MODELLING - SPECIFIC ACCOMPLISHMENTS
geoid or quasi-geoid determinations on a local or regional scale have
been carried out by members of the SSG in different sea/land test
areas using combinations of heterogeneous data sources referred to
high degree and order geopotential solutions. The methods employed
range from the classical numerical integration procedures, the
spectral FFT techniques and the stochastic least-squares collocation
algorithms to the input/output system theory algorithms in the
frequency domain (Abd-Elmotaal 2000, Andritsanos and Tziavos 2000,
Fotopoulos et al. 2000, Duquenne 2000, Rodriguez 1999, Toth et al.
2000, Tziavos 2000). The results obtained meet the today accuracy
demands of a wide number of applications related to surveying,
geodesy, geophysics and other disciplines of geosciences. The quality
of the geoid heights produced in land areas was assessed by
comparisons with corresponding heights at GPS benchmarks (see, e.g.,
Featherstone 2001, Marti et al. 2000, Toth et al. 2000). The use of
GPS in combined adjustments with gravimetric geoid heights is
discussed by Kotsakis and Sideris (2000). The estimated accuracy of
the determined geoid/quasi-geoid heights reached in some cases the
level of one decimeter and in pure marine geoid solutions found close
to one centimeter (Fernandes et al. 2000, Rodriguez 2000, Vergos et
al. 2001, Andritsanos 2000, Andritsanos et al. 2000). Gravimetric
geoid solutions were also computed on a national scale by different
authors and attempts were made to the direction of datum unification
(see, e.g., Featherstone 2000, Marti et al. 2001, Fotopoulos et al.
2000, Toth et al. 2000).
(2000) discussed problems occurring in linear signal estimation from
discrete gridded data and has drawn interesting conclusions related to
modern operational geodesy and practical applications like
local/regional geoid determination. Hwang and Lih-Shinn Hwang (2001)
computed an improved geoid model for Taiwan with an accuracy ranging
from 2 cm to 10 cm in order to assess the accuracy of orthometric
heights and detect vertical rates of land motion. Toth et al. (2001)
investigated the recovery of gravity and geoid in Hungary from torsion
balance data using collocation and spectral techniques. A thorough
comparative analysis on regional high-frequency geoid computations in
Canada using a synthetic gravity field is given by Novak et al.
simulation studies were carried out in the frame of modelling the long
wavelength part of the Earths gravity field taking advantage from
the new satellite gravity missions of CHAMP, GRACE and GOCE.
Tscherning (2001) has illustrated the advent of pure satellite gravity
models by the new missions. These models will considerably improve our
possibilities for computing precise quasi-geoidal differences. The
expected accuracy could be better enough than that obtained by EGM96.
effects of density variations on terrain corrections and the handling
of topography in practical geoid determination were studied by several
authors (see, e.g., Kuhn 2000, Omang and Forsberg 2000, Tziavos and
Featherstone 2000, Biagi and Sanso 2000). The geophysical dimension of
a regional quasi-geoid determination and its correlations with Moho
depths and other geophysical parameters have been studied in several
papers (see, e.g., Abd-Elmotaal 2000, Kuehtreiber and Abd-Elmotaal
2000, Toth et al. 2000). In the same frame and according to Molodensky
theory efficient ways of computing the G1 term and the influence of
the grid size of digital elevation models on quasi-geoid contribution
has been also investigated (Merry 2001, Amod 2001). Tsoulis (2000)
studied the spherical harmonic analysis of a global digital elevation
model using the Airy/Heiskanen and Pratt/Hayford isostatic models.
essential role of airborne gravimetry in combination solutions with
marine gravity observations, satellite altimetry derived and land
gravity for high resolution geoid computations is demonstrated in
several studies (Bastos et al. 2000, Olesen et al. 2000). The
increasing interest for new airborne gravity surveys during the last
two years contributed to the better knowledge of the geoid and sea
surface topography in different areas (Greenland, eastern
Mediterranean and Crete island, Azores islands, Corsica). The geoid
results reached the level of one decimeter or better in several cases
in terms of standard deviation of the differences between the computed
geoid heights and the corresponding heights derived from satellite
altimetry (Andritsanos et al. 2000, Fernandes et al. 2000, Rodriguez
1999, Rodriguez and Sevilla 2000). Several authors discussed the role
of satellite altimetry in gravity field modelling in self-seas and
coastal areas and pointed out inherent problems when working across
the land/sea boundary (see, e.g., Andersen and Knudsen 2000,
Andritsanos 2000, Hipkin 2000, Vergos et al. 2001). Pure altimetric
geoid solutions were carried out taking advantage from the most
accurate mission of TOPEX/Poseidon and the high resolution geodetic
missions of GEOSAT and ERS-1 (see, e.g., Fernandes et al. 2000, Vergos
et al. 2000, Andritsanos et al. 2000). Moreover, marine geoid
solutions were computed by combining altimetric data with shipborne
gravity data in areas presenting geodynamic and oceanographic interest
(see, e.g., Rodriguez 1999, Andritsanos 2000, Fernandez et al. 2000,
Vergos et al. 2000).
