SECTION III

DETERMINATION OF THE GRAVITY FIELD

DETERMINATION DU CHAMP DE PESANTEUR

President: R. Forsberg (Denmark)

Secretaries: M.G. Sideris (Canada)

(Commission III): I. Marson (Italy)

(Commission XII): H. Sünkel (Austria)

I-Terms of Reference

Section III, determination of the gravity field, is engaged in the determination and modelling of the earths gravity field variations. Knowledge of the gravity field variations is of prime importance for geodesy, geophysics and navigation, and efficient and accurate modelling of such variations is a key geodetic research activity. Within the modelling especially the precise determination of the geoid is of great contemporary significance due to requirements from satellite geodesy and oceanography.

The gravity field may be determined by a multitude of measurements: satellite tracking, absolute and relative gravimetry, gravity gradiometry, GPS-levelling, satellite altimetry, astronomical deflections of the vertical, etc. Currently the global determination of the gravity field is significantly improved due to advances in satellite techniques, the release of terrestrial gravity data from formerly classified sources, and the development of efficient airborne gravity survey techniques. Compared to just a few years ago, the global data coverage is improved to such a degree that the only major regions in the world void of data now are some tropical jungle regions and Antarctica.

The development of new global reference models, incorporating the advance in terrestrial data coverage and new satellite data, will be a major benefit to all geodesists in utilization of all these global gravity field data.

In the field of gravimetry, building up national and international networks of absolute and relative gravimetry continues to be a key element within the framework of Section III. With the increasing accuracy of these nets, now approcaching 10-9 g, the study of non-tidal gravity changes becomes increasingly important for geodynamic studies.

The developments in the the gravity field determination expressed in the formal IAG By-Laws, in which Section III is responsible for:

- absolute and relative terrestrial gravity measurements,

- gravity networks and control stations,

- non-tidal gravity variations,

- determination of the external gravity field and geoid the different gravity field data types, and

- reduction and estimation of gravity field quantities.

II-Structure

Commissions :

Commission III : International Gravity Commission

President : I. Marson (Italy)

Commission XII: International Geoid Commission

President : H. Sünkel (Austria)

Special Study Groups :

SSG 3.163 : Assesment and Refinement of Global Digital Terrain Models

Chairman : D. Arabelos (Greece)

SSG 3.164 : Airborne Gravimetry Instrumentation and Methods

Chairman : M. Wei (Canada)

SSG 3.165 : Global Gravity Field Determination and Evaluation

Chairman : N. Pavlis (USA)

SSG 3.166 : Local Gravity Field Modelling and Interpretation

Chairman : T. Basic (Croatia)

SSG 3.167 : Regional Land and Marine Geoid Modelling

Chairman : H. van Gysen (South Africa), from 1998 I.Tziavos (tziavos@topo.auth.gr), Greece.

International Services

International Gravimetric Bureau (BGI)

Director : G. Balmino (France)

International Geoid Service (IGeS)

Director : F. Sansò (Italy)

Commission III

International Gravity Commission

Commission Gravimétrique

Internationale

President: I. Marson (Italy)

Vice-Presidents: G. Boedecker (Germany)

J. Makinen (Finland)

Secretaries N. Courtier (Canada)

E. Klingele (Switzerland)

I. Program Of Activities

1.1 The International Gravity Commission (IGC) promotes scientific investigations of the gravity field of the Earth, its relationship to the form and processes of the Earth's interior and exterior and its variations with time. It does so through the concerted actions of its members to acquire homogeneous gravity anomaly coverage of the Earth. It regulates the operation of the Bureau Gravimetrique International (BGI), established to promote the free exchange of gravity data among the member countries for the benefit of not only geodesists but also geophysicists and geologists.

1.2 The Commission shall seek to collaborate with all international and national organizations concerned with the work of the IGC, particularly for the benefit of the developing countries.

1.3 The Commission will encourage and promote special campaigns to develop and compare instrumentations, techniques and procedure for the adjustment of the results of surveys.

1.4 The Commission will review its functions from time to time to ensure its activities are commensurate with the needs of the geodetic community.

1.5 The Commission will organize at least one meeting of a global nature in four-year period between the IUGG/IAG Assemblies. In addition, it would encourage regional meetings or workshops dedicated to specific problem where appropriate.

II. Functions

The functions of the IGC shall be :

a) To find solutions to problems requiring international cooperation in gravity research and review the results of such investigations ;

b) To recommend and coordinate international programmes for scientific investigation in gravity and related matters which call for concerted action by its members and interested organizations ;

c) To make recommendations to international organizations in matters which relate to the Commissions programme ;

d) To promote and make recommendations for the exchange of gravity data and the publication and dissemination of results of scientific investigations ;

e) to make recommendations to strengthen education and training programmes in gravity and its technology ;

f) to assist developing countries in gravity-related research and technology development ;

g) to promote scientific investigation of gravity on behalf of the international community, taking into account special interests and rights of countries concerning scientific research in the zones under their jurisdiction.

