HEIGHTING WITH GPS
This aim of this short section is to introduce an alternative technology, which can be very useful for heighting, where many points in a given area are required. The subject of satellite positioning is covered in depth.
The main advantages of using Global Positioning System (GPS) receivers for heighting are that no line of sight is required between instrument and target and the speed with which heights with their plan positions can be collected. The practical limit is usually the speed at which the GPS receiver can be moved over the ground.
Suppose the heights of some open ground are required with a density of not less than a point every 5 m and a vehicle with a GPS antenna mounted on its roof is available. If the GPS records 10 points every second, then the maximum theoretical speed of the vehicle would be 180 km per hour! Although it is unlikely that the surveyor would be travelling at such speeds over rough terrain this example illustrates the potential of the system.
On the negative side, GPS has limits as to the practical precision that can be achieved. Height of one instrument relative to another nearby instrument may be obtained with a precision of about 0.02 m. However, the heights are related to the World Geodetic System 1984 (WGS84) ellipsoidal model of the Earth and to be useful would need to be converted to heights above the local datum.
The relationship between WGS84 and the local datum will not be constant and will vary smoothly by up to 0.1 m per kilometre across the area of interest. Therefore external data will be required to apply the appropriate corrections to make the GPS derived heights useful to the surveyor. If the GPS antenna is mounted on a vehicle then the relationship between the antenna and the ground will vary as the vehicle bounces across.
the terrain adding further random error to the heights of individual points. This may not be so important if the purpose to the heighting is to determine volume of ground to be cut or filled, as the errors in individual heights will tend to cancel each other out. GPS equipment is rather more expensive than conventional levelling equipment or total stations.
GPS equipment works very well in a GPS friendly environment, i.e. where there is an open sky. It becomes much less useful if there are many obstructions such as tall buildings or under tree canopies. In such cases conventional techniques would be more appropriate.