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Global Positioning System

There are three basic sorts of GPS in use for field survey, each of different accuracy. The most basis, e.g., the Garmin eTrex & similar devices, have a maximum resolution of 6m horizontally & around 20m vertically. They are ideal for reconnaissance work like upland survey or for recording approximate 2D positions of features for future reference.

The next sort, DGPS, is nominally accurate to within 1m & utilise correction signals from maritime GPS beacons or EGNOS in Europe or WAAS in the Americas & Canada. In practice, provided a clear view of the sky is possible, resolutions of 0.3m are realisable. This sort of GPS is used for tracking geophysical instrumentation in certain circumstances, as shown in the photograph lower left.

For maximum resolution use of a sub-centimeter GPS is required which are physically different in that they have a roving GPS unit & a tripod-mounted base station GPS, the two communicating via a radio link. These work differently from the other two sorts in that as well as using the time signals from the GPS satellites to triangulate the position they also use the phase of the carrier waves. At the same time, they are capable of working in ordinary DGPS mode. In practice they are used two ways, either by setting up on a known 3D point & working to sub-centimeter resolution relative to that point or by the more usual autonomous method. For the autonomous method the instrument would be set up in DGPS mode & field data collected. Once the survey is complete, a subscription service (e.g. Ordnance Survey's Active Rinex in the UK) is used to correct the data & produce the true position to centimeter level global precision. In some countries, e.g., Ireland, it is possible to receive the Rinex-type correction signal in real time, some systems allow this to be done via a mobile phone.

Manufacturer

Leica Geosystems, United Kingdom

Web site

http://www.leica-geosystems.com