0.1.1. Working Group on Vertical Datum Standardization

A joint initiative of GGOS Focus Area Unified Global Height System, IAG Commission 2 (Gravity Field) and the International Gravity Field Service (IGFS).

Term: 2011 - 2015

Chair: Laura Sánchez (lm.sanchez@tum.de), Deutsches Geodätisches Forschungsinstitut der Technischen Universität München (DGFI-TUM).

Members: J. Ågren (Sweden), R. Cunderlík (Slovakia), N. Dayoub (Syria), J. Huang (Canada), R. Klees (The Netherlands), J. Mäkinen (Finland), K. Mikula (Slovakia), Z. Minarechová (Slovakia), P. Moore (United Kingdom), D. Roman (USA), Z. Šima (Czech Republic), C. Tocho (Argentina), V. Vatrt (Czech Republic), M. Vojtiskova (Czech Republic), Y. Wang (USA).

Motivation

During the last three decades, many initiatives related to the vertical datum unification have been developed in the frame of the International Association of Geodesy (IAG). They are basically oriented to define and realize a global reference level and to determine the connection (transformation) of the local height datums to the global one, i.e. all physical heights (or geopotential numbers) existing worldwide shall be referred to only one reference surface that is realized globally. In the past periods, 2003-2007 and 2007-2011, significant progresses were made by the IAG Inter-Commission Project 1.2 “Vertical Reference Frames” (IAG ICP1.2 VRF), whose principal results concentrate on the conventions for the definition and realization of a Global Vertical Reference System (GVRS) and the establishment of the corresponding Global Vertical Reference Frame (GVRF). The feasibility of the statements proposed in the conventions was evaluated in practice through the pilot project named “Realization of a World Height System”. At present both, conventions and pilot project achievements, provide guidance to establish a Global Vertical Reference System in practice. For the period, 2011-2015, activities and goals of the IAG ICP1.2 VRF were integrated into GGOS under the Focus Area Unified Global Height System (GGOS-FA-UHS). According to this, the main objective of the Working Group on Vertical Datum Standardization is to support the GGOS-FA-UHS in the realization of a unified global height system by bringing together different groups working on the vertical datum problem to avoid unnecessary duplicity in activities and to joint efforts guaranteeing a homogeneous advance in the reference frame issues not only worldwide, but also at national and regional level.

Objectives

The main purpose of the joint working group on Vertical Datum Standardization (JWG 0.1.1) was to provide a reliable W₀ value to be introduced as the conventional reference level for the realization of a Unified Global Height System. Although any W₀ value can arbitrarily be chosen, it is expected that this value is consistent with other defining parameters of geometric and physical models of the Earth. In this way, activities developed by JWG 0.1.1 are based on the state-of-the-art data and methodologies, especially on the newest available representations of the Earth’s surface and gravity field. Computations carried out by JWG 0.1.1 are documented in detail in order to guarantee the repeatability and reliability of the results. This documentation also supports the adoption of the obtained W₀ value as an official IAG/GGOS convention. An additional product is dedicated to provide guidance on the usage of W₀ in practice, in particular for the vertical datum unification.

Activities

1. To coordinate individual initiatives for a unified W₀ determination: At present (2011) there are at least four groups working on the estimation of a global W₀ value. The idea is to bring together these groups to elaborate an inventory describing individual methodologies, conventions, standards, and models presently applied in their W₀ computations.
2. To refine the W₀ estimation: it is proposed that each group makes a new W₀ computation following their own methodologies, but applying the most recent geodetic models (e.g. GOCE/GRACE gravity models, sea surface models derived from calibrated and combined satellite altimetry observations, etc.). For this, JWG 0.1.1 takes into account recommendations given by the different IAG components in the respective field of speciality. This analysis also includes an investigation about the time-dependence of W₀.
3. To make a proposal for a formal IAG/GGOS convention about W₀: It is expected that results obtained after applying the different methodologies considered in the previous item are very similar. In this way, after a rigorous reliability evaluation, the JWG 0.1.1 members shall formulate a unified agreement proposing the best estimate of W₀ as an IAG/GGOS official convention. This will be supported by a document containing the detailed computation of the recommended value.
4. To provide a standard about the usage of W₀ in the vertical datum unification: Based on the interchange of experiences within the WG, it is expected to generate a document describing the most appropriate strategy to connect (unify, transform) any local height system with the global W₀ reference level.

Results

The different computations carried out within the JWG 0.1.1 demonstrate that the 1998 W₀ value (62 636 856.0 ± 0.5 m²s^-2) is not in agreement (and consequently it is not reproducible) with the newest geodetic models describing geometry and physics of the Earth. This leads us to conclude that the 1998 W₀ value is not suitable as a conventional reference value and a better estimate for W₀ has to be adopted by the IAG. The recommended new value is 62 636 853.4 m²s^-2. It was officially adopted by the IAG in its Resolution No. 1, July 2015, as the conventional W₀ value for the definition and realization of the International Height Reference System (IHRS). A detailed description about the computation strategy of W₀, applied models, conventions and standards, as well as results is presented in the publication “A conventional value for the geoid reference potential W₀” (Sánchez L., Cunderlík R., Dayoub N., Mikula K., Minarechová Z., Šíma Z., Vatrt V., Vojtíšková M., J Geod (2016) 90:815-835, DOI: 10.1007/s00190-016-0913-x).

