Global positioning system

The Global Positioning System offers tremendous potential for variety of applications in the fields of surveying, mapping, engineering and geographic information. Many geodetic and even geodynamic aplications with high accuracy were demonstrated. Although the method of GPS positioning yields three dimensional coordinates (which can be transformed into two dimensional ellipsoidal curvilinear coordinates and ellipsoidal height) height information is seldom used. This three dimensional capability of GPS has created considerable opportunity for efficient, high accuracy determination.

However, GPS does not produce the same accuracy of height determination, compared with horizontal positions, due not only to inherent geometrical weakness, but also to several observational error sources which primarily affect the height component. Nevertheless, provided appropriate computational strategies are adopted, high accuracy height measurements can be obtained.

It is well known that ellipsoidal (GPS) height cannot be used like conventional heights derived from levelling. The reason for this is the fact that GPS height refers to an ellipsoid and not to the geoid (quasigeoid), as conventional heights (orthometric heights, normal heights) do. This difference in reference surface does not mean that ellipsoidal heights have no practical meaning. There are several applications in which ellipsoidal heights only are suitable to solve a specific problem in an appropriate way. This is particularly in tasks where height changes of individual points, e.g. of tide gauges, geodetic bench marks or some oil and gas platforms, are the focus of interest. Changes in ellipsoidal heights are highly correlated with changes in corresponding conventional heights, therefore, the results are independent of the chosen type of height. However, this kind of geodetic tasks is very rare in geodetic practice. The main purpose of the use of GPS technology is still the establishment of control networks for cadastral and engineering survey purposes. In these tasks conventional heights related to mean sea level are essential. Here measured ellipsoidal heights can be used for the determination of orthometric heights if the geoid heights are known with sufficient accuracy.

Дата добавления: 2015-07-19