In carrying all its functions, the IGC shall bear in mind the special needs and interests of developing countries.

III. Cooperation

The IGC shall give due attention to supporting the objectives of the international organizations with which it collaborates and which may request IGC to act, as appropriate, as the medium for discharging certain of their responsibilities in matters relating to gravity investigations. The IGC may also request these organizations to take its requirements into account in planning and executing their own programmes.

IV. Organization

4.1 The Assembly of the IGC shall be its principal organ and, without prejudice to the previsions of paragraph of this Article, shall make all decisions necessary to fulfil the functions of the IGC.

4.2 In accordance with the By-Laws of the IAG, the President of the Commission is appointed by the Council of the IAG. Every four years, the Assembly shall elect two Vice-Presidents and a Secretary at its ordinary session. They, along with the President, shall constitute the Executive Board. The President may appoint a second Secretary to assist with the operation of the Commission.

4.3 The Executive Board shall exercise the responsibilities delegated to it by the Assembly and act on its behalf in the implementation of decisions of the Assembly.

4.4. The Executive Board shall review the effectiveness of the operating structure of the Commission and make recommendations to the Assembly on structural or other changes necessary.

V. Working Procedures

5.1 The Assembly shall be convened in ordinary session every four years. Extraordinary sessions may be convened at the discretion of the Executive Board or at request of at least five National Committees.

5.2 Each Country belonging to the IAG and represented at the Assembly shall have one vote, but may accompany its representative to sessions of the Assembly with alternates and advisers as it deems necessary.

5.3 The IGC either through its Assembly or by Executive action subject to the approval of the Assembly may create, for the examination and execution of specific projects, working groups or other subsidiary bodies composed of experts interested in such projects.

VI. Financial Support

6.1 The programmes endorsed and coordinated by the IGC and recommended to its Members Countries for their concerted action shall be carried out with the aid of the resources of participating member Countries, in accordance with the obligations that each Country is willing to assume.

6.2 The expenditure of the IGC shall be financed from funds appropriated for this purpose by IAG or other organizations of the IUGG.

6.3 Contributions to the Commission from outside the structure of the IUGG may be accepted and established as trust in accordance with the financial regulations of the IAG. Such funds will be allocated to programmes of the Commission in accordance with any special wishes of the donor or decisions of the Assembly or the Executive Board.

6.4 Funds so allocated shall be expended by a Secretary of the Commission who will be accountable in this matter to the Central Bureau of the IAG.

VII. Sub-Commissions

7.1 The International Gravity Commission can activate regional Sub-Commission to assist the Commission on specific topics.

7.2 The terms of reference for the Sub-Commissions are :

a) to act as a regional centre for maintaining a catalogue of information of the International Gravity Standardization Network (IGSN71), the International Absolute Gravity Base Station Network (IAGBN) and other gravity base stations in the area, including marine gravity bases, and to facilitate the supply of this information to BGI.

b) To coordinate the maintenance, revision and extension of the network of international gravity base stations in the region,

c) To collaborate with, and where necessary to coordinate assistance to, countries in the area wishing to set up and maintain national gravity reference networks.

d) To assist the BGI in obtaining a good regional coverage of surface gravity data in the area.

e) To undertake, in consultation with the President of the IGC, other activities as required in the pursuit of the objectives of the Commission.

f) To act as regional agency of IGC.

7.3 The coordinators are requested to organize the Bureau of each Sub-Commission as appropriate, based on regional support by parent agencies.

7.4 Membership of each Sub-Commission is envisaged to comprise:

a) Representatives of member Countries of the IUGG in the area concerned, as provided in the IAG Rules for Commissions.

b) Representatives of countries in the area who are not members of the IUGG, to be appointed in consultation with representatives of Member Countries.

7.5 Communication and Reporting

a) The Sub-Commission shall communicate to its members by circular letters, with information copies to the President of the IGC, the Director of BGI, the President of IAG, the President of Section III, and Presidents of eventual other Commissions of the IAG interested.

b) The Sub-Commission shall report to the President of IGC as required.

VIII. Work To Be Performed

8.1 Bureau Gravimetrique International

The Commission supports the continuing operation of the BGI with the emphasis on expanding its gravity data base to include national areas for which no data have been released and to improve coverage on land and on the oceans.

8.2 Absolute Gravity Measurements.

a) Intercomparisons of transportable absolute gravity meters at locations to be determined and in conjunction with the observations of the IAGBN, in order to investigate systematic errors.

b) Further development of the IAGBN

c) Collection of new absolute gravity measurements, regional and local network connections and adjustments.

8.3 International Gravity Standardization Net

Improvements and extensions of the existing net where necessary, new regional adjustment where necessary, combined adjustment of absolute and relative gravity data on fundamental networks.

