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Commonly known as GPS, Global Navigational Satellite Systems are becoming a part of most people’s everyday lives. From giving you directions when driving to tracking your exercise routes, GPS provides a method of locating yourself quickly and easily.
Traditionally, GPS refers to the US Government “Navstar” navigational satellite system. Other systems are also in place including the Russian Glonass satellite system, the European “Galileo” satellite system, China’s “Beidou” and Japan’s regional “Quasi-Zenith Satellite system”. Other countries including France and India are developing their own regional satellite systems. This diversity in systems has led to the introduction of the generic term GNSS to replace the traditional term GPS, which is proprietary.
Global Navigational Satellite Systems work by having a series of satellites in orbit around the earth. The satellites all send out a constant radio signal, which is detected by a receiver. The receiver makes some incredibly complex calculations based on these signals and from these calculations, determines the location of the receiver relative to a mathematical model of the earth, called a geoid.
Due to the nature of the satellite constellation, the signal and the limitations in accuracy of receivers, a single hand-held GNSS receiver will give your location to within about 7 to 15 metres horizontally, and about 12 to 35 metres for height. For navigational purposes, this level of accuracy is perfectly acceptable, and will deliver you to your destination – provided you have satellite signal!
Surveyors in NSW performing cadastral (boundary) surveys are required to guarantee any distance and bearing shown on their plans to a specific accuracy – or better. Currently, the Surveying and Spatial Regulation 2006 requires distances to be shown to an accuracy of “10mm plus 15 parts per million or better“ – meaning on a line 1 km long (1000 metres) an acceptable accuracy for distance would be plus or minus 25mm.
Angular measurements are read in degrees, minutes and seconds – 360° is a full circle. Each degree of angle is divided into 60 minutes and each minute is divided in to 60 seconds. In real terms, one minute of arc (60 seconds) represents about 30mm of distance on a line 100 metres long. The Surveying and Spatial Information Regulation 2006 specifies an accuracy of “20 seconds plus 10 seconds or 2 minutes, whichever is the lesser” where n is the number of traverse stations, or points occupied in the survey. On a single line 1km long (1000 metres) an acceptable accuracy for angle would be 22 seconds, which equates to 38mm. Picture a target 1000 metres away, then imagine heat haze and sheer distance and you can start to appreciate how this accuracy is actually quite good.
So, given the limitations in the accuracy of satellite systems, how do surveyors use GPS/GNSS?
In order to increase the accuracy beyond the 7 to 15 metre navigational accuracy, a surveyor uses two receivers. One receiver is set on a fixed point (A), and the other is used to measure remote points (B). A link between the two receivers means that at any given point in time, the coordinates of point A are compared with the coordinates of point B, giving a relative bearing and distance from point A to point B, to survey accuracy.
The problem with using GNSS for these surveys is twofold. Firstly, GNSS receivers give you coordinates for your position – and we need to show distances and angles on the plan. Secondly, the coordinates given represent the geoid model of the earth and don’t translate directly to distances on the ground.
“Ground distances” are measured between two points on the ground, using a tape or surveying Electronic Distance Machine (EDM). The distance between two coordinates from a GPS/GNSS receiver will differ from the “ground distance”. This is because the geoid model of the earth does not match perfectly with the physical shape of the earth. In order to convert measurements of coordinates from a GPS/GNSS received to “ground distances”, a surveyor must connect with his GPS/GNSS to points with known “ground distances” between them. A scale factor is then applied to try to match the GPS/GNSS coordinates to the “ground distances”.
Current surveying GNSS equipment utilises both the GPS and Glonass signals, and is able to provide measurements over large distances to excellent accuracy. Provided this information is linked to “ground distances”, the surveying GNSS often offers a faster solution than by measurement with EDM. On shorter distances, the accuracy requirements of the legislation often mean that GNSS is not a viable option. Adopting the correct rigorous GNSS measurement techniques means that more time is taken to verify measurements. The benefits of using GNSS in a boundary survey must be balanced against the rigorous checks required, and of course using a satellite system means you must have a clear view of the sky!
Not all surveys use GPS – some because it is not physically possible, others because it is not financially practical. Advances in surveying equipment have increased the useability and accuracy of GNSS measurement, which can be expected to continue. In conjunction with other surveying techniques, GNSS is a valuable tool for surveyors.
Eric Smith
Registered Surveyor