Geodetic datum
[1] Datums[note 1] are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography.
A vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL).
A three-dimensional datum enables the expression of both horizontal and vertical position components in a unified form.
Since the rise of the global positioning system (GPS), the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications.
Before GPS, there was no precise way to measure the position of a location that was far from reference points used in the realization of local datums, such as from the Prime Meridian at the Greenwich Observatory for longitude, from the Equator for latitude, or from the nearest coast for sea level.
A standard datum specification (whether horizontal, vertical, or 3D) consists of several parts: a model for Earth's shape and dimensions, such as a reference ellipsoid or a geoid; an origin at which the ellipsoid/geoid is tied to a known (often monumented) location on or inside Earth (not necessarily at 0 latitude 0 longitude); and multiple control points or reference points that have been precisely measured from the origin and physically monumented.
Because Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS 84 can.
[4] The spherical nature of Earth was known by the ancient Greeks, who also developed the concepts of latitude and longitude, and the first astronomical methods for measuring them.
These methods, preserved and further developed by Muslim and Indian astronomers, were sufficient for the global explorations of the 15th and 16th Centuries.
However, the scientific advances of the Age of Enlightenment brought a recognition of errors in these measurements, and a demand for greater precision.
This led to technological innovations such as the 1735 Marine chronometer by John Harrison, but also to a reconsideration of the underlying assumptions about the shape of Earth itself.
The subsequent French geodesic missions (1735-1739) to Lapland and Peru corroborated Newton, but also discovered variations in gravity that would eventually lead to the geoid model.
More ambitious undertakings such as the Struve Geodetic Arc across Eastern Europe (1816-1855) and the Great Trigonometrical Survey of India (1802-1871) took much longer, but resulted in more accurate estimations of the shape of the Earth ellipsoid.
Improving measurements, including the use of early satellites, enabled more accurate datums in the later 20th century, such as NAD 83 in North America, ETRS89 in Europe, and GDA94 in Australia.
A horizontal datum is a model used to precisely measure positions on Earth; it is thus a crucial component of any spatial reference system or map projection.
Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas.
For example, in Sydney there is a 200 metres (700 feet) difference between GPS coordinates configured in GDA (based on global standard WGS 84) and AGD (used for most local maps), which is an unacceptably large error for some applications, such as surveying or site location for scuba diving.
A geodetic reference datum is a known and constant surface which is used to describe the location of unknown points on Earth.
Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas.
The geoidal height at Meades Ranch was assumed to be zero, as sufficient gravity data was not available, and this was needed to relate surface measurements to the datum.
"This datum, designated as NAD 83…is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin."
WGS 84 is used by the DoD for all its mapping, charting, surveying, and navigation needs, including its GPS "broadcast" and "precise" orbits.
At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS 84 was redefined again and was more closely aligned with International Earth Rotation Service (IERS) frame ITRF 94.
WGS 84 is the default standard datum for coordinates stored in recreational and commercial GPS units.
Examples of map datums are: The Earth's tectonic plates move relative to one another in different directions at speeds on the order of 50 to 100 mm (2.0 to 3.9 in) per year.
