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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 2.4 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