LAND/MARINE GEOID
MODELLING - SPECIFIC ACCOMPLISHMENTS
Different
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).
Kotsakis
(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.
(2000).
Several
simulation studies were carried out in the frame of modelling the long
wavelength part of the Earth’s 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.
The
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.
The
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).
FUTURE WORK
The
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:
·
Refinement
of the procedures used for the computation and evaluation of the
regional geoid/quasi-geoid solutions and their errors.
·
Investigation
of the comparison and combination techniques between geoid heights and
GPS/levelling heights.
·
More
systematic analysis on the contribution of the new satellite gravity
missions to the improvement of the long-wavelength part of a geoid/quasi-geoid
determination.
·
High-resolution
marine geoid solutions by combining satellite altimetry, airborne and
sea gravimetry data for oceanographic applications.
REFERENCES
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