geographical distribution of the members of the SSG made difficult
their close cooperation and common research. However there was a
collaboration between different members on an individual basis. The
research carried out by the members of SSG during the last two years
was mainly addressed in its different targets, promising results were
reported and important conclusions were drawn with respect to regional
geoid modelling. However, additional work should be done within the
next two years until the General Assembly of IUGG in Saporo, Japan, in
1993. Some suggestions for future work are summarized as follows:
of the procedures used for the computation and evaluation of the
regional geoid/quasi-geoid solutions and their errors.
of the comparison and combination techniques between geoid heights and
systematic analysis on the contribution of the new satellite gravity
missions to the improvement of the long-wavelength part of a geoid/quasi-geoid
marine geoid solutions by combining satellite altimetry, airborne and
sea gravimetry data for oceanographic applications.
Abd-Elmotaal, H.: A
gravimetric geoid for Egypt derived by FFT techniques. Presented at
Gravity, Geoid and Geodynamics 2000, Banff, Canada, July 31-August
Vening Meinesz inverse isostatic problem with local and global Bouguer
anomalies. Journal of Geodesy,
74/5, 390-398, 2000.
Amod, A.: The use of spectral
methods in quasi-geoid determination. Ph.D. Thesis, Dept. of
Geomatics, University of Cape Town, 2001.
Andersen, O.B. and P. Knudsen: The role of satellite altimetry in gravity
field modelling in coastal areas. Phys.
Chem. Earth (A), vol. 25, No. 1, 17-24, 2000.
Andritsanos, V.D., and I.N. Tziavos: Estimation of gravity field
parameters by a multiple input/output system. Phys. Chem. Earth (A), vol. 25, No. 1, 39-46, 2000.
Andritsanos, V.D., G.S. Vergos, I.N. Tziavos, E.C. Pavlis and S.P.
Mertikas: A high-resolution geoid for the establishment of the GAVDOS
multi-satellite calibration site. Proceedings
of Gravity, Geoid and Geodynamics 2000, Banff, Canada, July
31-August 4, 2000.
Andritsanos, V.D., M.G. Sideris and I.N. Tziavos: Quasi-stationary sea
surface topography estimation using input/output method and error
analysis of satellite altimetry. Accepted for publication in Journal of Geodesy.
Andritsanos, V.D.: Optimal
combination of terrestrial and satellite data using spectral methods
for applications to geodesy and oceanography. Ph.D. Thesis, Dept.
of Geodesy and Surveying, Aristotle Univ. of Thessaloniki, 2000.
Bastos, L., C. Cunha, R. Forsberg, A. Olesen, A. Gidskehaug, L. Timmen
and U. Meyer: On the use of airborne gravimetry in gravity field
modelling: experiences from the AGMASCO project. Phys.
Chem. Earth (A), vol. 25, No. 1, 1-8, 2000.
Biagi, L. and F. Sanso: A new algorithm for fast RTC computation.
Presented at Gravity, Geoid and
Geodynamics 2000, Banff, Canada, July 31-August 4, 2000.
Duquenne, H.: The geoid in New Caledonia. Private communication, 2001.
Featherstone W.E.: Towards unification of the Australian height datum
between the mainland and Tasmania using GPS and the AUSgeoid98 geoid
model. Geomatics Research
Australasia, 73, 30-40, 2000.
Featherstone W.E.: Absolute and relative testing of gravimetric geoid
models using GPS and orthometric height data. Accepted for publication
in Computers and Geosciences,
Fernandes, M.J., L. Bastos, J. Catalao: The role of multi-mission ERS
altimetry in the determination of the marine geoid in the Azores, Marine
Geodesy, 23, 1-16, 2000.
