SC 2.2: Geoid, Physical Height Systems and Vertical Datum Unification

Chair: George Vergos (Greece)
Vice-Chair: Rossen S. Grebenitcharsky (Saudi Arabia)

Terms of Reference

A global height reference frame with high accuracy and stability is required to determine the global changes of the Earth. A major step towards this goal was taken by the IAG resolution (No. 1) for the definition and realization of an International Height Reference System (IHRS), adopted at the IUGG 2015 meeting in Prague and the IAG resolution (No. 3) for the establishment of the International Height Reference Frame (IHRF), adopted at the IUGG 2019 meeting in Montreal. Given the work carried out for the general methodological scheme for geoid and potential determination, the data prerequisites and practical studies, it has become apparent that the IHRS should be globally realized with common standards in terms of the processing strategy. Moreover, the use of all available data sources, e.g., GNSS-derived heights, satellite altimetry, topography/bathymetry, local gravity (terrestrial, airborne and marine) as well as the latest global geopotential models, should be employed in order to properly model the high-frequency part of the gravity field spectrum. Such combination of heterogenous data has been deemed a mandatory in order to reduce the omission error as well as to properly model the contribution of topography. Traditional levelling might also be integrated on a regional or local scale. Finally, the unification of local/national vertical reference frames to regional ones and their link to the IHRF is of main importance, employing local geoid realizations and datum definitions. The IAG SC 2.2 aims at bringing together scientists and geodesists concerned with methodological questions in geoid and potential determination, who in different ways contribute to reach the above-mentioned goal of a global height system realization and unification. It includes topics (state of art methodologies for processing, analyzing, utilizing data, unifying datums, etc.)  and ranging from regional gravimetric geoid determination to the realization and implementation of IHRS in view of the existing regional/local/national height system realizations and 3D vertical datum (geoid) definitions.


The IAG Sub-Commission 2.2 (SC2.2) promotes and supports scientific research related to methodological questions in geopotential, geoid and height determination, both from the theoretical and practical perspectives. The former refers in particular on methodological questions and practical numerical applications contributing to the realization of IHRS with the required sub-centimeter accuracy, the combination of local/regional vertical reference frames and their unification to the IHRF. This includes for instance:

  • Realization of the International Height Reference System (support to JWG Joint Working Group x.x.x. “Implementation of the International Height Reference Frame (IHRF)”)
  • Height system unification at regional scales and unification to the IHRF.
  • Studies on W0 determination.
  • Studies on data requirements, data quality, distribution and sampling rate to reduce the omission error to the sub-centimeter level in different parts of the world.
  • Contributions of alternate data sources, such as altimetry sea surface heights and GNSS geometric heights to geopotential modeling and geoid determination at reference benchmarks.
  • Investigation of the theoretical framework required to compute the sub-centimeter geoid (support of ICCT SG: Geoid/quasi-geoid modelling for realization of the geopotential height datum)
  • Investigation of the error budget od potential determination and vertical reference frames unification (support to Commission 2 WG: Error assessment of the 1 cm geoid experiment)
  • Investigation and benchmarking of alternative regional geoid determination methods and software.
  • Studies on theoretical and numerical problems related to the solution of the geodetic boundary value problems (GBVPs) in geoid determination,
  • Studies on time variations of the gravity field and heights due to Glacial Isostatic Adjustment (GIA) and land subsidence.
  • Development of relativistic methods for potential difference determination using precise atomic clocks (support of Joint Working Group 2.1).
  • Investigating the role of traditional levelling in future regional/local height system realizations combined with all available data linked to Earth’s geopotential determination.
  • Investigating the utilizations of already defined national and regional geoid models together with new types of Geodetic Earth Observations (GEOs) and based on theoretical and practical developments linked to mixed GBVPs.

Program of Activities 

  • Organizing meetings and conferences.
  • Organization of local/regional workshops for the promotion of IHRF related studies.
  • Inviting the establishment of Special Study Groups on relevant topics.
  • Reporting activities of SC2.2 to the Commission 2.
  • Communication/interfacing between different groups/fields relevant to the realization of IHRS.
  • Conceptual and methodological support to working groups for national & regional vertical datums and reference frames definitions as realizations of IRHS.

Working Groups of Sub-Commission 1.1

JWG 2.2.1: Error assessment of the 1 cm geoid experiment

(joint with ISG, IGFS

Chair: Martin Willberg (Germany)
Vice-Chair: Tao Jiang (China)

Terms of Reference

The realization of the International Height Reference System (IHRS) will be based on reference stations, which are calculated from a local combination of global gravity field models and regional gravity observations. This process, which is called regional geoid/gravity modeling is realized with different philosophies and theories inside the geodetic community. However, the final quality of the IHRS depends heavily on the consistency of all included modeling methods. Consequently, in the previous IAG period, within JWG 2.2.2 an effort was made to evaluate the differences in various regional geoid/gravity modeling approaches and standardize them to a specific degree. 

Within ‘the 1 cm geoid experiment’ 15 participating groups have calculated a regional geoid/gravity model in the area of Colorado, US. Thereby, all groups used identical input data sets consisting of terrestrial and airborne gravity observations. After two iterations within ‘the 1 cm geoid experiment’, differences of a few centimeters remain between the final results of the contributing groups. Manifold reasons might be responsible for this difference, but major aspects are assumed to be differences in the procedure, the topographic reduction, and the individual data handling. Until now, the quality assessment of individual solutions was mainly analyzed by their variation from a joint mean value, which was interpreted as reference. However, the Colorado area contains a geoid slope validation survey (GSVS), where positions, gravity values and deflections of the vertical were measured with very high quality along a set of 223 benchmarks. These values and the results of the GSVS processing are not yet published, but will be open to the public soon, thereby providing an improved reference (for this JWG). 

The objectives of this JWG are to validate the results, to identify and quantify potential error sources, and to develop and improve methods for deriving realistic error estimates of the gravity potential values at the IHRS stations. Once available, the differences of individual geoid/height anomaly results to the improved reference from the GSVS shall be analyzed. The most possible reasons for these differences should be worked out. Especially, a modified computation set-up shall be defined, which enables to separate method-related and data-driven error contributions. This will be important to quantify the error level caused by different regional gravity modelling methods. If possible, these errors shall be further reduced within an additional iteration step. Furthermore, different methods should be examined for their ability to estimate an appropriate error budget for the final results. This is of great importance, as the assessment of the total error is needed for the reference stations in the IHRS. Lessons from this JWG should then be transferred to the JWG for ‘Implementation of the International Height Reference Frame – IHRF’.

Program of Activities

  • Analyze the difference of various solutions to the GSVS reference
  • Quantify the main error contributors of regional geoid modeling
  • Estimation of the total error budget
  • Derive recommendations for the IHRS realization from the viewpoint of regional gravity field modeling
  • Organize conference sessions
  • Report activities


Chair: Martin Willberg (Germany)
Vice-Chair: Tao Jiang (China)
Laura Sánchez (Germany)
Yan Ming Wang (USA)
Vassilios Grigoriadis (Greece)
Marc Véronneau (Canada)
Sten Claessens (Australia)
Qing Liu (Germany)
Rene Forsberg (Denmark)
Hussein Abd-Emotaal (Egypt)
Koji Matsuo (Japan)
Bihter Erol (Turkey)
Jonas Ågren (Sweden)
Kevin Ahlgren (USA)
Matej Varga (Czech/Croatia)
Riccardo Barzaghi (Italy)
Representative person USP (Brazil)

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