Gravity Reference Systems

• Introduction
• Gravimeter
• German Gravity Reference Systems
• European Gravity Reference Systems
• Global Gravity Reference Systems

German Gravity Reference System

To ensure long-term reliability of the German gravity reference system as to level and scale, absolute gravity measurements are performed regularly in combination with the permanent registrations of the Superconducting Gravimeters in Wettzell, Bad Homburg and Moxa (Friedrich-Schiller-University of Jena). Usually, the measurements are performed twice a year and therefore allow monitoring the calibration factors and instrumental drifts of the Superconducting Gravimeters (SG). Through the combination of the data, instrumental zero point drifts of the Superconducting Gravimeter and secular gravity changes at the station can be separated from each other.

Deutsches Schweregrundnetz 1994 (DSGN94)

The German Gravity Reference Network with 30 absolute gravity points determined in 1994 defines the datum of the national gravity networks of the German National Survey. It was determined after the German reunification on decision of the German Geodetic Commission (DGK) by resurveying of the German Fundamental Gravity Network 1976 and extending it to the new Länder.

The German Fundamental Gravity Network 1994 – DSGN94 (click to enlarge)

BKG was assigned performing and evaluating these measurements. On all main stations of the Fundamental Gravity Network observations were carried out using the absolute gravimeter FG5-101. For checking the absolute gravity measurements in the new Länder, determining the vertical gradients on the main points as well as for linking the main points with the reference points, gravity differences were measured by means of four relative gravimeters. In addition, the "Institut für Erdmessung" (Institute for Earth Measurement - IfE) of the University of Hannover performed measurements on five stations using the JILAG-3 absolute gravimeter. The gravity values on the absolute gravity stations can be given with an accuracy of ±50 nm/s².

The comparison with the DSGN76 showed a mean displacement of 160 nm/s² between the two networks which is referred to less accurate absolute gravimeter measurements at four primary network stations (Hamburg, Braunschweig, Wiesbaden, München) in 1976. A change in scale could not be derived from the comparison.

The results and point descriptions of the German Fundamental Gravity Network DSGN94 have been published in Series B, No. 309, of the German Geodetic Commission. The point descriptions of the DSGN94 are also available as download - Download point description - PDF, 16 MByte, only in German.

BKG is responsible for maintaining and updating the German Fundamental Gravity Network DSGN94 and included the stations in its monitoring cycle.

Deutsches Hauptschwerenetz 1996 (DHSN96)

The German Primary Gravity Network 1996 (DHSN96) of the German National Survey constitutes an extension of the German Primary Gravity Network 1982 - DHSN82 with the inclusion of current gravity measurements of the Länder Berlin, Brandenburg, Mecklenburg-Vorpommern, Sachsen, Sachsen-Anhalt, and Thüringen.

German Primary Gravity Network 1996 - DHSN96 (click to enlarge)

The DHSN82 was observed in the western Länder during the years 1978 to 1982. The Länder Berlin, Brandenburg, Mecklenburg-Vorpommern, Sachsen, Sachsen-Anhalt, and Thüringen performed the measurements necessary for the enlargement in the years 1996 ff. according to the guidelines adopted on the AdV Meeting 63/78. The DHSN96 includes the stations of the Primary Gravity Network 1994 of the Federal Republic of Germany (DSGN94).

BKG computed the DHSN96 network in accordance with the guidelines formulated by the Working Group Fundamental Geodetic Survey of the AdV. With the adjustment of the measurements of the DHSN82 in the level DSGN94 for the old Länder a mean level displacement of –0.197 ±0,003 µm/s² to the DHSN82 resulted (level DSGN76). The level displacements, which were computed from identical points of the single Länder, differ by between -0.28 ±0.02 µm/s² (Schleswig-Holstein) and -0.15 ±0.01 µm/s² (Baden-Württemberg, Bayern). The DSGN76) is seen responsible for this level displacement. For this reason, the AdV Plenum agreed on its 105th Meeting in October 1999 in Berlin, that the DHSN96 (gravity status 130) in the old Länder must be computed by subtracting 0.19 µm/s2 from the gravity values of the DHSN82 (gravity status 100).

Computation of the gravity values of the DHSN96, part "New Länder", was done by adjustment of the 1st order gravimeter measurements of the years 1996/97 with connection to 15 absolute points of the DSGN94.

Besides the absolute points, this network consists of 91 new points located in the new Länder and 23 control points of the DHSN82 along the Länder boundaries. The quadratic mean of the standard deviations of the adjusted gravity values amounts to ±0.05 µm/s².

GOCE Gravity Field Analysis II (GOCE GRAND II)

Within the scope of the research program "Geotechnologies" the "GOCE Gravity Field Analysis II" (GOCE-GRAND II) project was dealt with jointly with other German partners. The acronym GOCE stands for "Gravity Field and steady-state Ocean Circulation Explorer" – which is a satellite gravity field mission of the European Space Agency and GRAND for "Gravity Field Analysis of Germany".

Points of the DHSN96 for which the absolute gravity values were re-determined in the frame of the GOCE GRAND II project (click to enlarge)

The objectives of the BKG project part consisted in verifying and improving gravity data existing in Germany and, on this basis, in combining terrestrial geoid models with satellite gravity field models as well as in validating the results. The terrestrial gravity measurements complement the satellite gravity field models by short-wave parts of the geoid (degree and order >250). From 2006 to 2008 measurements at 100 field stations of the gravity control network had been performed with the absolute gravimeter A10-012 all over Germany for checking the existing terrestrial data, they were completed in 2008. For ensuring the measuring accuracy comparisons at the reference station Bad Homburg were performed regularly.

The results of the field measurements are suited to examine the gravity database existing at the BKG. This database forms the basis for the computation of a quasigeoid model of Germany. On the basis of the now available absolute gravity measurements the homogeneity of data of various origin, the measuring techniques and the accuracy are checked. Systematic differences in the level can thus be detected and eliminated and a strategy for weighting within the database can be derived. The GOCE GRAND II project was supported by the Federal Ministry of Education and Research within the special program Geotechnologies.

Absolute gravity measurements within the frame of the renewal of the German First Order Levelling Network - DHHN

Points of the DHHN, for which in the frame of the renewal of this network the absolute gravity values were determined (click to enlarge)

From 2009 to 2011 in the frame of the renewal of the German First Order Levelling Network - DHHN92 absolute gravity measurements with two absolute gravimeters A10 of the BKG have been carried out on 100 field stations of the First Order Levelling Network. These works were commissioned by the Working Committee of the Surveying Authorities of the States of the Federal Republic of Germany - AdV. The AdV Project "DHHN Renewal 2006-2011" comprises the GNSS measurement of about 250 mostly newly established points, re-measurements of about 80% of the levelling lines of the DHHN92 by precise levelling and the absolute gravity measurements at 100 of the 250 stations (see also Renovation of national height reference system).

At the absolute gravity stations of the DHHN, relative gravity measurements for determining the vertical gravity gradient were performed additionally. For ensuring the measuring accuracy of the absolute gravimeters applied, repeated comparisons at the reference station in Bad Homburg and checks of the instrument standards (Laser, atomic clock) were performed. After evaluation of the data the results are forwarded to the DHHN Project Group and to the commissioning Federal States, to enter the data into the respective databases conform to the data modell of German surveying authorities rules.

Since 2009, supplementary absolute gravity measurements have been carried out for the Länder to establish a network of long-term stable Geodetic Fundamental Network Points. It is expected that for additional 90 points of the 250  DHHN sites absolute gravity measurements can be obtained.