1. Preface and highlights

According to the by-laws of IAG, Section III is responsible for the determination and modelling of earths’s gravity field variations. This activity includes absolute and relative gravity measurements, gravity networks and control stations, non-tidal gravity variations, determination of the geoid and external gravity field from the multitude of gravity field data available (satellite measurements, gravimetry, deflections of the vertical, gradiometry, GPS-levelling etc.), and the reduction and estimation of gravity field quantities. It is a special challenge for Section III to encourage international cooperation in the area of gravity field mapping, and work towards the ultimate goal of physical geodesy: to have a complete coverage of reliable gravity field data over the entire earth.

The period 1995-99 has seen a healthy development in geodetic gravity field science. In the period the new joint NASA-OSU-NIMA spherical harmonic reference model EGM96 has been released, with testing carried out by an international evaluation panel as a special working group (headed by M. G. Sideris) of IGeC. The new model has included significant new terrestrial data sets, improved satellite altimetry and -tracking, and ensures good fits to satellite orbits, and will be a good base for numerous activities, especially the need for good geoid models. Similar models have since been developed by other researchers in both Europe and Russia (GAO-98).

It is a very important step forward that new gravity field satellite missions have finally been approved, and will make even better long-wavelength earth models in the future. The German CHAMPS satellite is planned for launch in 1999, the NASA GRACE mission in 2002, and (probably) the ESA GOCE mission in 2004. In the future it will be a challenge for Section III to utilize these satellite data along with other gravity field information and digital terrain models to obtain the best possible gravity field and geoid models, and linking the vertical datums on the continents to a uniform world height datum.

Other major developments have been the computations of large-scale geoid models for many major continental regions (e.g., Europe, US, Canada, Australia ..) by FFT methods, and in some countries the subsequnt GPS/levelling fitting, allows height determination down to 1 cm in some countries. The old dream of a 1 cm geoid is thus a reality .. at least relatively, and in areas with good gravity coverage, good vertical control and not too excessive topography.

On the data collection sides developments in absolute gravimetry continues to push routine gravity measurement accuracy (to the micro-gal level), allowing geodynamic gravity applications (e.g., monitoring of land uplift and mass changes, by co-location with geodynamic GPS measurements). Airborne gravimetry has matured in the period, and is now a routine tool for geodetic data collection, with major programs in the period taking place over the Arctic regions. Research into the use of airborne gravity for geoid determination has flourished, as has the push towards new sensor systems (INS units and gradiometry).

On the theoretical side topics such as wavelets, optimized kernels, faster collocation, improved BVP solutions continue to give new insight into improved gravity field modelling. For geoid determination IGeS geoid schools have widened international IAG cooperation in gravity field modelling. During the period IGeS geoid schools were held in Rio de Janeiro (1997) and in Milano (1999), attracting a large percentage of students from countries which are rarely active otherwise in IAG activities.

The availability of digital terrain models (land elevations, bathymetry etc.), import auxillary data for local gravity field modelling, have been improved in the period. Many countries have released dense DEM data for geodetic scientific use, and several projects have been underway to compile global DEM’s at the 1 km level. Developments in Synthetic Aperture Radar projects (notably the planned NASA/NIMA Shuttle SAR topography mission), and new laser altimetry satellite missions, and regional compilations of ERS SAR data, promises to generate very dense DEM data in the coming years. It is antipated that much of this data will be publically available. It will be a challenge for Section III to coordinate these data in a suitable (thinned) format, to make data accessible with a limited work effort for global or regional gravity field approximation.
 

2. Section III organisation and meetings 1995-99

IAG Section III is divided into two Commissions (International Gravity Commission, IGC, and the International Geoid Commission, IGeC), which again has topical working groups (IGC) or regional working groups (IGeC). A number of Special Study Groups (SSG’s) have been established in order to coordinate research in well-defined topical areas. Whereas Commissions are longer term structures, SSG’s are concentrating on time-limited activities, typically over a 4-year period between IUGG general assemblys.

The Section III steering committee consists of the Section President (Rene Forsberg), Section Secretary (Michael G. Sideris, Canada), the presidents of the Commissions (I. Marson and H. Sunkel), and the SSG presidents (see below). The Steering Committee has mainly been communicating by E-mail, with more formal meetings in Rio (1997) and Trieste (1998).

A main task of Section III has been the preparation of the IAG scientific meetings in Rio de Janeiro, 1997 (proceedings "Geodesy on the Move - Gravity, Geoid, Geodynamics and Antarctica" published in the Springer Verlag IAG Series), and the IUGG symposium G3 in Birmingham, 1999.

Special workshops, sponsored by Section III, include the Intercomparison of Absolute Gravimetres (Sevres, 1997) and the "Airborne Gravity and the Polar Gravity Field" workshop held in Kangerlussuaq, Greenland, 1998. Relevant other activities include the IGeS geoid schools (Rio, 1997; Milano, 1999) and the Summer School of Theoretical Geodesy (Como, 1996) which to a large degree covered Section III-relevant science. Regional geoid workshops include a.o. the "Second Continental Workshop on the Geoid in Europe" (Budapest, 1998), geoid sessions at annual EGS and AGU meetings, and the yearly Canadian Geoid Workshops (Ottawa, 1997-99).

Commission reports, and the reports of the affiliated services (Bureau Gravimetrique, BGI; and International Geoid Service, IGeS) follows in the sequel.

The Commissions of Section III - IGeC and IGC - will in 1999 be joined into one commission: "International Gravity and Geoid Commission - IGGC", as decided by the IAG Executive Committee. The merger of IGC and IGeC are natural, since there is quite a large number of common scientists in the two Commissions, and topics are strongly interrelated. IGC and IGeC has held joint General Assemblys since 1994, during the present period in Tokyo 1996 ("Gravity, geoid and Marine Geodesy" (GRAGEOMAR) - Proceedings published in Springer Verlag IAG series), and in Trieste 1998 (IGC/IGeC General Assembly, proceedings in prep.). Both these meetings has attracted several hundred participants from around the world.

The Study Groups of Section III has all been working more or less succesfully, and generated new research, as evident from the chairman reports in the following. The following study groups were formed in 1995:

- SSG 3.163: Assessment and refinement of DTM's (chairman D. Arabelos, Greece)

- SSG 3.164: Airborne gravimetry instrumentation and methods (M. Wei, Canada)

- SSG 3.165: Global gravity field determination and evaluation (N. Pavlis, USA)

- SSG 3.166: Local gravity field modelling and interpretation) (T. Basic, Croatia)

- SSG 3.167: Regional land and marine gravity field modelling (H.van Gysen, South Africa)

The SSG 3.167 was taken over by I. Tziavos, Greece, after the death of Herman van Gysen, who was taken away by serious disease at a far too young age.

The SSG 3.166 was discontinued due to lack of activity by the IAG executive in 1997, and replaced by a new SSG:

- SSG 3.177: Synthetic modelling of earth's gravity field (W. Featherstone, Australia)

Finally in late 1998 a new SSG was formed and approved, to accomodate new gravity cooperation project in the Arctic region:

- SSG 3.178: Arctic Gravity Project (R. Forsberg, Denmark)

The reports of the SSG’s follow in the sequel.

3. Acknowledgements