ABSTRACT
At the time of going to press, satellite surveying relies mainly on a system called global positioning system (GPS), which was originally set up as a military navigation aid by the USA in the mid-1980s, but which has now become a significant tool for civilian use in general and surveyors in particular. Using so-called differential GPS (DGPS), in which data recorded by a receiver at a ‘known’ station are combined with data recorded simultaneously by a second receiver at a new station which might be 30 km away, it is possible to find the position of the second receiver to within about 5 mm. The advantage of GPS compared to all earlier methods of surveying is that the two stations do not need to have a line of sight between them. This means that national networks of ‘known’ stations (provided round Great Britain by the Ordnance Survey) no longer need to be located on high hilltops or towers but can, for instance, be positioned on the verges of quiet roads.
