0.1.2. Working Group on Strategy for the Realization of the International Height Reference System (IHRS)

Joint Working Group (JWG) of

- GGOS Focus Area Unified Height System,
- IAG Commission 2 (Gravity field),
- IAG Commission 1 (Reference Frames),
- IAG Inter-commission Committee on Theory (ICCT), and the
- International Gravity Field Service (IGFS).

Term: 2015 - 2019

Chair: L. Sánchez (Germany)

Members: J. Ågren (Sweden), M. Amos (New Zealand), R. Barzaghi (Italy), S. De Freitas (Brazil), W. Featherstone (Australia), T. Gruber (Germany), J. Huang (Canada), J. Ihde (Germany), G. Liebsch (Germany), J. Mäkinnen (Finland), U. Marti (Switzerland), P. Novák (Czech Republic), M. Poutanen (Finland), D. Roman (USA), D. Smith (USA), M. Véronneau (Canada), Y. Wang (USA), M. Blossfeld (Germany), J. Böhm (Austria), X. Collilieux (France), M. Filmer (Australia), B. Heck (Germany), R. Pail (Germany), M. Sideris (Canada), G. Vergos (Greece), C. Tocho (Argentina), H. Denker (Germany), D. Avalos (Mexico), H. Wziontek (Germany), M. Varga (Croatia), I. Oshchepkov (Russia), D. Blitzkow (Brazil), A.C.O.C. Matos (Brazil), J. Bouman (Germany), H.A. Abd-Elmotaal (Egypt), K. Matsuo (Japan), S. Claessens (Australia), R. Forsberg (Denmark), T. Jiang (China), V.N. Grigoriadis (Greece), D.A. Natsiopoulos (Greece), Q. Liu (Germany), M. Willberg (Germany), B. Erol (Turkey), M. Serkan Isik (Turkey), S. Erol (Turkey).

Motivation

The IAG Resolution No. 1 released during the IUGG 2015 General Assembly outlines five conventions for the definition of the International Height Reference System (IHRS). The definition is given in terms of potential parameters: the vertical coordinates are geopotential numbers (-Delta_W_P = C_P = W₀ - W_P) referring to the equipotential surface of the Earth's gravity field realized by the conventional value W₀ = 62 636 853.4 m²s^-2. The spatial reference of the position P for the potential W_P = W(X) is given by coordinates X of the International Terrestrial Reference Frame (ITRF). This Resolution also states that parameters, observations, and data shall be related to the mean tidal system/mean crust.

At present, the main challenge is the realization of the IHRS; i.e., the establishment of the International Height Reference Frame (IHRF). It is expected that the IHRF follows the same structure as the ITRF: a global network with regional and national densifications, whose geopotential numbers referring to the global IHRS are known. According to the GGOS objectives, the target accuracy of these global geopotential numbers is 1 x 10-2 m²s^-2. In practice, the precise realization of the IHRS is limited by different aspects; for instance, there are no unified standards for the determination of the potential values W_P, the gravity field modelling and the estimation of the position vectors X follow different conventions, the geodetic infrastructure is not homogeneously distributed globally, etc. This may restrict the expected accuracy of 1 x 10^-2 m²s^-2 to some orders lower (from 10 x 10^-2 m²s^-2 to 100 x 10^-2 m²s^-2). Consequently, the next step is to outline the minimum set of fundamentals needed for a reliable and sustainable realization of the IHRS.

The primary requirement of the IHRF reference network is its collocation with:

• fundamental geodetic observatories (to make feasible the connection between position vectors X, gravity potential W, international atomic time TAI, and absolute gravity g);
• geometrical reference stations of different densification levels (presumable with GNSS to allow access to the IHRF also in remote areas);
• reference tide gauges and local levelling networks (to precisely know the local geopotential numbers and to allow the global vertical datum unification);
• gravity reference stations.

