Height Reference Systems

• Introduction
• The German height reference system
• The European height reference system 
• Geoid and Gravity field modelling 

The German height reference system

View of network DHHN92

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 levelling networks of the old and the new federal states (Länder). This linkage was established by means of 16 lines of levelling, 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 levelling 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 levelling 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 –50mm and +40mm for the DHHN85 (DHHN92 – DHHN85). The extreme values of the differences are caused by the stresses between the different parts of the networks and occur at the boundary between the old and new Länder. In most areas the differences lie below 1cm.
• In the new Länder the differences are considerably larger in contrast to the former Height System HN76 (DHHN92 – HN76 between +12cm and +15cm) since the height level of the HN76 refers to a different height datum (Kronstadt tide gauge).

Differences DHHN92-DHHN85 in mm

Differences DHHN92-HN76 in mm

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

Integration of measurements 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, this results in a permanent loss in quality caused by the deterioration of the network and destroyed points. 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 to renewal partly the DHHN in the years 2006 – 2011. 

Renovation of national height reference system

In the original network design the new measuring of about 60% of the DHHN92 was provided.  Meanwhile the densification of the original network design by the measuring of additional, so-called optional lines was decided in many federal states. With level of March 2008 the network of 14000 planned originally km of line length grown on approximately 21000 km, this corresponds approx. 80% of the lines of the DHHN92. In addition to the levelling also GNSS measurements being carried out at approx. 250 points and at approx. 100 points absolute gravity measurements.

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
• combination with GNSS epoch simultaneous measurements for modeling of geoid
• creation of a topical dataset for scientific work (e.g. determination recent vertical crust movements)

Renovation of DHHN: levelling, GNSS- and absolute gravity points

The process and evaluation of all these work get planned and coordinate by a project team of the AdV.

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

• 2006-2011
  carrying out of re-measurements of levelling lines

• Summer 2008
  GNSS measurements on 250 points nearby levelling lines, campaign with by 35 measuring teams, 2 x 24 h on each point
  
• 2009 / 2010
  absolute gravity measurements on 100 points

The evaluation of the levelling will be independently of each other carried out in 2 computing centers (surveying office of state Nordrhein-Westfalen and BKG).