Geodetic datum

Geodesy

A Geodetic Datum is an adjustment to position of origin and shape of the planet, used by cartographers and satellite navigation systems to translate positions indicated on their products to their real position on earth.

Examples of common Geodetic Datums are:

• World Geodetic System 1984 (WGS84) - the datum of GPS
• Ordnance Survey of Great Britain 1936 (OSGB36) - the usual datum of British nautical charts and Ordnance Survey maps until 2001

The difference between WGS84 and OSGB36 is 140 metres / 450 feet, which for most navigational purposes is an insignificant error. For other applications, such as surveying and dive site location for SCUBA divers, 140 metres is an unacceptably large error.