The JWG 0.1.1 also supported the activities of the GGOS Bureau of Products and Standards and compiled the chapter “4.6 Height systems and their realizations” of the “Inventory of standards and conventions used for the generation of IAG products” published in the IAG Geodesist’s Handbook 2016.

Publications

Sánchez L., Cunderlík R., Dayoub N., Mikula K., Minarechová Z., Šíma Z., Vatrt V., Vojtíšková M.: A conventional value for the geoid reference potential W₀. Journal of Geodesy, 10.1007/s00190-016-0913-x, 2016

Ihde J., Barzaghi R., Marti U., Sánchez L., Sideris M., Drewes H., Foerste C., Gruber T., Liebsch G., Pail R.: Report of the Ad-hoc Group on an International Height Reference System (IHRS). In: Drewes H., Hornik H. (Eds.) Travaux de l'AIG 39, IAG Reports 2011-2015, 2015

Sánchez L., Dayoub N., Cunderlík R., Minarechová Z., Mikula K., Vatrt V., Vojtísková M., Síma Z.: Report of Joint Working Group 0.1.1: Vertical Datum Standardization (JWG 0.1.1). In: Drewes H., Hornik H. (Eds.) Travaux de l'AIG 39, IAG Reports 2011-2015, 2015

Cunderlík R.: Determination of W₀ from the GOCE Measurements Using the Method of Fundamental Solutions. In: International Association of Geodesy Symposia, first on-line, doi: 10.1007/1345_2015_39, 2015

Cunderlik R., Minarechová Z., and Mikula K.: Realization of WHS Based on the Static Gravity Field Observed by GOCE. In: Marti U. (ed,), Gravity, Geoid and Height Systems. IAG Symposia Series 141: 211-220, doi: 10.1007/978-3-319-10837-7_27, 2014

Sánchez L., Dayoub N., Cunderlík R., Minarechová Z., Mikula K., Vatrt V., Vojtísková M., Síma Z.: W₀ estimates in the frame of the GGOS Working Group on Vertical Datum Standardization. In: Marti U. (Ed.) Gravity, Geoid and Height Systems (GGHS2012), IAG Symposia 141: 203-210, 10.1007/978-3-319-10837-7_26, 2014

Dayoub N., Edwards S.J. and Moore P.: The Gauss-Listing potential value W0 and its rate from altimetric mean sea level and GRACE, Journal of Geodesy, 86(9): 681-694, doi: 10.1007/s00190-012-1547-6, 2012

Sánchez L.: Towards a vertical datum standardization under the umbrella of Global Geodetic Observing System. Journal of Geodetic Science 2(4): 325-342, Versita, 10.2478/v10156-012-0002-x, 2012

Reports

Report of the Joint Working Group 0.1.1: Vertical Datum Standardization (JWG 0.1.1) for the term 2013-2015. In: Drewes H., Hornik H. (Eds.) Travaux de l'AIG 39, IAG Reports 2011-2015, 2015

Report of the Joint Working Group 0.1.1: Vertical Datum Standardization (JWG 0.1.1) to the International Gravity Field Service, 2015

Report of the Joint Working Group 0.1.1: Vertical Datum Standardization (JWG 0.1.1) to the Executive Committee of the International Association of Geodesy, 2013

Report of Joint Working Group 0.1.1: Vertical Datum Standardization (JWG 0.1.1) for the term 2011-2013. In: Drewes H., Hornik H. (Eds.) Travaux de l'AIG 38, IAG Reports 2011-2013, 2013

Joint Working Group 0.1.1: Vertical Datum Standardization (JWG 0.1.1) meeting summary, 2012

Presentations

A new best estimate for the conventional value W₀ - Final Report of the WG on Vertical Datum Standardization -, IUGG2015 General Assembly, Prague, Czech Republic, June 27, 2015

Towards a new best estimate for the conventional value of W₀, 3rd IGFS General Assembly, Shanghai, China, June 30-July 6, 2014

Conventional reference level for a global unified height system, IAG 150 years, Potsdam, Germany, September 1, 2013

Vertical datum standardization: a fundamental step towards a global vertical reference system, AGU Meeting of the Americas, Cancun, Mexico, May 16, 2013

Towards a Vertical Datum Standardization, SIRGAS annual meeting 2012, Concepción, Chile, October 31, 2012

Splinter meeting of the Working Group Vertical Datum Standardization, GGHS 2012, Venice, Italy, October 11, 2012

Report on the activities of the Working Group Vertical Datum Standardization, GGHS 2012, Venice, Italy, October 9, 2012

Towards a Vertical Datum Standardization, AOGS-AGU (WPGM) Joint Assembly, Singapore, August 13-17, 2012