8.4 Marine Gravity Data

Connection of harbour sites to existing gravity base stations networks, comparisons with satellite altimetry.

8.5 Gravity Variation with Time

Coordination of absolute gravity measurements at superconducting gravimeter sites in order to study instrumental effects and non-tidal gravity variations with time

IX. Working Groups

The Commission has established four Working Groups in order to perform a specific task and to assist the BGI:

WG2 - World Gravity Standards

Chairman: G. Boedecker (Germany)

WG6: Intercomparison of Absolute Gravimeters

Chairman: L. Robertsson (France)

WG7: Global Gravity Monitoring Network

Chairman: B. Richter (Germany)

WG8 : Relative Gravity Network for 1997 Absolute Gravimeter Intercomparison

Chairman: M. Becker (Germany)

X. National Representatives :

P. Steinhauser (Austria)

O. Francis (Belgium)

R. A. Gibb (Canada)

L. Kubackova (Czech Rep.)

R. Forsberg (Denmark)

S. Riad (Egypt)

J.J. Walch (France)

E. Groten (Germany)

E. Lagios (Greece)

R. Hipkin (Great Britain)

G Csapo (Hungary)

B.C. Roy (India)

C. Morelli (Italy)

H.N. Nyapola (Kenya)

Rin Ryong Un (Korea)

S. Takemoto (Japan)

J. Ning (People's Rep. of China)

M. Rodrigues Lisboa (Portugal)

P. Medvedev (Russia)

D. Miskovic (Slovenia)

R. Vieria Diaz (Spain)

R.J. Kleywegt (South Africa)

A. Haller (Sweden)

E. Klingele (Switzerland)

J. Zid (Tunisia)

H. Demirel (Turkey)

J. Faller (USA)

Commission XII

International Geoid Commission

President : H. Sünkel (Austria)

Secretaries: D.G. Milbert (U.S.A.)

W. Kearsley (Australia)

1. Main Objectives

The International Geoid Commission (IGeC) is considered the primary activity center of the IAG for information related to the determination of the geoid for geodetic, geophysical and oceanographic purposes. The International Geoid Service (IGeS), as the working arm of IGeC, established at the Politecnico di Milano, is operational since January 1, 1993.

The International Geoid Commission accomplishes the following tasks:

- data collection

- coordination of computational efforts (merging of geoids)

- execution of geoid determinations through computer centers financed nationally or by international organizations

- execution of computational tasks of regional character

- evaluation and exchange of tested software

- organization of vertical positioning campaigns for geoid control

- organization of contacts to oceanographers in need of geoid data

- organization of expert meetings

The International Geoid Service accomplishes the following tasks:

- issuing a Bulletin to present IGS and its programs

- collecting geoids computed globally or locally

- implementation of such geoid solutions in a worldwide geoid data file

- organization of International Geoid Schools

- preparation of a standard software package for the computation of the gravity field, collecting already existing software packages; a special "call for software" will be issued where a sequence of well defined tasks will be identified as well as format specifications for the input and output files provided

- the scientific program of IGeS will essentially consists in the participation in international projects such as

- studies for the realization of a dedicated gravity field mission

- studies for the use of altimeter data for geoid research

2. Program of Activities

- The Commission will identify centers having data needed for geoid determinations. Such centers include the Bureau Gravimétrique International (gravity data), Technical University Delft (altimeter data), Istituto Geografico Militare Italiano (vertical deflection data), The Ohio State University (Earth gravity field models). Other centers collecting related data such as topographic data, station positions, etc. will also be identified.

- The Commission will make available selected software for geoid computations.

- The Commission will help to coordinate geoid computations by active communication with countries and groups involved with such work.

- The Commission will prepare a newsletter at usually six-month intervals which will be sent to all active country representatives and interested parties.

- The Commission will organize at least one meeting of a global nature in the four-year period between the IUGG/IAG Assemblies.

In addition, it would encourage regional meetings where appropriate.

The Commission will provide information to groups and disciplines needing geoid data.

- The Commission will issue a national report about geoid related activities in its member countries for the past four-year period to be submitted at the IUGG/IAG Assemblies. The report for the period 1991 - 1995 is available as a postscript file and may be fetched using the following computer address:

anonymous ftp

host: ftp-geomatics.tu-graz.ac.at [129.27.91.2]

directory: /pub/IGeC

file: rep91-95.ps

3. Sub-Commissions

The Commission has established three Sub-Commissions, the Sub-Commission for the Geoid in Europe (chairman: Martin Vermeer, Finland), the Sub-Commission for the Geoid in South America (chairman: D. Blitzkow, Brazil), and the Sub-Commission for Geoid in South-East Asia (chairman: A.H.W. Kearsley, Australia). The objectives of the Sub-Commissions are in general agreement with the objectives of IGeC with special emphasis on the respective regions.

