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DGPS Surveyors

A Differential Global Positioning System (DGPS) is a GPS upgrade that improves location accuracy in the spectrum of operations of each system, from the nominal GPS accuracy of 15 metres to roughly 1-3 centimetres in the best implementations.

Each DGPS broadcasts the difference between the positions given by the GPS satellite system and known fixed positions via a network of fixed ground-based reference stations. Receiver stations may correct their pseudo ranges by the same amount as the difference between measured satellite pseudo ranges and actual (internally computed) pseudo ranges broadcast by these stations. The digital correction signal is usually broadcast locally over shorter-range ground-based transmitters.

The principle of a DGPS survey that uses two receivers at the same time is as follows:

GCPs are sites on the surface of our globe with known X,Y (e.g. latitude and longitude) and Z coordinates (e.g. height above mean sea level in meters). GCPs must be exceedingly accurate in order to be used in the ortho-rectification process or for any Survey application. A low-cost handheld global positioning system (GPS) unit or a smart phone will not be able to collect them. A differential GPS unit, which is sometimes referred to as a survey-grade GPS unit, is required. These devices function by locking on to a number of GPS satellites in space, which triangulate your location on the ground.
The baselines are a three-dimensional line that connects the two sites where each pair of GPS antennae is located. Because most GPS mistakes affect each receiver approximately equally, the post-processed readings allow for more exact location because the inaccuracies may be cancelled out in the computations. Some GPS receivers can compute differential GPS measurements in real time provided they receive a correction signal from a separate radio receiver, such as in Real Time Kinematic (RTK) surveying or navigation.

"C&G SURVEY TECHNICAL SERVICES" has 15 survey grade, dual frequency Trimble & Leica RTK DGPS (Differential Global Positioning System) receivers viz.  To acquire the best results, all GCPs are observed in static mode for a sufficient length as per industry requirements, using the triangulation method. For applications requiring ortho rectification of satellite pictures with a relatively low accuracy of 2 to 5 cm, the control points can alternatively be produced using the RTK approach.