THANKS to work by Ordnance Survey in Southampton, users of satellite navigation equipment can instantly convert readings to match precise locations on OS maps.

Satellites which are part of the global positions system give co-ordinates for the surface level position of the user's receiver known as ETRS89, which is part of the world geodetic systems. But Ordnance Survey mapping is based on national grid co-ordinates.

"Users of GPS equipment and Ordnance Survey mapping know that the co-ordinates used for each do not match,'' said Jan Lepowski of Ordnance Survey. "Features surveyed in one co-ordinate system appear to be in a different location in the other.

"Neither is wrong. They are just different. But the difference can be up to 200 metres on the ground and it varies in a complex way across Britain.

"Height measurement is also different. That's because Ordnance Survey mapping shows height above mean sea level, but GPS heights are relative to an imaginary height reference covering the whole world.

"In Britain the imaginary height reference is between 30 and 40 metres below sea level.'' The problems relating to both horizontal and height differences have been solved with the launch of two new products.

"To create the transformations, literally millions of calculations have had to be made and are now incorporated into computer programmes so the two systems can be match up anywhere in Britain, '' said Mr Lepowski.

Virtually every business and administrative body in Britain using mapping in its work relies on the National Grid, but with the development of GPS equipment it has become critical have access to instant transformation of the satellite readings.

The transformation is being licensed to companies which sell software and hardware for GPS navigation and geographical information systems so that it can be incorporated into their own software. The data is not being licensed direct to individual users.

To use transformations effectively, Ordnance Survey is developing a national GPS network which offers precise positioning right down to centimetre accuracy. This consists of between 20 and 30 continuously operating active GPS stations and 900 passive GPS stations. This new network is being linked to a network of triangulation sta-tions on hill-tops across Britain. The introduction of the national GPS network will make it possible to obtain up-to-the-minute readings relating to any of the stations from Ordnance Survey via the Internet.

This should lead to greater efficiency in detailed surveying as it will become possible for surveyors to work individually rather than in pairs.

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