4. Executive Committee

The Commission has established an Executive Committee which is composed of the following members:

D. Blitzkow (Brazil)

B. Heck (Germany)

A.H.W. Kearsley (Australia)

D.G. Milbert (U.S.A.)

F. Sansò (Italy)

M. Sideris (Canada)

H. Sünkel (Austria)

H.G. van Gysen (South Africa)

M. Vermeer (Finland)

5. National Representatives

N. Lopez (Argentine)

W. Kearsley (Australia)

* E. Erker (Austria)

* P. Pâquet (Belgium)

J.A. Zelaya (Bolivia)

* D. Blitzkow (Brazil)

* M. Sideris (Canada)

H.T. Hsu (China)

K. Colic (Croatia)

* M. Pick (Czech Republic)

* P. Knudsen (Denmark)

S.R. Archiniegas-Ortega (Ecuador)

* D. Alnaggar (Egypt)

* M. Vermeer (Finland)

* H. Duquenne (France)

* B. Heck (Germany)

* D. Arabelos (Greece)

* G. Papp (Hungary)

B.S. Rajal (India)

J. Supomo (Indonesia)

E. Mogilowski (Israel)

B. Benciolini (Italy)

* Y. Fukuda (Japan)

N. Yahya-Sagarat (Jordan)

A.S. Lwangasi (Kenya)

Chan Peng Yue (Malaysia)

G.L. Strang van Hees (Netherlands)

W.I. Reilly (New Zealand)

C. Ezeigbo (Nigeria)

D. Solheim (Norway)

R.B. Feir (Philippines)

A. Lyszkowicz (Poland)

M.R. Lisboa (Portugal)

* P.P. Medvedev (Russia)

* M.J. Sevilla (Spain)

* H.G. van Gysen (South Africa)

L. Sjöberg (Sweden)

* B. Bürki (Switzerland)

J. Saburi (Tanzania)

C. Wichiencharoen (Thailand)

E. Ayhan (Turkey)

A.H. Dodson (U.K.)

* S. Nerem (U.S.A.)

(* = confirmed)

Special Study Group 3.163

Assessment And Refinement

Of Global Digital Terrain Models

Chairman : D. Arabelos (Greece)

I- Terms Of Reference

Precise gravity field modelling requires the combined manipulation of all available data especially in areas with a strong gravimetric signal. Among other data types, the information concerning the visual topography and its isostatic compensation is substantial in order to smooth the gravity field. Such information is strongly related to the effect of the topography on the various quantities related to the gravity field. For the computation of the various kinds of topographic reductions a digital terrain model that comprises the appropriate coverage and resolution is essential. In local scale applications precise and high resolution local DTMs are usually available in national geodetic data banks. On the other hand, for regional or global scale computations, global DTMs are available in different resolution. According to common experience, these models are affected by systematic and random errors. The aim of this study group will be the assessment and refinement of the global DTMs in different ways: (i) by detecting systematic and random errors (2), by predicting the bathymetry in sea areas with a good coverage of sea gravimetry or satellite altimetry, (3) by incorporating new data in the existing models.

II- Program Of Activities

- Comparisons between the global DTMs in various test areas.

- Assessment of the effects of the various systematic and random errors in different types of topography.

- Refinement of the global DTMs taking advantage of the local (national scale) high resolution DTMs. - Detection of possible shift of coordinates and of gross errors of the global models, by comparing global DTMs with local models of the same resolution.

- Incorporation of new data to the existing models.

- Enhancement of the DTM over ice sheets using satellite and airborne altimetry, GPS, SAR interferometry, etc.

- Tests in order to assess the quality of the improved DTMs. These tests will include prediction experiments in the gravity field by taking into account the topography/ bathymetry in terms of the well known reductions (e.g., residual terrain modelling). The ground truth should be used to investigate the prediction results' quality in both cases i.e. using the original or the improved DTM.

- Prediction of bathymetry by inverting the gravity field in areas with a good coverage with gravity measurements. In case of areas that lack of satisfactory surface data, this data shall be recovered by an inversion of satellite altimetry data.

- Combination of other existing geophysical information is optional. The smoothing effect of the resulting model of bathymetry on other kinds of data, related to the gravity field, such as altimeter data, could be a measure to the quality of the model.

III- List Of Members

D. Arabelos (Greece) - Chairman

R. Barzaghi (Italy)

H. Denker (Fed. Rep. of Germany)

S. Ekholm (Denmark)

Y. Fukuda (Japan)

C. Green (United Kingdom)

R. Haagmans (The Netherlands)

A. M. Hittelman (USA)

W. Kearsley (Australia)

P. Knudsen (Denmark)

L. Li (China)

R. Salman (USA)

D. Sandwell (USA)

G. Sarrailh (France)

H.-G. Schenke (Fed. Rep. of Germany)

H. Sünkel (Austria)

C.C. Tscherning (Denmark)

G.-C. Tsuei (Republic of China)

I.N. Tziavos (Greece)

Corresponding Members :

M.G. Sideris (Canada)

M. Vermeer (Finland)

Special Study Group 3.164

Airborne Gravimetry Instrumentation and Methods

Chairman : M. Wei (Canada)

Objective :

The research areas of the study group are: testing and further development of operational airborne gravity systems; development of new airborne gravity system concepts; software development and comparison; comparative analysis of flight test results.