Objectives

The objectives of the WG on Strategy for the Realization of the International Height Reference System (IHRS) are:

• To define the standards and conventions required to establish an IHRF consistent with the IHRS definition. A main issue is the high-precise modelling of the time-dependent changes of the vertical coordinate (which also reflect time variations of X and W).
• To formulate minimum requirements for the IHRF reference stations.
• To develop a strategy for collocation of IHRF reference stations with existing geometrical reference stations at different densification levels.
• To identify the geodetic products associated to the IHRF and to describe the elements to be considered in the corresponding metadata.
• To review the processing strategies for the determination of the potential values W_P and to recommend an appropriate computation procedure based on the accuracy level offered by those strategies.
• To review different approaches for the vertical datum unification and to provide guidance for the integration of the existing local height systems into the global IHRS/IHRF.

Results

The main results of this working group are:

• Establishment of a global reference network for the IHRS realization: the International Height Reference Frame (IHRF). A preliminary reference network for the IHRF was completed in April 2019. This network comprises 170 stations well-distributed world-wide, materialized by GNSS continuously operating stations and co-located with VLBI (30 sites), SLR (40 sites), DORIS (35 sites), absolute gravity – IGRF (77 sites), tide gauges (26 sites), and national levelling networks (23 sites).
• Evaluation of different strategies for the determination of potential values as IHRS/IHRF reference coordinates, being the main action the Colorado Experiment. The objective of the Colorado Experiment was the computation of the geoid, the quasi-geoid and potential values using the same input data and the own methodologies (software) of colleagues involved in the gravity field modelling. This action was conducted by
  • GGOS JWG: Strategy for the realization of the IHRS (chair: L Sánchez)
  • IAG JWG 2.2.2: The 1 cm geoid experiment (chair: YM Wang)
  • IAG SC 2.2: Methodology for geoid and physical height systems (chair: J Ågren)
  • ICCT JSG 0.15: Regional geoid/quasi-geoid modelling - Theoretical framework for the sub-centimetre accuracy (chair: J Huang)
• The Colorado Experiment started at the IAG/IASPEI Scientific Assembly (Aug 2017, Kobe). First results were discussed at the GGHS2018 Symposium (Sep 2018, Copenhagen). A second computation was ready for the EGU2019 (Apr 2019, Vienna) and some refinements (third computation) were delivered in Jun 2019. The results were extensively discussed at the IUGG2019, Symposium G02: Static Gravity Field and Height Systems (July 2019, Montreal). In addition, a Journal of Geodesy special issue on Reference Systems in Physical Geodesy is in preparation (as of Sep 2020). This special issue will include key contributions for the establishment of the IHRS/IHRF and the IGRS/IGRF (International Gravity Reference System and Frame) as well as the scientific description of the individual solutions contributing to the Colorado Experiment.
• Strategy for the integration (transformation) of existing vertical datums into the IHRS/IHRF (read more).

An additional important output is the IAG Resolution No. 3, 2019 for the Establishment of the International Height Reference Frame (IHRF) adopted by the IAG Council at the 27th General Assembly of the International Union of Geodesy and Geophysics in Montreal, Canada, July 8-18, 2019

Publications

Ihde J., Sánchez L., Barzaghi R., Drewes H., Foerste Ch., GruberT., Liebsch G., Marti U., Pail R., Sideris M.: Definition and proposed realization of the International Height Reference System (IHRS). Surveys in Geophysics, 10.1007/s10712-017-9409-3, 2017

Sánchez L., Sideris M.G.: Vertical datum unification for the International Height Reference System (IHRS) . Geophysical Journal International 209(2), 570-586, 10.1093/gji/ggx025, 2017

Sánchez L., Martí U., Ihde J.: Le chemin vers un système de référence altimétrique global et unifié. Association Française de Topographie, Revue XYZ 150: 61-67, 2017

Sánchez L., Ågren J., Huang J., Wang Y.M., Forsberg R.: Basic agreements for the computation of station potential values as IHRS coordinates, geoid undulations and height anomalies within the Colorado 1 cm geoid experiment, 2018

Presentations

Sánchez L.: Activities and plans of the GGOS Focus Area Unified Height System. GGOS Days 2019, Rio de Janeiro, Brazil, 2019

