Describe Modern engineering surveying uses GPS in an increasing number of situations. Indeed it is often the primary method of survey. Global datums are established by assigning Cartesian coordinates to various positions throughout the world. Observational errors in these positions will obviously be reflected in the datum.
The WGS84 was established from the coordinate position of about 1600 points around the globe, fixed largely by TRANSIT satellite observations. At the present time its origin is geocentric (i.e. the centre of mass of the whole Earth) and its axes virtually coincide with the International Reference Pole and International Reference Meridian. Designed to best fit the global geoid as a whole means it does not fit many of the local ellipsoids in use by many countries. In Great Britain, for instance, it lies about 50 m below the geoid and slopes from east to west, resulting in the geoid–ellipsoid separation being 10mgreater in the west than in the east.
It is also worth noting that the axes are stationary with respect to the average motions of this dynamically changing Earth. For instance, tectonic plate movement causes continents to move relative to each other by about 10 cm per year. Local movements caused by tides, pressure weather systems, etc., can result in movement of several centimetres. The result is that the WGS84 datum appears to move relative to the various countries. In Great Britain, the latitudes and longitudes are changing at a rate of 2.5 cm per year in a north-easterly direction. In time, this effect will be noticeable in large-scale mapping.
Historically, the majority of local datums were made accessible to the user by means of a TRF of points coordinated by triangulation. These points gave horizontal position only and the triangulation point, monumented by pillars in the UK, were situated on hilltops. A vertical TRF of benchmarks, established by spirit levelling, required low-lying, easily traversed routes. Hence, there were two different but loosely connected systems.
ATRF established by GPS gives a single three-dimensional system of easily accessible points that can be transformed to give more accurate position in the local system. As WGS84 is continually changing position due to tectonic movement, the local system must be based on WGS84 at a certain time. Thus, we have local datums like the North American Datum 1983 and the European Terrestrial Reference System 1989 (ETRS89). In 1989 a high precision European Three-Dimensional Reference Frame (EUREF) was established by GPS observations on 93 stations throughout Europe (ETRF89). The datum used (ETRS89) was consistent with WGS84/1TRF2000 and extends into Great Britain, where it forms the datum for the Ordnance Survey National GPS Network.
The OS system will be briefly described here as it illustrates a representative model that will be of benefit to the everyday user of GPS. The National GPS network TRF comprises two types of GPS station consisting of:
• Active network: this is a primary network of about 60 continuously observing, permanent GPS receivers whose precise coordinates are known. Using a single dual frequency receiver and data downloaded from these stations, which are located within 100 km of any point in Britain, precise positioning can be achieved to accuracies of 10 mm.
• Passive network: this is a secondary network of about 900 easily accessible stations at a density of 20–35 km. These stations can be used for control densification or in kinematic form using two receivers to obtain real-time positioning to accuracies of 50–100 mm.
The coordinates obtained by the user are ETRS89 and can be transformed to and from WGS84/1TRF2000 by a six-parameter transformation published by IERS on their internet site. Of more interest to local users would be the transformation from ETRS89 to OSGB36, which is the basic mapping system for the country. The establishment of the OSGB36 TRF by triangulation has resulted in variable scale changes throughout the framework, which renders the use of a single Helmert transformation unacceptable. The OS have therefore developed a ‘rubber-sheet’ transformation called OSTN02, which copes not only with a change of datum but also with the scale distortions and removes the need to compute local transformation parameters by the inclusion of at least three known points within the survey.