to the NAD83 datum used in North America, is a common standard datum. WGS 84 is the 1984 revision of the World Geodetic System. ...

## Vertical datums

A vertical datum is used for measuring the elevations of points on the earth's surface. Vertical datums are either tidal, based on sea levels, or geodetic, based on the same ellipsoid models of the earth used for computing horizontal datums. For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ...

In common usage, elevations are often cited in height above sea level; this is a widely used tidal datum. Because ocean tides cause water levels to change constantly, the sea level is generally taken to be some average of the tide heights. Mean lower low water — the average of the lowest points the tide reached on each day during a measuring period of several years — is the datum used on most nautical charts, for example. Whilst the use of sea-level as a datum is useful for geologically recent topographic features, sea level has not stayed constant throughout geological time, so is less useful when measuring very long-term processes. For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ... The tide is the regular rising and falling of the oceans surface caused by changes in gravitational forces external to the Earth. ... In mathematics, there are numerous methods for calculating the average or central tendency of a list of n numbers. ...

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. One example of a geodetic datum is NAVD88, used in North America, which is referenced to a point in Quebec, Canada. Beginning in 1963, a terrorist group that became known as the Front de libÃ©ration du QuÃ©bec (FLQ) launched a decade of bombings, robberies and attacks on government offices and at least two murders by FLQ gunfire and three violent deaths by bombings. ...

On nautical charts, depths of water are relative to chart datum which is generally the lowest tide caused by gravity alone. A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. ... The chart datum is the theoretical level of water in any tidal area during the lowest possible astronomical tide (LAT). ... It has been suggested that Law of universal gravitation be merged into this article or section. ...

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