Activities :

Hardware comparison - Investigation of different approaches to airborne gravity such as scalar gravimetry and vector gravimetry.

Establishment of a test range for comparative airborne tests. Software comparison on standard data sets made available by the SSG.

Error model improvement - Error models of acceleration measurements using scalar gravimeters or inertial systems, orientation errors using different attitude stabilization techniques, analysis of disturbing accelerations using GPS or other sensors.

Estimation and data processing - Investigation of different filtering methods including IIR and FIR digital filters, shaping filters, Kalman filters, wave filter technique and systems theory.

Analysis - Analyzing and evaluating test results from road or airborne tests made available by the SSG. Spectral analysis of airborne gravimetry with respect to spectral range of interest for gravity determination, the effect of system dynamics and flight conditions.

Members :

R.E. Bell (USA)

G. Boedecker (Germany)

J. Brozena (USA)

I. Colomina (Spain)

J. Czompo (Canada)

X. Dong (China)

B. Eissfeller (Germany)

R. Forsberg (Denmark)

W. Gumert (USA)

K. Hehl (Germany)

P. Jones (UK)

B. Kearsley (Australia)

E.E. Klingele (Switzerland)

O.S. Salychev (Russia)

J. Segawa (Japan)

I.N. Tziavos (Greece)

M. Van Gelderen (Netherland)

M. Wei (Canada) - Chairman

Corresponding members :

M.E. Halliday (USA)

G.W. Hein (Germany)

P. Holota (Czech)

C. Jekeli (USA)

W. Keller (Germany)

Y.C. Li (Canada)

K.P. Schwarz (Canada)

L. Wu (Canada)

J. Zhang (Canada)

Special Study Group 3.165

Global Gravity Field Determination

and Evaluation

Chairman : N. K. Pavlis (USA)

I- Terms Of Reference

Global models of the Earth's gravitational potential provide information required for a variety of geodetic, geophysical and oceanographic investigations and applications. High resolution gravitational models (extending to degree and order 360) are necessary to provide a reference surface for local or regional detailed geoid computations. Currently the accuracy of these models represents a limiting factor for oceanographic applications aiming to determine the absolute dynamic topography of the sea surface at increasingly finer resolution. The realization of a Global Vertical Datum depends strongly on the accuracy of global geopotential models.

The development of global high resolution gravitational models currently relies on the combination of information obtained from the analysis of satellite tracking data, terrestrial and airborne gravimetry and satellite altimeter data. It is therefore a task requiring expertise over a wide range of observational, modelling and analysis techniques. The theoretical aspects of the problem, despite the attention that they have received for many years, are still posing interesting questions. There is a persistent need for more rigorous and complete functional description of the data and for more efficient analytical and numerical methods for the set-up and solution of the very large least-squares adjustment problems involved. A dedicated geopotential mapping satellite mission, which may be launched in the near future, will undoubtedly introduce new challenges to the problem of global geopotential modelling.

There is a definite requirement that global geopotential models are accompanied by error estimates that accurately reflect the quality of the models.

Estimating the errors associated with these solutions is a difficult task given the large number of heterogeneous data based upon which these solutions are

developed. A better representation of the stochastic properties of large data sets characterized by geographically varying accuracy and long wavelength systematic errors (such as gravity anomaly data bases), will make a significant contribution towards the improvement of the models.

The evaluation of global geopotential models is based on a variety of comparisons with information independent of the solutions. These comparisons help identify problem areas and therefore constitute a critical part of the continuing effort for the improvement of the models.

The SSG aims to stimulate and coordinate research on these topics and facilitate the exchange of information among groups and individuals working in this area.

II- Program Of Activities

The proposed list of activities and research topics is as follows :

1. Modelling and estimation techniques. This includes :

- functional representation of various data types

- consideration of systematic effects

- efficient techniques for high degree harmonic analysis/synthesis

- alternative techniques for the development of high degree combination solutions

- alternative forms of gravity field representation

2. Improvement on the consideration of the error properties of large data sets used in the development of global gravity models (e.g., consideration of correlated errors among the gravity anomalies in global 30'x30' data bases).

3. Design and set-up of a data base that may include :

- published global gravitational models

- independent data which may be used for evaluation of existing and future gravity models (e.g., GPS/

Levelling-derived geoid undulations)

The SSG, in close cooperation with other bodies of the IAG such as the International Geoid Service, should decide the content and format and consider the logistics involved in establishing and maintaining such a database.

