ISG - Projects
Integration and validation of local geoid estimates
Commission 2 Joint Working Group 2.4 (joint with ISG, IGFS, ICGEM)
Regional geoid estimates (in areas having e.g. extension
of some degrees) can give a detailed description of the
high frequency geoid features. They are based on local
gravity databases and high resolution DTMs that allow to
reconstruct the high frequency spectrum of the gravity
field, thus improving the global geopotential model
representation. Local geoid estimates are computed
following well-defined estimation methods that can give
reliable results. These estimates are frequently used in
engineering applications to transform GPS derived
ellipsoidal heights into normal or orthometric heights.
Despite the fact that methodologies in geoid estimation
have a sound basis, there are still some related issues
that are to be addressed.
In comparing local geoid estimates of two adjacent areas
inconsistencies can occur. They can be caused by the
different global geopotential models used in representing
the low frequency part of the gravity field spectrum
and/or the method that has been adopted in the geoid
estimation procedure. Biases due to a different height
datum can also be present. Thus proper procedures should
be proposed and assessed to homogenize the two local
solutions.
Validation of regional geoid is another issue that is to
be better standardized. Usually the validation is based on
GPS/levelling data that are compared with the geoid
estimates. Differences between GPS/levelling and
geoid/quasi-geoid values are then fitted with polynomial
surfaces to account for reference frames discrepancies.
Statistics of the post-fit residuals are then considered
as the estimates of the geoid precision. In this respect,
some issues related to the fitting procedure could be
better defined and standardized.
Finally, another question to be investigated is the
definition of procedures for local geoid estimates in
areas with sparse gravity data. The interactions existing
among the maximum degree of the global geopotential model,
the DTM resolution, the local gravity database mean
spatial density, the estimation geoid grid step should be
studied to define some general best-practice rules.
The objectives of the Working Group are to:
- Study and define methodologies for merging local geoid
solutions
- Discuss and define proper procedure to assess the geoid
estimation precision
- Compare different geoid estimation methods
- Define general rules for geoid estimation in areas with
sparse gravity data
The Working Group activities will be developed following
the objective stating above. Particularly, based on the
geoid solution available at ISG, numerical tests will be
carried out. Members will be required to participate in
these tests with their own software/methodologies. Results
of these tests will be discussed through the ISG website
and in face-to-face meeting to be held in connection with
major geodesy related congresses.
Members of the JWG 2.2.1 are: M. Reguzzoni (Chair, ISG,
Italy), G. Vergos (Vice-Chair, AUTh, Greece), G. Sona
(ISG, Italy), R. Barzaghi (IGFS, Italy), F. Barthelmes
(ICGEM, Germany), M.F. Lalancette (BGI, France), T. Basic (UniZG, Croatia), H.
Yildiz (General Command of Mapping, Turkey), N. Kuhtreiber
(TUG, Austria), H. Abd-Elmotaal (Minia University, Egypt),
W. Featherstone (Curtin University, Australia), Jianliang
Huang (NRC, Canada), Cheinway Hwang (National Chiao Tung
University, Taiwan), Shuanggen Jin (Chinese Academy of
Sciences, China), G. Guimaraes (UFU, Brazil).
GEOMED-2 project
The main scope of the proposed GEOMED-2 project
is the determination of a high-accuracy and
high-resolution geoid model for the Mediterranean Sea
employing land and marine gravity data and GOCE/GRACE
based Global Geopotential Models (GGMs). The processing
methodology will be based on the well-known
remove-compute-restore method following both stochastic
and spectral methods for the determination of the geoid
and the rigorous combination of heterogeneous data.
Within a pre-processing step, all available gravity
observations for the wider Mediterranean basin will be
collected, validated, homogenized and unified in terms
of their horizontal and gravity system, so as to derive
a gravity data base that will be used for the
determination of the geoid. The so-determined geoid
model will form the basis for height-system unification
within the Mediterranean Sea and to derive
high-resolution models of the Mean Dynamic Topography
(MDT) to be used in estimating the circulation in the
Mediterranean Sea. The validation of the produced models
will be carried out with GPS/Levelling and drifter data,
respectively.
The GEOMED-2 project is sponsored by the European Space
Agency (ESA) and all the participating Institutions. It
starts at the beginning of 2015 and it will end by
November 2016. The final delivered products will be the
geoid estimate in the Mediterranean area, the Mean
Dynamic Topography(MDT) of the Mediterranean Sea and the
implied circulation model.
Apart from the IGFS, BGI and ISG services, the project
partners are:
- Politecnico di Milano (Italy)
- GET, SHOM and OCA/Geoazur (France)
- Aristotle University of Thessaloniki (Greece)
- DTU Space (Denmark)
- General Command of Mapping (Turkey)
- University of Zagreb (Croatia)
- University of Jaen (Spain)
Relief model of Mediterranean area built from SRTM and EMODnet
Present and Future Activities
Beyond institutional activities of ISG, it
is worth mentioning the following programs:
- participation to the International ESA
Gradiometric Mission (GOCE)
- participation to the GOCE Global Models
validation
- computation of improved geoids for
Italy and the Mediterranean area
- participation within GGOS to the study of the
height datum unification problem
- study of improved methodologies for
the determination of the geoid at global and local level
- organization of International Geoid School,
possibly one school every two years