# Fundamental point

“Kilomètre zéro”, the fundamental point of France in front of the main entrance of the Notre-Dame de Paris cathedral

In classical geodesy, a fundamental point , also known as a central point , was the central measurement point of a country.

From its accurately determined astronomic coordinates ( astronomical latitude and longitude ) starting, by was triangulation (large-scale triangle measurement) a measurement power over the whole land spread .

In the last few decades the importance of fundamental points has decreased because the surveying systems of the states are being converted to intercontinental ellipsoids such as the WGS 84 .

## Classic definition

The fundamental point represents - together with a reference ellipsoid and the precise direction of a long triangle side - the terrestrial reference system of a national survey. The reference system and its orientation are called geodetic datum , the fundamental point and the measuring network form its implementation.

## The classic method of land surveying

The storage (fixing) of a reference ellipsoid relative to the earth's body takes place in that the geographical latitude and longitude of the fundamental point are determined astronomically and equated with its ellipsoidal latitude / longitude . Together with the exact direction to a distant triangulation point, this makes the axis of rotation of the ellipsoid parallel to the axis of rotation of the real earth .

However, the center of the ellipsoid does not coincide with the center of the earth , because the gravitational field of each region has certain irregularities (see geoid ). The center points can differ by a few 100 meters - depending on the location of the fundamental point in the region and its geological conditions.

## Fundamental points in Europe

The Panthéon of Paris is the fundamental point of the French NTF system ( Nouvelle triangulation de la France )

Some fundamental points in Europe are:

The direction-giving side of the triangle usually has a length of around 30 km; the one for Austria-Hungary (Hermannskogel - Hundsheimer Berg ) was chosen to be unusually long at 60 km in order to increase the directional accuracy of the very extensive network. The long sight , the observation of which required particularly good weather conditions , runs across the plain of the Vienna Basin .

In the 1920s, Germany, Austria and other countries switched to the 3 ° stripe system of the Gauß-Krüger projection , but the earlier fundamental points play a role, e.g. B. in the analysis of surveying networks and the digitization of older data from the land registry . In the near future, the transition to an EU -uniform reference system with 6 ° wide meridian strips will take place.

## Modern definitions

With the emergence of satellite geodesy , it was possible for the first time to derive geocentric earth models whose characteristics are based on the different mass distributions of the earth's mass . Since the geodetic calculation surface in the form of the ellipsoid is now derived from a large number of measurements and no longer from a single point without reference to the geocenter , classical fundamental points have largely lost their meaning. Because of the manifold possibilities of determination, one no longer speaks of the geodetic datum, but of the system .

In Germany, the Potsdam datum - with the Rauenberg fundamental point , the Bessel ellipsoid and the conforming Gauß-Krüger mapping - is currently being replaced by the ETRS89 system with the GRS80 ellipsoid and the UTM mapping .

Internationally, the WGS 72 and above all WGS 84 systems have gained great importance with regard to GPS . Many states have converted their national surveys to these geocentric systems.

## literature

• Bernhard Heckmann: Introduction of the ETRS89 / UTM position reference system when switching to ALKIS. In: German Association for Surveying, Landesverein Hessen eV and Thüringen eV Mitteilungen 1, 2005, , p. 17 ff., Online .
• NIMA - National Imagery And Mapping Agency: Department of Defense World Geodetic System 1984. Its definition and relationships with local geodetic systems. Technical Report, TR 8350.2. 3rd edition, amendment 1. National Imagery and Mapping Agency, Bethesda MD 2000.
• Bernhard Heck: Calculation methods and evaluation models for national surveying. Classic and modern methods. H. Wichmann, Karlsruhe 1987, ISBN 3-87907-173-X ( Wichmann book series ).