Height Reference Systems

• Introduction
• The German height reference system
• The European height reference system 
• Determination of the height reference surface of Germany
• GCG2011 - German Combined QuasiGeoid 2011

The German height reference system

View of network DHHN92 (click to enlarge)

The German height reference system is presently realized through the Deutsche Haupthöhennetz 1992 - DHHN92 (German mean height reference system 1992). After the German Reunification the DHHN92 was created by the linkage of the respective 1^st order leveling networks of the old and the new federal states (Länder). This linkage was established by means of 16 lines of leveling, which were measured during the years 1990 to 1992. Besides the LGN Niedersachsen, BKG functioned as one of the two computing centers which independently from each other conducted the adjustment of the DHHN92, each using different program systems.

The DHHN92 consists of 757 leveling lines with a total length of 30 908 km, 287  loops and 469  nodal points. The heights are normal heights referenced to the level of the Normaal Amsterdams Peil (NAP). The height level is realized by fixing the geopotential height of the point 3614/5 (Wallenhorst church) from the adjustment of the European leveling network UELN73/86.

The DHHN92 was introduced in the single Länder (states of the Federal Republic) at different times. The differences as compared with the previously valid height systems amount 

• in the old Länder to between –50 mm and +40 mm for the DHHN85 (DHHN92 – DHHN85). The maximum differences are caused by stresses between the different parts of the networks and occur at the boundary between the old and new Länder. In most areas the differences are below 1 cm.
• In the new Länder the differences are considerably larger in contrast to the former Height System HN76 (DHHN92 – HN76 between +12 cm and +15 cm) since the height level of the HN76 refers to a different height datum (Kronstadt tide gauge).

Differences DHHN92-DHHN85 in mm (click to enlarge)

Differences DHHN92-HN76 in mm (click to enlarge)

Detailed descriptions of the definitions of the different height systems as well as transformation parameters for application between the systems are available in the Information system of European Coordinate Reference Systems CRS-EU.

Integration of observations obtained from three different epochs (HN76 1974 – 1982, DHHN85 1977 – 1988, connecting measurements 1990 – 1992) leads to strains and stresses in the network. In addition, a permanent loss in quality is caused by the deterioration of the network and an ongoing damage of benchmarks. For this reason the "Arbeitsgemeinschaft der Vermessungsverwaltungen der Länder der Bundesrepublik Deutschland - AdV" (Working Committee of the Surveying Authorities of the States of the Federal Republic of Germany) decided in April 2005 a partly renewal of the DHHN in the years 2006 – 2011. 

Renovation of national height reference system

In the original network design about 60% of the DHHN92 should be reobserved.  Meanwhile many federal states decided a further densification of the original network design by the observation of additional, so-called optional lines. In March 2008 the network of 14000 km leveling lines which were planned originally has been enlarged to approximate 21000 km which corresponds to approximate 80% of the lines of the DHHN92. In addition GNSS measurements at about 250 points and absolute gravity observation at approximate 100 points have been carried out.

Objectives of the modernization of the German height reference system are:

• verification of the official height reference system DHHN92 for the uncovering of height changes and tensions
• imbedding of the DHHN in a future integrated spatial reference system
• generation of GNSS epoch simultanous measurements for geoid modeling
• creation of a topical dataset for scientific research (e.g. determination of recent vertical crustal deformations)

Renovation of DHHN: levelling, GNSS- and absolute gravity points (click to enlarge)

A project team of the AdV plans and coordinates the procedure and analyses within all activities.

• 2005
  decision of the AdV on the renewal of the DHHN

• 2006 – 2011
  reobservation of leveling lines

• Summer 2008
  GNSS observation at 250 points located close to leveling lines, campaign with by 35 surveyor teams, 2 x 24 h observation period on each point
  
• 2009 / 2010
  absolute gravity observations at 100 points

The analyses of the leveling observations will be done independently at 2 computing centers (surveying office of state Nordrhein-Westfalen and BKG).