1065 - OGP Surveying and Positioning Committee

Annex F______________________________________________________________________________________________________________Page F1Annex F - A guide to co-ordinate system geodesyIntroductionCo-ordinates describing a position on the earth are only unambiguous if the co-ordinatesystem they are referenced to is identified. Many geographical co-ordinate systems exist. Inthe UK and on the UKCS, three geographic co-ordinate systems are frequently encountered.OSGB36 is the basis for the onshore National Grid used for Ordnance Survey of Great Britainmapping, ED50 is used in the North Sea and WGS 84 for the GPS system which is now inregular use for navigation and surveying. The same co-ordinate values on these systemsdescribe different positions on the Earth which may be several hundred metres apart. Thecorollary of this is that latitude and longitude are only unique when the co-ordinate system towhich they belong is given.Locations with same geographical values on three differentCo-ordinate systems51d 30m 45sN0d 20m 20sW(OSGB36)XX51d 30m 45sN0d 20m 20sW(WGS84)51d 30m 45sN X0d 20m 20sW100 metres(ED50)Figure F-1 - relative disposition of locations with the same co-ordinate values.In the case of a business park near London, three different office buildings have the same coordinatevalues on these three systems - see figure F-2.Locations with same geographical coordinate valuesOSGB36xxWGS84xED50100 metresFigure F-2____________________________________________________________________________________________UKOOA Guidance Notes on use of co-ordinate systems in data management on the UKCS - December 1999