The introduction of the techniques employed in space geodesy means that studies of the Earth’s orientation in outer space have also incorporated in the geodetic discipline.
Measurements of the planetary system with the aid of geodesy help us to understand geophysical processes. Such understanding is crucial in turn for being able to describe what has happened to the Earth in earlier times and its future fate. The discipline is therefore fundamental for monitoring the effects of climate change, and provides the basis for both Earth observation and the acquisition of geographical data.
The Norwegian Mapping Authority (NMA) monitors the Earth system and has access to comprehensive data from such sources as global positioning system (GPS) stations across the whole of Norway, Earth observation satellites and its own geodetic Earth observatory at Ny-Ålesund in Svalbard. Applications for this information include calculating changes in sea level, land uplift and the Earth’s rotation.
Film on geodesy
Everything happens somewhere, and everything has a position. Geodesy allows that exact location to be pinpointed with an accuracy down to a millimetre. Learn more in the film "What is geodesy" (english text).
The Earth’s crust is in constant change and motion. Its surface is continuously being displaced. Most people know about ocean tides – where the sea rises and falls twice a day because of the gravitational pull of Moon and Sun. Fewer are aware that the same forces also cause the crust to deform. Known as the Earth tide, this effect means the surface of the planet moves up and down by a total of 30 centimetres. And that distance is roughly twice as great at the equator.
Displacement of mass and pressure
Ocean tides also deform the Earth’s crust. At high water, the seabed and surrounding areas will be depressed by the greater water mass. This change could amount to several centimetres, and must be taken into account when determining a precise location. Similarly, other forms of mass or pressure also deform the crust. Examples includes air pressure, glaciers, snow, groundwater and human structures.
Many of these phenomena are periodic, and almost identical from day to day or year to year. Others are more unpredictable and harder to describe.
Continuous changes also occur. One example is plate tectonics. The Earth’s crust is formed of continental plates which move in relation to each other. Europe and North America, for example, are separating by a few centimetres every year. Similarly, the Pacific plate is moving towards and under America. When two plates collide in this way, big stresses build up which are occasionally released to cause earthquakes and major deformation.
Climate change and sea level
Where climate change is concerned, interest concentrates on the long-term trends such as alterations in sea level. Little happens from day to day, but the effects can be dramatic over a century. Nothing precise can be said about sea level without knowing the extent of land uplift. This can be as much as one centimetre a year in Scandinavia, where the crust has been bouncing back slowly since the Ice Age came to an end 10 000 years ago.
Systems for measuring movement
A key challenge for geodesy is establishing a system which permits precise measurements in a world where everything is in motion. This is known as the frame of reference, which provides immobile fixed points to measure against. Quasars are remote celestial objects which provide excellent fixed points, and can be observed with the aid of very long baseline interferometry (VLBI).
A number other techniques with different strengths and weaknesses are also needed to determine this frame of reference. They include the GPS and satellite laser ranging (SLR). Together with VLBI, these represent the geometric techniques. Water level gauges measure sea level, while various gravimetric and satellite methods determine gravity and its changes. Satellites can measure sea level, ice thickness and the gravitational field. All these tools are necessary for creating the best possible frame of reference and thereby for the ability to say something about how the Earth and its climate are developing.
A number of these techniques have been brought together at the NMA’s Ny-Ålesund observatory, providing a unique opportunity to compare and combine their various results.