III- List Of Members

Members

D. Blitzkow (Brazil)

J.Y. Chen (China)

T. Gruber (Germany)

C. Jekeli (USA)

A.H.W. Kearsley (Australia)

J.-M. Lemoine (France)

A.N. Marchenko (Ukraine)

R.S. Nerem (USA)

N.K. Pavlis (USA) - Chairman

K. Seltz (Germany)

M.G. Sideris (Canada)

G. Sona (Italy)

H. Suenkel (Austria)

I.N. Tziavos (Greece)

W. Wiejak (Poland)

Corresponding Members

R. Biancale (France)

W. Bosch (Germany)

H. Denker (Germany)

B. Heck (Germany)

E.C. Pavlis (USA)

R.H. Rapp (USA)

P. Schwintzer (Germany)

Special Study Group 3.166

Local Gravity Field

Modelling and Interpretation

Chairman : T. Basic (Croatia)

Terms of Reference

The research areas of the study group are:

1. Use of more detailed and more accurate gravity field data on the local scale.

2. Use of terrain and geophysical (density and seismic) information.

3. Modelling problems with heterogenous gravity field data.

4. Application of different approximation methods (collocation, FHT, ..).

5. Interpretation of results, especially the influence of geological contribution.

6. Comparison to available regional and global solutions.

Membership

Members:

T. Basic (Croatia) - Chairman

D. Behrend (Germany)

W. E. Featherstone (Australia)

A. Kenyeres (Hungary)

N. Kuetreiber (Austria)

D. G. Milbert (USA)

J. Simek (Czech Republic)

G. Strykowski (Denmark)

I. N. Tziavos (Greece)

W. Wiejak (Poland)

Corresponding members:

M. Brkic (Croatia)

H. Denker (Germany)

D. A. Smith (USA)

Special Study Group 3.167

Regional Land and Marine Geoid Modelling

Chairman : H. van Gysen (South Africa)

Objectives And Programme

Regional modelling of the geoid is a traditional part of the activities of Section III, and much has been achieved in this area, both as regards method (especially as regards approximation and numerical methods), and in results. That the interest in geoid modelling continues is due not only to its significance to practical geodetic surveying tasks and the needs of scientific investigations in other fields, but also because there remain unmet and new challenges within the topic itself, arising from new data sources, new theoretical methods, and new computational possibilities.

The objectives of SSG3.167 reflect this duality between past and future. It seeks in part to consolidate what has already been achieved, and to work towards addressing open and new questions. In consolidating the current state of knowledge in geoid modelling, it is appropriate to seek answers the following questions :

* To what extent is there agreement about the various elements of regional geoid modelling: data reductions and data preparation (including data gridding and block averaging) ; what theoretical model to use (which BVP, how to include terrain and non-linear effects); and what numerical techniques? To what extent is it possible to prescribe or recommend a standard procedure ?

* Are there any substantive differences between the modelling procedures for land and marine geoids, and are there real difficulties in working across the land/sea divide ?

* How is the quality of the geoid product assured? Is there agreement on validation procedures and measures of quality ?

* In what form should regional geoids be published - as maps, gridded heights, function coefficients, using data compression techniques ?

Looking to open issues, the following questions present themselves :

* What is the best way of working with heterogeneous data ?

* Are GPS-derived geoid heights forever to be relegated to a validation-only role? Are there new techniques for a common adjustment of GPS and geoid heights? What is the impact of GPS in studying the compatibility of neighbouring datums through geoid determination ?

* Are there new solutions of the GBVP that hold the promise of better theoretical geoid models ?

* Are there new approximation and numerical techniques that hold the promise of a closer representation or more efficient computation ?

* What can be done to improve regional marine geoids, so that they can better serve the needs of oceanographic studies ? In particular, what is the contribution of an accurate regional- scale marine geoid solution on sea surface topography studies ?

Are there lessons that geodesists can learn from the oceanographers' technique

of 'synthetic' geoid modelling ?

The cast of these questions is deliberately quite wide. It will be the task of the members of the study group, working together, to sharpen the focus, to identify the extent of common agreement, and to identify the key issues that remain to be tackled - doing all these things using the Internet. A World-Wide Web home page for the study group is being set up; it will serve not only as an information resource (containing news, a bibliography of recent publications, abstracts, short reports, geoid images, and links to sites where source data or results can be obtained), but will also serve as a forum for the members of the study group (and others). Members of the study group will be asked at intervals for their views on the questions above, and on issues identified by the group, and to contribute summaries of results and images of projects they are working on. These contributions will be published on the SSG's home page.

