1.1 Vertical Reference System
1.2 Ellipsoidal Heights
Global and local geoids differ in their origin: global geoids consider only the long- wave and middle-wave part of the earth’s gravity field, whilst local geoids, in addition, also consider the short-wave part of the gravity field, resulting in higher resolution and, hence, better local accuracy.
Global geoids are used when consistent, orthometric heights, over long distances (continent or earth surveying), are required. Currently, the world’s best global geoid model is the Earth Gravitational Model (EGM) 200812. It was determined using satellite tracking, gravity anomalies and satellite altimetry. Its accuracy is in the range of ± 0.05m (oceans) and ± 0.5m (on land). This accuracy is higher in flat regions than in topographically mountainous terrain, such as the Alps. In aviation, elevation values have long been referenced to MSL; ICAO Annex 15 [Reference 4] requires that EGM-96 is used as the global gravity model as EGM- 2008 was not available when the requirements for a global gravity model requirement were introduced through Amendment 33 in 2004. The accuracy of EGM-96 is sufficient for terrain and obstacle elevations. This is because it meets the accuracy requirements of aviation and because height information is primarily used in context.
For local engineering applications and cadastre-surveying, global geoids are not as accurate as needed. For such applications, local geoid models are calculated, developed using local field measurements. They offer centimetre accuracy over several hundred kilometres, with a high resolution. Local geoids are not suitable for height comparison over large distances since they are based on different origins and reference heights (different equipotential levels).
1.5 Normal Heights
1.6 Graphical Representation of Different Reference Surfaces
Recent Developments in Vertical Reference Systems
2.1 European Vertical Reference System
2.2 Modernised National Vertical Reference Frames
To eliminate inaccuracies, as well as torsion in the vertical reference, national geodetic agencies have started, often in combination with new horizontal reference frames, to rebuild the vertical reference frame, taking into account very accurate geoid or quasi-geoid models. The results are strict orthometric or normal heights which provide the base for a new national height reference frame. This allows the simple combination of GPS measurements (ellipsoidal heights) and levelling since the geoid undulation is precisely known for each horizontal co- ordinate.
Source: Eurocontrol Terrain and Obstacle Data Manual