Three-dimensional systems

Concurrent with the increased use of satellite-based surveying techniques (GPS/GNSS), the importance of three-dimensional reference systems has also grown. Using the new, globally adapted three-dimensional reference systems, positions can be unambiguously determined and used as uniform positional references on a global scale. WGS 84 and SWEREF 99 are examples of three-dimensional reference systems.

Two-dimensional systems

Our traditional reference systems can be considered to be two-dimensional since they have no direct connection to the height component. This is true for example for the Swedish RT 38, RT 90, regional and local reference systems. There are also plane co-ordinates, through projections of the national reference systems such as RT 90 2.5 gon V 0:-15 or SWEREF 99 TM.

Height systems

Heights above sea level are referred to a height system. A height system is made up of a network of well-founded and accurately measured points -benchmarks - and has a well-defined vertical datum (zero point). Through the years, Sweden has had three national height systems: RH 00, RH 70 and RH 2000. The phenomenon of postglacial land uplift, to which the Nordic countries, amongst others, are exposed, is a factor that influences height systems.

The geoid

Normally when we speak of height above sea level it is more strictly the height above the geoid to which we refer. The geoid is the geopotential surface that best connects to the average sea surface and its hypothetical extension under the continents. To convert GPS heights (ellipsoidal heights) to heights above sea level in Sweden, we use a geoid model called SWEN08_RH2000.

Gravity systems

Most geodetic surveying methods are, in one way or another, influenced by the size and direction of gravity. Gravity values are given in a so-called gravity system. The official Swedish national gravity system is called RG 82.

Postglacial land uplift

As early as the 18th century it was observed that the Earth's surface was rising in the Nordic area. At that time it was believed that this was due to a lowering of the sea level and the phenomenon was called "water diminution". Today, we know that the land is rising after having been heavily loaded by the kilometre-thick ice during the last Ice Age. The land uplift can be modelled using sea level observations, repeated levelling and continuous observations at permanent GNSS reference stations.