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SCAR Report No 21, January 2002

Analysis of Regional Geoid Estimation in Victoria Land

R.Barzaghi(1), A.Borghi (1),A.Capra(2), S.Gandolfi(3)
(1)DIIAR Dept.-Polytechnic of Milano (2)Polytechnic of Bari
(3)DISTART Dept.- University of Bologna

Introduction

The improvement in the knowledge of Antarctic Geoid is currently one of the main purposes of the international scientific community. This is a hard task due to the lack of gravimetric observations which are directly connected to the particular characteristics (climatic and hazardous) of the Antarctic continent. Each nation still owns gravimetric and geodetic data on non-public databases and partially for this reason, it is quite difficult to obtain reliable geoid estimates. In order to define a project for the estimation of a high precision Geoid, the international scientific community needs a global geodetic and gravimetric database for the Antarctic continent (Capra & Gandolfi 2001).

The actual situation in geoid knowledge in Antarctica can be summarized in the following points

a precise geoid (<1 m precision) is not available at the moment
some discrepancies (more than 1m) between observed values and OSU91A and EGM96 are present;
a relatively poor distribution of geodetic and geophysical data, overall gravity data is present.

Taking into account the state of the art equipment available, some preliminary analysis has to be made in order to evaluate the possibility of estimating a really high precision geoid of Antarctica.
The impact of global model and observed data on high-resolution geoid estimation in a coastal area of Victoria Land, Antarctica was performed and some preliminary analysis for verifying data compatibility and reliability were made.
Moreover the effectiveness in gravity data reduction of available global geopotential models, ice thickness and digital terrain model data in the coastal area of Victoria Land (85 < φ < -65; 130 < λ < 181) was tested. This analysis allows us to point out some constraints on the possible high-resolution geoid computation.
One of the main problems is the recovery of gravimetric data from the scientific community. In particular, for the historical data as a lot of information about the survey characteristics were lost.

 

Fig 1 - Investigated area

 

Fig 2 - The ice thickness (a) and gravity data measurements (b) performed by different groups in Antarctica

Ice surface topography has been taken from BEDMAP, the cartography from ADD (Antarctic Digital Database), the ice density mean values from literature, and Airborne Gravity data regarding the Northern part of Victoria Land has been provided from Reitmayr et al. (1963 - 1995) (USG: A.P. Crary (1963), BEN: S.B. Smithson (1972), ROB: E.S. Robinson & J.F. Splettstoesser (1984)). The EGM96 and the GPM98CR global models have been chosen for reducing the gravity data.

Statistics of the gravity data set

In order to check the quality of the available data, some preliminary statistical test has been performed.
As for the gravity data, the most significant results are listed in Table.1

.

Tab. 1 - Statistics of the gravity data set (values in mGal)

The geopotential model EGM96 were computed up to degree 360 while the geopotential model GPM98CR (Wenzel, 1998) to degree 720
These are very poor results that are naturally confirmed by a campaign-based analysis that is described in Table 2. The table also indicates that only few campaigns show an efficient reduction using EGM96 geopotential model.
The reasons for this behaviour are quite difficult to find both for the unreliability of the global models in the Antarctic continent and for the poor description of the gravimetric survey.
In Table 3 a test on the reduction of the masses above sea level is reported (no bathymetric information are taken into account).

 

Tab. 2 Statistics of the gravity data set for each gravimetric campaign (values in mGal)

(Blue colour: campaign with an efficient reduction using the geopotential model EGM96)

Tab. 3 Summary of the statistics on the reduction of the masses above sea level (values in mGal)

Conclusions and future perspectives

Geodetic and gravimetric data collection was carried out and a test to check available gravity data for geoid computation was done. A very poor efficiency of the used global potential models in reducing the existing gravity data was proved. Moreover the correlation between ice thickness/DTM data and gravity appears to be not sufficient for a reliable data reduction.
In the future, a deeper investigation into the different data sets will be necessary in order to verify data compatibility before starting any kind of geoid estimation.

Bibliography

Antarctic Digital Database: http://www.nerc-bas.ac.uk/public/magic/add_main.html

BEDMAP project: http://www.antarctica.ac.uk/aedc/bedmap/

Capra A. and Gandolfi S., (2001) &emdash; A project for Archiving a Managing Physical Geodesy data in Antarctica. SCAR Report International Council of Science, No. 20, May 2001, 6-9.

Reitmayr, G. (1995): The Gravity Fields of Victoria Land and the Adjacent Oceans, Antarctica. 7th International Symposium Antarctic Science, Siena, 10-15.9.1995