Achievements of the GGOS Focus Area Unified Height System

Abstract The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) promotes the standardisation of height systems worldwide. The GGOS Focus Area Unified Height System (GGOS-FA-UHS) was established to lead and coordinate the efforts needed towards the establishment of a global standard for the precise determination of physical heights. During the 2011–2015 term, various discussions focused on the best possible definition of a global unified vertical reference system, resulting in the IAG Resolution for the Definition and Realisation of an International Height Reference System (IHRS), which was adopted at the 2015 General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Prague, Czech Republic. During the period 2015–2019, activities were undertaken to investigate the best strategy for the implementation of the IHRS; i.e., the establishment of the International Height Reference Frame (IHRF). A preliminary selection of stations for the IHRF reference network was made and different calculation methods for the determination of potential values as IHRF coordinates were evaluated. For the period 2019–2023, the objectives of the GGOS-FA-UHS focused on (i) compiling detailed standards, conventions and guidelines to support a consistent determination of the IHRF at global, regional and national levels; (ii) coordinating with regional/national experts in gravity field modelling the computation of a first IHRF solution; and (iii) designing an operational infrastructure that will ensure the long-term sustainability and reliability of the IHRS/IHRF. This infrastructure was approved by the IAG Executive Committee in December 2023 and will operate under the responsibility of the International Gravity Field Service (IGFS). With these objectives achieved, the GGOS-FA-UHS completed its goals and was closed during the IUGG 2023 General Assembly in Berlin, Germany. This paper presents a comprehensive report on the activities and achievements of the GGOS-FA-UHS.