A Global Standard

A Global Standard

A significant consequence of the development of GPS has been the establishment of this one system as a global standard for time, position, and datum. Time and position are easy to appreciate, but sometimes overlooked is the role of GPS in establishing a single worldwide datum. It turns out that specifying coordinates in terms of latitude and longitude is not unique. When surveyors first set out to measure latitude and longitude, they selected a set of reference points (and a model of Earth’s shape) called the local datum. Because different points (and sometimes different shape models) are used in different places, coordinates measured using, for example, the North American Datum of 1927 (NAD 27) will be different from coordinates measured using the National Geodetic Vertical Datum of 1929 (NGVD 29). The British and the Greeks each use a local datum consistent with earlier survey work in Britain and Greece. This system works well as long as all position information is local, such as determining the correct boundaries of a piece of real estate. When precise calculations need to be performed on a global scale, however, the plethora of inconsistent local datum conventions becomes a hefty burden. The position of an oil well in the North Atlantic surveyed using NAD 27 might be tens of meters off from the exact same latitude and longitude when surveyed using Ordnance Survey Great Britain 1936 (OSGB 36).

GPS works on a single global datum called the World Geodetic System 1984 (WGS 84). If all coordinates are expressed using this datum (sometimes, the latitude and longitude are then called the GPS coordinates), there will be no confusion. With the proliferation of GPS devices, the use of WGS 84 “GPS coordinates” has become the world’s de facto standard. This is somewhat unfortunate for England’s historical Greenwich Meridian. This famous line, which is marked at the observatory in Greenwich, England, is no longer the point of zero degrees longitude. As determined by the WGS 84 GPS coordinates, the world’s meridian, or line of zero longitude, is now some 30 meters to the east of the famous Greenwich meridian. The Greenwich meridian is in fact not the world’s meridian, but rather just the meridian of the OSGB 36 datum.

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