Sánchez L., Ågren J., Huang J., Wang Y.M., Mäkinen J., Denker H., Ihde J., Abd-Elmotaal H., Ahlgren K., Amos M., Barzaghi R., Bašic T., Blitzkow D., Carrion D., Claessens S., Erol B., Erol S., Filmer M., Forsberg R., Grigoriadis V.N., Serkan Isik M., Jiang T., Li X., Liu Q., Matos A.C.O.C., Matsuo K., Novák P., Pail R., Pitonák M., Roman R., Schmitd M., Sideris M., Varga M., Vergos G.S., Véronneau M., Willberg M., Zhang V., Zingerle P.: Advances in the realisation of the International Height Reference System. GGOS Days 2019 and Symposium SIRGAS2019, Rio de Janeiro, Brazil, 2019

Sánchez L.: Status of the International Height Reference Frame - IHRF. IERS/GGOS Unified Analysis Workshop, Paris, France, 2019

Sánchez L.: The International Height Reference System (IHRS) and its realisation, the International Height Reference Frame (IHRF). Workshop for the Implementation of the GGRF in Latin America, Buenos Aires, Argentina, 2019

Sánchez L., Barzaghi R., Vergos G.: Operational infrastructure to ensure the long-term sustainability of the IHRS/IHRF. IUGG General Assembly 2019, Montreal, Canada, 2019

Sánchez L., Sideris M., Ihde J.: Activities and plans of the GGOS Focus Area Unified Height System. IUGG General Assembly 2019, Montreal, Canada, 2019

Sánchez L., Ågren J., Huang J., Wang Y.M., Mäkinen J., Denker H., Ihde J., Abd-Elmotaal H., Ahlgren K., Amos M., Barzaghi R., Bašic T., Blitzkow D., Carrion D., Claessens S., Erol B., Erol S., Filmer M., Forsberg R., Grigoriadis V.N., Serkan Isik M., Jiang T., Li X., Liu Q., Matos A.C.O.C., Matsuo K., Novák P., Pail R., Pitonák M., Roman D., Schmitd M., Sideris M., Varga M., Vergos G.S., Véronneau M., Willberg M., Zhang C., Zingerle P.: IHRF2019: the first realisation of the International Height Reference System. IUGG General Assembly 2019, Montreal, Canada, 2019

Wang Y.M., Forsberg R., Sánchez L., Ågren J., Huang J., et al. Report on the Comparisons of Refined Colorado Geoid Models. IUGG General Assembly 2019, Montreal, Canada, 2019

Sánchez L.: GGOS Focus Area Unified Height System: Status report, ongoing activities, outlook. GGOS Coordinating Board Meeting, Vienna, Austria, 2019

Sánchez L.: GGOS Focus Area Unified Height System: Status report, ongoing activities, outlook. GGOS Days 2018, Tskuba, Japan, 2018

Sánchez L., Sideris M.: Vertical Datum Unification for the International Height Reference System (IHRS). International Symposium on Gravity, Geoid and Height Systems 2018 (GGHS2018), Copenhagen, Denmark, 2018

Sánchez L., Ågren J., Huang J., Véronneau M., Wang Y.M., Roman D., Vergos G., Abd-Elmotaal H., Amos M., Barzaghi R., Blitzkow D., Matos A.C.O.C., Denker H., Filmer M., Claessens S., Oshchepkov I., Marti U., Matsuo K., Sideris M., Varga M., Willberg M., Pail R.: Advances in the establishment of the International Height Reference Frame (IHRF). International Symposium on Gravity, Geoid and Height Systems 2018 (GGHS2018), Copenhagen, Denmark, 2018

Wang Y.M., Forsberg R., Sánchez L., Ågren J., Huang J.: Report on Colorado geoid comparisons. International Symposium on Gravity, Geoid and Height Systems 2018 (GGHS2018), Copenhagen, Denmark, 2018

Sánchez L., Madzak M.: GGOS activities related to the implementation of the International Height Reference System (IHRS). European Geosciences Union General Assembly 2018, Vienna, Austria, 2018

Sánchez L.: GGOS Focus Area Unified Height System: Ongoing activities. GGOS Coordinating Board Meeting, Vienna, Austria, 2018

Sánchez L.: GGOS Focus Area Unified Height System: Report, ongoing activities, outlook. GGOS Days 2017, Vienna, Austria, 2017

