Datum

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This article describes a concept from surveying and geodesy. Datum is also the singular form of data.

A datum (plural datums) is a reference from which measurements are made. In surveying and geodesy, a datum is a reference point on the earth's 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. In engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made.

1 Horizontal datums
2 Vertical datums
3 Engineering Datums
4 External links

[edit] Horizontal datums

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 and the default for coordinates displayed in most commercial GPS units. The NAD27 datum is commonly used on older USGS topographic maps. The user of a GPS unit should normally change the datum it uses to display coordinates to match the datum of the map on which the coordinates are being plotted.

[edit] 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.

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. While 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. 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 WGS84, GRS80 or NAD83 use a theoretical surface that may differ significantly from the geoid.

On nautical charts, depths of water are relative to chart datum which is generally the lowest tide caused by gravity alone.