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| From
1990 to 2003, the Institut für Erdmessung (IfE) was acting as the
computing center of the IAG (International Association of Geodesy) Geoid and
Gravity Commission (Sub-commission Europe). Within the framework of these
activities, interim solutions were produced about once-a-year (e.g., see Torge
and Denker 1991, Denker and Torge 1993, Denker et al. 1995, Denker and Torge
1998, Roland and Denker 2005). However, only the geoid and quasigeoid model
EGG97 (European Gravimetric Geoid 1997; Denker and Torge 1998) was provided to
external users. The investigations were partly funded by the German Research
Foundation DFG.. |
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| The
geoid and quasigeoid model EGG97 is based on all high-resolution gravity and
terrain data available in 1997 as well as the global geopotential model EGM96
from NASA/NIMA (lmax=360). Approximately 3 million gravity and 700 million
terrain data existed at that time. The resolution of the gravity data was at
least 10 km for the entire European land area, and the digital terrain models
had different resolutions varying from about 30 m to a few km. Furthermore,
altimetric data were included in marine areas. The combination of all data sets
was done by spectral combination (Wenzel 1982) in connection with the
remove-restore technique. The model EGG97 was calculated in a grid with a
spacing of 1.0' x 1.5' as well as 10' x 15', covering the area from 25°N to
77°N and 35°W to 67.4°E; the number of grid points is 3,120 x 4,096
= 12,779,520. The grid data were archived together with an interpolation
program on a CD-ROM. All data providers received the CD-ROM free of charge,
while other users have to pay a fee (differently for scientific and commercial
users).. |
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| The
evaluation of the EGG97 model by GPS and levelling data clearly showed the
existence of long wavelength geoid errors with a magnitude of 0.1 to 1.0 ppm,
while over shorter distances up to about 100 km the agreement is at the level
of 1 - 2 cm, provided that the quality of the input data is sufficient (Torge
und Denker 1999). The existing long wavelength discrepancies are mainly due to
corresponding errors in the underlying global geopotential model and the
terrestrial gravity data. Recent re-computations with updated gravity field
data sets and GRACE geopotential models resulted in significant accuracy
improvements, showing that continental geoid computations with an accuracy
potential of about 3 - 5 cm are possible in areas with good input data (Denker
2006). Furthermore, systematic geoid errors can be modeled using empirical corrector surfaces and GPS and levelling control points (Denker 1998, Denker et al. 2000). This approach for the combination of gravimetric geoid models with GPS and levelling data gives the possibility to compute a height reference surface with cm accuracy (e.g., tested in France, Germany, etc.). An actual example for this approach is the combined quasigeoid model for Germany (German Combined (Quasi)Geoid 2005, GCG2005; Liebsch et al. 2006), which is based on two independent computation procedures applied within a cooperation project between the "Bundesamt für Kartographie und Geodäsie" (BKG) in Frankfurt/Main and the "Institut für Erdmessung" (IfE). Contact person: (for further informations and orders) Dr.-Ing. Heiner Denker Institut für Erdmessung Universität Hannover Schneiderberg 50 D-30167 Hannover Tel.: 0511-762-2796; Fax: 0511-762-4006 Email: denker@ife.uni-hannover.de or http://www.gfy.ku.dk/~iag/egg97.html |
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Publications:
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Digital terrain models
EGG97
Gravity data
EGG97
| last change
21.11.2006 by webmaster Institute of Geodesy |
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| Schneiderberg 50 · D-30167 Hannover · Tel.: (++ 49) 511 762 2795 · Fax.: (++ 49) 511 762 4006 |
| email: office · email: webmaster |