Sánchez L., Denker H., Pail R., Lieb V., Huang J., Roman D., Ågren J., Amos M., Ihde J., Barzaghi R., Sideris M., Oshchepkov I., Blitzkow D., Matos A.C.O.C., Piñon D., Avalos D., Freitas S.R.C., Luz R.: A first approximation to the International Height Reference Frame (IHRF). Joint Scientific Assembly of the International Association of Geodesy and the International Association of Seismology and Physics of the Earth’s Interior (IAG-IASPEI 2017), Kobe, Japan, 2017

Mäkinen J.: The permanent tide and the International Height Reference System IHRS. Joint Scientific Assembly of the International Association of Geodesy and the International Association of Seismology and Physics of the Earth’s Interior (IAG-IASPEI 2017), Kobe, Japan, 2017

Vergos G.S., Tziavos I.N.: Establishing an IHRS reference station. Joint Scientific Assembly of the International Association of Geodesy and the International Association of Seismology and Physics of the Earth’s Interior (IAG-IASPEI 2017), Kobe, Japan, 2017

Sánchez L., Čunderlík R., Mikula K., Minarechová Z., Dayoub N., Šíma Z., Vatrt V., Vojtíšková M.: A conventional value for the geoid reference potential W₀. Unified Analysis Workshop 2017 (UAW2017), Paris, France, 2017

Sánchez L., Ihde J., Pail R., Gruber T., Barzaghi R., Marti U., Ågren J., Sideris M., Novák P.: Towards a first realization of the International Height Reference System (IHRS). European Geosciences Union General Assembly 2017, Vienna, Austria, 2017

Sánchez L. Sideris M.G.: Vertical datum unification for the International Height Reference System (IHRS). European Geosciences Union General Assembly 2017, Vienna, Austria, 2017

Marti U., Sánchez L.: Le système de référence altimétrique global. Commission Géopositionnement, Conseil national de l’information géographique, Paris, France, 2017

Sánchez L., Ihde J., Pail R., Barzaghi R., Marti U., Ågren J., Sideris M., Novák P.: Strategy for the Realization of the International Height Reference System (IHRS). Symposium SIRGAS 2016, Quito, Ecuador, 2016

Sánchez L.: Invitación a la realización del Sistema de Referencia Internacional de Alturas (IHRS) en la Región SIRGAS (in Spanish). Meeting of the SIRGAS Directing Council, Quito, Ecuador, 2016

Sánchez, L. International Height Reference System (IHRS): Required measurements and expected products. GGOS Days 2016, Cambridge (MA), USA, 2016

Sánchez L., Ihde J., Pail R., Barzaghi R., Marti U., Ågren J., Sideris M., Novák P.: Strategy for the Realization of the International Height Reference System (IHRS). GGHS2016: International Symposium on Gravity, Geoid and Height Systems 2016, Thessaloniki, Greece, 2016

Sánchez L.: Working Group on the Strategy for the Realization of the International Height Reference System (IHRS): Brainstorming and definition of action items. Splinter meeting at the International Symposium on Gravity, Geoid and Height Systems 2016, 2016

Sánchez L.: GGOS Focus Area 1: Unified Height System: Present activities, GGOS Coordinating Board Meeting, TU Vienna, Vienna, Austria, 2016

Sánchez L., Ihde J., Barzaghi R., Drewes H., Foerste Ch., Liebsch G., Marti U., Sideris M.: Establishment of an International Height Reference System in the frame of GGOS. Symposium SIRGAS 2015, Santo Domingo, Dominican Republic, 2015

Ihde J., Sánchez L.: Physical Height and the GGRF, Earth Gravity Field and the GGRF, GGRF an Integrated Approach. GGOS Days 2015, Frankfurt a.M., Germany, 2015

Business meetings

Business meeting at the GGHS2018 Symposium, Copenhagen, Denmark, September 18, 2018 (Meeting summary).

Business meeting at the IAG-IASPEI 2017 Joint Scientific Assembly, Kobe, Japan, July 31, 2017 (Meeting summary).

Business meeting at the GGHS2016 Symposium, Thessaloniki, Greece, September 21, 2016 (Meeting summary).