Abstract
<p>The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), providing precise geodetic infrastructure and expertise for monitoring the System Earth, promotes the standardization of height systems worldwide. The GGOS Focus Area "Unified Height System” (GGOS-FA-UHS, formerly Theme 1) was established at the 2010 GGOS Planning Meeting (February 1 - 3, Miami, Florida, USA) to lead and coordinate these efforts. Starting point was the results delivered by the IAG Inter-Commission Project 1.2 "Vertical Reference Frames" (IAG ICP1.2 VRF), which was operative from 2003 to 2011. During the 2011-2015 term, different discussions focussed on the best possible definition of a global unified vertical reference system resulted in the  IAG resolution for the "Definition and realization of an International Height Reference System (IHRS)” that was approved during the 2015 General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Prague, Czech Republic. The immediate objectives of the GGOS-FA-UHS are (1) to outline detailed standards, conventions, and guidelines to make the IAG Resolution applicable, and (2) to establish the realization of the IHRS, i.e., the International Height Reference Frame (IHRF). This contribution summarizes the main achievements of the GGOS-FA-UHS and provides a review of the open questions that need to be resolved in the near future.</p>
Highlights
Most of the existing physical height systems do not fulfil the accuracy requirements of modern Geodesy
A current main objective of the International Association of Geodesy (IAG) is the implementation of an integrated Global Geodetic Reference System (GGRS) that simultaneously supports the consistent determination and monitoring of the Earth’s geometry, rotation, and gravity field changes with high accuracy worldwide (IAG, 2017)
This objective is in accordance with the resolution adopted by the United Nations General Assembly on a Global Geodetic Reference Frame (GGRF) for Sustainable Development (A/RES/69/266) on February 26, 2015
Summary
Quasi-geoid and potential values using the same input data and the own methodologies (software) of colleagues involved in the gravity field modelling. A Journal of Geodesy special issue on Reference Systems in Physical Geodesy is in preparation 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. (2) With the support of the IAG Commission 2, the IGFS and the ICCT to promote the study of - quality assessment in the determination of potential values; - determination of potential changes with time Ẇ; - realisation of the IHRS in marine areas With this purpose, following working/study groups are established for the term 2019-2023: - IAG Commission 2 working group Error assessment of the 1-cm geoid experiment, chaired by M Willberg (Germany) and T Jiang (China). Aspects to be considered are: - Updates of the IHRS definition and realisation according to future improvements in geodetic theory and observations. - Regular updates of the IHRF (e.g. IHRFyyyy) according to new stations, coordinate changes with time, improvements in the estimation of reference coordinates and modelling of the Earth’s gravity field, etc. - Support in the realisation and utilisation of the IHRS/IHRF at regional and national level. - To guarantee an organisational and operational infrastructure to ensure the sustainability of the IHRF
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