To encourage compatibility of national and global precise positioning of GPS surveys in Great Britain, Ordnance Survey is providing access at no cost to the new National GPS Network, via a website first available in June 2000. By requiring the use of this infrastructure to provide ETRS89 control station coordinates at a stated level of precision, survey clients can be assured that any commissioned survey will be precisely consistent with any other, even if the two are widely spaced in location and time. All such surveys will also be precisely consistent with the Ordnance Survey large-scale base mapping, using the precise National Grid Transformation and National Geoid Model. This is now the Ordnance Survey recommended ‘standard method’ of creating new National Grid coordinate datasets, eventually to replace completely the traditional national control networks of triangulation stations and height bench marks.

This paper compares the differences between the rhumbline courses and distances calculated by three methods: the classical formula, precise formula and by software contained in a typical GPS receiver. It concludes that, while the calculations made by the GPS receiver are generally more accurate than the classical method, navigators must use Tables of Latitude Parts to calculate rhumbline distances accurately.