JSG T.26

Introduction

The geopotential height datum is realized by a gravimetric geoid/quasi-geoid model. The geoid/quasi-geoid model can now be determined with the accuracy of a few centimetres in a number of regions around the world; it has been adopted in some as a height datum to replace spirit-levelling networks, e.g., in Canada and New Zealand. A great challenge is the 1-2 cm accuracy anywhere to be compatible with the accuracy of ellipsoidal heights measured by the GNSS technology. This requires an adequate theory and its numerical realization, to be of the sub-centimetre accuracy, and the availability of commensurate gravity data and digital elevation models (DEMs).

Geoid/quasi-geoid modelling involves the combination of satellite, airborne and surface gravity data through the remove-compute-restore method, employing various modelling techniques such as the Stokes integration, least-squares collocation, spherical radial base functions or spherical harmonics. Satellite gravity data from recent gravity missions (GRACE and GOCE) enable to model the geoid components with the accuracy of 1-2 cm at the spatial resolution of 100 km. Airborne gravity data are covering more regions with a variety of accuracies and spatial resolutions such as the US GRAV-D project. They often overlap with surface gravity data which are still essential in determining the high-resolution geoid model. In the meantime, DEMs required for the gravity reduction have achieved higher spatial resolutions with a global coverage. In order to understand how accurately the geoid model can be determined, the 1 cm geoid experiment was carried out in a test region in Colorado, USA by more than ten international teams. The state-of-the-art airborne data was provided for this experiment by US NGS. The test results reveal that differences between geoid models by these teams are at the level of 2-4 cm in terms of the standard deviation with a range of decimetres. Reducing these differences is necessary for realization of geopotential height datums and the International Height Reference System (IHRS). This will require a thorough examination and assessment of both methods and data.

Objectives

• Adoption of physical parameters such as GM.
• Determination and adoption of W0.
• Geo-center convention with respect to the International Terrestrial Reference Frame (ITRF).
• Adoption of a Geodetic Reference System.
• Identification of data requirements and gaps.
• Gravity data gridding methods.
• Downward continuation of high-altitude airborne gravity data.
• Spatial and spectral modelling of topographic effects considering mass density variation.
• Combination of satellite, airborne and surface gravity data.
• Separation between the geoid and quasi-geoid.
• Estimation of data and geoid/quasi-geoid model errors.
• External validation data and methods for the geoid/quasi-geoid model.
• Dynamic geoid/quasi-geoid modelling.
• New geodetic boundary-value problems.

Program of Activities

• Involving and supporting new generation of geoid modellers.
• Organizing splinter meetings in coincidence with major IAG conferences and a series of online workshops.
• Circulating and sharing information, ideas, progress reports, papers and presentations.
• Organizing a session at the Hotine-Marussi Symposium 2022.
• Supporting and cooperating with IAG commissions, services, GGOS and other study and working groups on gravity modelling and height system, in particular GGOS IHRS working group, and International Service for the Geoid (ISG).

Members

Jianliang Huang (Canada), chair
Jonas Ågren (Sweden)
Riccardo Barzaghi (Italy)
Heiner Denker (Germany)
Bihter Erol (Turkey)
Christian Gerlach (Germany)
Christian Hirt (Germany)
Juraj Janák (Slovakia)
Tao Jiang (China)
Robert W. Kingdon (Canada)
Xiaopeng Li (USA)
Urs Marti (Switzerland)
Ana Cristina de Matos (Brazil)
Pavel Novák (Czech Republic)
Laura Sanchez (Germany)
Matej Varga (Croatia)
Marc Véronneau (Canada)
Yanming Wang (USA)
Xinyu Xu (China)