Fotopoulos, G., C. Kotsakis and M.G. Sideris: A new Canadian geoid model
in support of levelling by GPS. Geomatica,
54.1, 53-62, 2000.
Hipkin, R.: Modelling the geoid and sea-surface topography in coastal
areas. Phys. Chem. Earth (A),
vol. 25, No. 1, 9-16, 2000.
Hwang , C. and L.S. Hwang: Use of geoid for assessing trigonometric
height accuracy and detecting vertical land motion. Draft, 2001.
Kotsakis, C.: Multiresolution
aspects of linear approximation methods in Hilbert spaces using
gridded data. Ph.D. Thesis, Dept. of Geomatics Eng., University of
Calgary, UCGE Rep. Nr. 20138,
Kotsakis, C. and M.G. Sideris: On the adjustment of combined GPS/levelling/geoid
networks. Journal of Geodesy,
73/8, 412-421, 1999.
Kuehtreiber, N. and H.A. Abd-Elmotaal: Gravimetric geoid computation for
Austria using seismic Moho data. Presented at Gravity, Geoid and Geodynamics 2000, Banff, Canada, July 31-August
Kuhn, M.: Geoidbestimmung unter Verwendung verschiedener Dichtehypothesen. Ph.D.
Thesis, DGK, Reihe C, Heft Nr. 520, Munich 2000.
Marti, U., A. Schlatter and E. Brockmann: The possibilities of combining
levelling with GPS measurements and geoid information. Presented at
the EGS XXVI Gen. Assembly,
Nice, France, 26-30 March, 2001.
Merry, C.L.: Personal communication,
Novak, P., P. Vanicek, M. Veronneau, S. Holmes, W.E. Featherstone: On the
accuracy of modified Stokess integration in high-frequency
gravimetric geoid determination. Journal
of Geodesy, 74/9, 644-654, 2000.
Olesen, A.V., R. Forsberg, K. Keller, A. Gidskehaug: Airborne gravity
survey of Linkoln Sea and Wandel Sea, north Greenland, Phys. Chem. Earth (A), vol. 25, No. 1, 25-30, 2000.
Omang, O.C.D. and R. Forsberg: How to handle topography in practical
geoid determination: three examples. Journal
of Geodesy, 74/6, 458-466, 2000.
Rodriguez, G., M.J. Sevilla, C. de Toro: Crossover analysis in the
Canary-Azores region of ERS-1 altimetric data. Bollettino di Geofisica Teorica ed Applicata, 40, 3-4, 1999.
Rodriguez, G.: Geoid studies using
terrestrial and marine gravity data (in Spanish). Ph.D. Thesis,
Universitad Complutense de Madrid, 1999.
Toth, Gy., Sz. Rosza, V.D. Andritsanos, J. Adam and I.N. Tziavos: Towards
a cm-geoid for Hungary: recent efforts and results. Phys.
Chem. Earth (A), vol. 25, No. 1, 47-52, 2000.
Toth, Gy., Sz. Rosza, J. Adam and I.N. Tziavos: Gravity field recovery
from torsion balance data using collocation and spectral methods.
Presented at the EGS XXVI Gen.
Assembly, Nice, France, 26-30 March, 2001.
Tscherning, C.C.: Geoid determination after the first satellite gravity
missions (draft version). To appear in the Festschrift volume for
Prof. Torge, 2001.
Tsoulis, D.: A comparison between the Airy/Heiskanen and the Pratt/Hayford
isostatic models for the computation of potential harmonic
coefficients. Journal of Geodesy,
74/9, 637-643, 2000.
Tziavos, I.N. and W. Featherstone: First results of using digital density
data in gravimetric geoid computation in Australia. Proceedings of Geodynamic,
Gravity and Geoid 2000, Banff, Canada, July 31-August 4, 2000.
Tziavos, I.N.: The geoid in the Mediterranean-Recent results and future
plans. Invited paper, presented at the FIG
Symposium Mediterranean Surveyors in the new millennium,
Malta, Sept. 18-21, 2000.
Vergos, G.S., F.A. Bayoud, M.G. Sideris and I.N. Tziavos: High-resolution
geoid computation by combining shipborne and multi-satellite altimetry
data in the eastern Mediterranean sea. Presented at the EGS
XXVI Gen. Assembly, Nice, France, 26-30 March, 2001.