Survey Technology

The primary instrument of the surveyor is the theodolite an instrument consisting of two full-circle protractors, one in the horizontal plane and one in the vertical plane. It is used for measuring angles between points to an accuracy of an arc-second. Measuring these angles in a full circle (clockwise or anti-clockwise) is termed a round. Measuring two arcs, one clockwise the other anti-clockwise, is called an arc of observations. In observing the triangulation network, the following observation were used: At first-order points: 8 to 16 arcs. At second-order points: 6 to 12 arcs. At third-order points: 4 to 6 arcs. At fourth-order points: 2 arcs.

The innovation of Electronic Distance Measuring made it possible to measure distances electronically. The instrument sends out a pulse of optical light (invisible light). The light is reflected off a reflector (basically a mirror), and returns to the EDM. The time taken for the light to travel to the reflector and back is accurately measured. The time is halved and multiplied by the speed of light to give as the distance to the reflector.

Various instrument are available today. Generally distances accurate to millimetre level, up to 5 kilometres away, can be obtained.

This Office primarily uses the instrument for fourth order work, such as Town Survey schemes.

The Doppler system was the first artificial satellite-based navigation system introduced to the geodetic fraternity.

The system consisted of a receiver which tracked and recorded data from the satellites in the system constellation. Effectively, the satellite sends down its position data on a navigation message (broadcast ephemeris). The receiver records the frequency of the received message. The sent frequency is contained in the message. With this information it is possible to determine the time taken for the signal to travel from the satellite to the receiver.

The time taken, multiplied by the speed of light, gives the range to the satellite. Obtaining sufficient information, which is 30 to 40 passes of a satellite, (taking between 3 to 6 days), it is possible to compute a co-ordinate of the ground position to an accuracy of between 2 metres to 5 metres. Using translocation techniques (vector between points, to get relative rather than absolute position), it is possible to obtain accuracies in the order of 0.5 metres.

This technology has subsequently been superceded by the more accurate Global Positioning System.

This Office has used Doppler techniques. The results identified gross errors in our geodetic network, but could not be used to improve the network as its accuracy was not an improvement when compared to the overall network.

The GPS system came into operation in the 1980's. The system is similar to the Doppler system in that a navigation message is sent from the satellite, encoded with the time sent. The receiver has a built in clock. The time of the received measured is recorded and the distance to the satellite computed. The advantage of this system is that here at least 24 satellites are employed, orbiting at a distance of approximately 20,000 kilometres from the earth. The number of satellites allows the errors in the system be negated and accurate absolute positions to be computed in a matter of a few hours. The accuracy of this position is of the order of 30 metres. However, the big advantage of GPS is the ability to compute relative or differential position (translocation techniques) in the order of 2 centimetres plus two parts per million of the distance between them.

The advent of this technology has made it possible to consider geodetic networks at the continental and global level.

This Office uses the GPS technique to the preference of others. GPS has provided the means with which to remove distortions in the South African geodetic network, and as a direct result of the GPS technology, the new South African Datum, based on the WGS84 ellipsoid, has been identified for implementation on the 1st January 1999.