Members :

O. Andersen (Denmark)

R. Barzaghi (Italy)

D. Behrend (Germany)

E. de Min (The Netherlands)

W. Featherstone (Australia)

R. Hipkin (UK)

B. Kearsley (Australia)

P. Knudsen (Denmark)

J. Krynski (South Africa)

M. Kuhn (Germany)

J. Li (Canada)

C. Merry (South Africa)

D. Milbert (USA)

G. Papp (Hungary)

B. Shaofeng (China)

I. Tziavos (Greece) Co-chairman

H. van Gysen (South Africa) Chairman

Tsuei Gwo-Chyang (Taiwan)

M. Vermeer (Finland)

J. Zhiheng (France)

Associate Members :

W. Wiejak (Poland)

H. Denker (Germany)

M. Pearse (Australia)

D. Blitzkow (Brazil)

International Gravimetric Bureau

Bureau Gravimétrique International

(BGI)

Director : G. Balmino (France)

1.Objectives and Terms of Reference

The main task of BGI is to collect, on a world-wide basis, all gravity measurements and pertinent information about the gravity field of the Earth, to compile them and store them in a computerised data base in order to redistribute them on request to a large variety of users for scientific purposes. The data consists of: gravimeter observations (mainly location - three co-ordinates, gravity value, corrections, anomalies...), mean free air gravity values, gravity maps, reference station descriptions, publications dealing with the Earth's gravity field. Other data types are sometimes used for data validation and geophysical analysis, such as satellite altimetery derived geoid height and gravity anomalies, digital terrain models, spherical harmonic coefficients of current global geopotential models.

BGI has been developing various algorithms and software for data validation and analysis, as well as its own data management system. A large number of services are offered to the users (see below)..

All kinds of gravity data can be sent to BGI, with or without restrictions of redistribution to be specified by the contributors, sometimes in the form of a protocol of usage.

2. Structure and membership

- BGI is one of the offices of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS). It may also be considered as an executive office of the International Gravity Commission.

- It has a Directing Board composed of the following members :

Voting members :

I. Marson (Italy) IGC President

G. Boedecker (Germany) IGC Vice-President

J. Makinen (Finland) IGC Vice-President

G. Balmino (France) BGI Director

R. Forsberg (Denmark) Section III President

J.E. Faller (USA) elected

E. Groten (Germany) elected

P.P. Medvedev (Russia) elected

S. Takemoto (Japan) elected

Non voting members :

L. Robertsson (France) Chairman of WG6

B. Richter (Germany) Chairman of WG7

M. Becker (Germany) Chairman of WG8

N. Courtier (Canada) Secretary

E. Klingelé (Switzerland) Secretary

Ex officio members :

H. Sünkel (Austria) Presid.of Commission XII

F. Sansò (Italy) Director IGeS

P. Pâquet (Belgium) FAGS representative

The central office is located in Toulouse, France, in the premises of the Observatoire Midi-Pyrénées, of which it is one of the services. The other supporting organizations are : The Centre National d'Etudes Spatiales, the Bureau de Recherches Géologiques et Minières, the Institut Géographique National, the Centre de la Recherche Scientifique (via the Institut National des Sciences de l'Univers). There exists a covenant between these agencies to guarantee their support to the BGI.

The address of the office is :

Bureau Gravimétrique International

18, Avenue Edouard Belin

31401 Toulouse Cedex 4, France

Phone : 33-6133-2980

FAX: 33-6125-3098

Email: balmino@pontos.cst.cnes.fr

3. The BGI Bulletin d'Information

The office issues a Bulletin d'Information twice a year (generally in June and December).

It contains :

- general information in the fieled of the Bureau itself, about new available data sets,

- contributing papers in gravimetry,

- communications at meetings dealing with gravimetry (e.g. IGC meeting).

Every four years, an issue (which may be an additionnal one) contains the National Reports of Activities in Gravimetry.

The full catalogue of the holdings is issued every two years.

The Bulletin is sent free of charge to individuals and institutions which currently provide information and/or data to the Bureau. In other cases, information and subscription prices can be obtained on request.

There exist 79 issues and about 350 subscribers as of mid-1996.

4. Providing data to BGI

Essential quantities and information for gravity data sumission are :

(a) position of the site :

- latitude, longitude (to the best possible accuracy)

- elevation or depth :

for land datra : elevation of the site (on the physical surface of the Earth)

for water stations : water depths

(b) measured (observed) gravity, corrected to eliminate the periodic gravitational effects of the Sun and the Moon, and the instrumentation driftt.

(c) Reference (base) station(s) used. For each reference station (a site occupied in the survey where a previously determined gravity value is available and used to help establish datum and scale for the survey), give name, reference station number (if known), brief description of location of site, and the reference gravity value used for that station. Give the datum of the reference value; example : IGSN71.

Give supplementary elevation data for measurements made on towers, on upper floor of buildings, inside of mines or tunnels, atop glacial ice. When applicable, specify whether gravity value applied to actual measurement site or it has been reduced to the Earth's physical surface (surface topography or water surface). Also give depth of actual measurement site below the water surface for underwater measurements.

For marine gravity stations, gravity value should be corrected to eliminate effects of ship motion, or this effect should be provided and clearly explained.

