GIS Files 2: Geographical data

2.2: Surveying and remote sensing (3)

The Global Positioning System (GPS)

The development of the GPS, by the United States Department of Defense, is revolutionising the world of surveying. It enables positioning of objects on or above the earth's surface in an absolute sense, not just in relation to other nearby features (as in the use of photogrammetry described previously, in which locations are defined relative to the known position of certain features in the image).

GPS can be used almost anywhere in the world, 24 hours a day, in all weathers. A constellation of 24 satellites orbit the earth and send signals that can be picked up by GPS receivers. GPS measurements are taken by computing the distance between the receiver and the satellite. If a receiver picks up signals from four or more satellites, a 3-dimensional position can be calculated. Certain methods can be used to increase the accuracy of the position to the 1cm level, either in real time or afterwards during post-processing.

GPS measurements are obtained in the GPS coordinate system: World Geodetic System 1984 (WGS84). Users should be aware that this position usually needs to be converted into the local coordinate system for the region, OSGB36® in Great Britain, enabling GPS to be used alongside the local mapping. GIS data collectors can make use of the free GPS service provided to locate map objects and features directly in the field. Full details of this are available on Ordnance Survey's GPS web site (www.gps.gov.uk).

Ordnance Survey uses GPS to locate map detail. However, there are many other uses for GPS, such as navigating boats, planes or cars, monitoring the stability of structures, and providing location information for recreational users.

Ordnance Survey's GPS web site

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