File:Maybush Triangulation Pillar and the Ordnance Survey HQ - geograph.org.uk - 1714856.jpg

English: Maybush Triangulation Pillar & the Ordnance Survey HQ The Ordnance Survey's own description for this triangulation pillar advises that users should seek permission before use so I trust they would have indulged us on the day! The pillar is in the grounds of the Ordnance Survey headquarters and is on a bund at the rear of the site beyond the car parks. The main cluster of the Ordnance Survey's offices is beyond. The co-ordinates of the centre of the brass plate set in the top of the pillar are Eastings 438532.250, Northings 114863.209, converted from the ETRS89 Geodetic co-ordinates of Latitude 50° 55' 54.9458 North, Longitude 1° 27' 10.9765 West using the OSTN02/OSGM02 transformation parameters. The ellipsodal height of the flush bracket on the side of the pillar is 78.997 metres, which converts to 32.616 metres above mean sea level on the orthometric height datum, using the same transformation parameters. Without getting into too much detail this height datum is based on a theoretical mean sea level where the gravitational pull is the same throughout, an equipotential surface. It is not quite the same height datum as the familiar Ordnance Survey Bench Mark height datum, and the Ordnance Survey's OSBM height for the flush bracket is published as 32.650 metres, showing a difference of 0.034 metre between the two datums at this point. This difference between the two datums varies from location to location.