Additional informations are optional, but welcome.

5. Services

The most frequent service BGI can provide is data retrieval over a limited area. Data are sent on diskettes or printouts or transfered electronically. Data coverage plots may also be provided, usually over 20°*20° areas. Cases of massive data retrieval requests may be considered; they are studied and may be processed in a specific way. The simplest way for users is to acquire the open files of the BGI data base which are on two CDs.

Other services include :

- data screening,

- provision of gravity base station information,

- data evaluation and gridding,

- computation of mean values,

- contouring,

-supply of, or information on existing maps (catalogue available)

The costs of the services have been established in view of the categories of users-mostly contributors of measurements and scientists, and also considering the large amount of our host organizations. The charging policy is explained in detail in the Bulletin d'Information.

Some of the services may be provided free of charge upon request, to data contributors, individuals working in universities, such as students, and generally to any person who can contribute to the BGI activities on a data or documentation exchange basis.

6. Program of activities for the next four years

- continue publication of the Bulletin d'Information

- continue data collection, archiving and distribution : emphasis will be on those countries which have not, or seldom, contributed to the BGI data bank. First priority is then given to careful data evaluation; Land data and marine data are validated using different software. Satellite altimetry derived free-air anomalies are to be more and more frequently used to validate sea measurements.

- assist IGC in setting up the International Absolute Gravity Data Base Station (IAGBN), and assit in the intercomparisons of instrument

- establish simple procedures for the collection and archiving of absolute measurements.

- Link with the commission for the Geoid in data preparation in view of geoid computations and evaluations to be performed by the International Service for the Geoid.

-Assist in promoting satellites techniques to improve our global knowledge of the Earth's gravity field : satellite-to-satellite tracking, satellite gradiometry, etc...

International Geoid Service

Service International pour le Géoïde

(IGeS)

Director : F. Sansò (Italy)

Objectives

The main tasks of IGeS are:

- to collect data referring to the geoid on a worldwide scale,when possible to validate them and to disseminate them upon request among the scientific community; other auxiliary data can also be collected by IGeS, when useful for the geoid determination, and might be made available with the sharp exclusion of gravity anomalies data,

- to collect, test and, when allowed, to distribute software for the geoid determination,

- to conduct researches on the best procedures for the geoid determination, possibly from different sources conveniently combined,

- to provide the international community with technical schools where consolidated techniques of geoid determination, be demonstrated and students trained in the use of the relevant software,

- to produce, at least once per year, an IGeS Bulletin on geoid related matters.

Data and software given to IGeS remain property of the source which can dictate the conditions of use and restrict their distribution. IGeS itself can indeed perform geoid computations within different projects, but not in economic competition with Firms or Public Organizations institutionally devoted to that.

Structure

The Service is for the moment provided by a Main Centre, at the Politecnico of Milano, and by individual scientists, called advisors, though in future more Centres could join the organization. IGeS is related to IAG, being the operative arm of the International Commission for the Geoid, operating within IAG - Section III. As such it has a Directing Board which receives a report and defines the long term program of the Service.

The Directing Board is composed by :

President of Section III

Secretaries of Section III

Director of BGI

Director of IGeS;

in this way a strong link is created between the two services of Section III, namely IGeS and BGI.

The Director of IGeS is nominated by the President of the International Geoid Commission, upon recommendation of the past Directing Board.

The IGeS-Main Centre is supported by Italian authorities which nominate its Director, upon recommendation of the International Geoid Commission. Its structure, tools and activities are illustrated in the IGeS reports to the International Geoid Commission. In the present period Director of IGeS as well as of its main centre is Fernando Sansï (Italy). The IGeS advisors are individual members of IGeS, which have had an outstanding activity in the field of geoid determination and also can represent IGeS in both research and teaching activities.

At present, beyond the members of the Directing Board, the following distinguished scientists are IGeS advisors:

C.C. Tscherning (Denmark)

M. Sideris (Canada)

W. Kearsley (Australia)

J. Milbert (USA)

H. Denker (Germany)

M. Vermeer (Finland)

D. Arabelos (Greece)

M. Sevilla (Spain)

B. Benciolini (Italy)

R. Barzaghi (Italy)

The list is open and nominations are welcome by IGeS Director.

Finally within the structure of IGeS, Working Groups can be established for specific purposes, limited in time. At present one W.G. is functioning on "Validation of Global Models" chaired by M. Sideris (Canada).

Programs till 1999

Beyond usual activities of IGeS, the following programs are worth of specific mention:

1) Geomed: improvement of the existing geoid and SST in the Mediterranean area,

2) Italgeo 2000: improved gravimetric geoid for Italy, with implementation of integrated procedures to combine gravimetric and GPS-leveling data,

3) Improved methodologies for the determination of global models,

4) Organization of 2 international geoid school, of which at least one in a developing country.
Changed 1998.07.09 by CCT.