The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), taking care of providing a precise geodetic infrastructure for monitoring the System Earth, promotes the standardization of height systems worldwide. With the purpose to bring together existing initiatives and to address the activities to be faced, the GGOS Focus Area Unified Height System (formerly Theme 1) was established during the GGOS Planning Meeting 2010 (February 1 - 3, Miami/Florida, USA). Starting point were 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 term 2011-2015, the GGOS Focus Area Unified Height System (GGOS-FA-UHS) was chaired by Michael Sideris (University of Calgary, Canada) and Johannes Ihde (Bundesamt für Kartographie und Geodäsie, Germany). The main result is the compilation of the IAG resolution for the "Definition and realization of an International Height Reference System (IHRS)" 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) the outlining of 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). For the term 2015-2019, this Area is chaired by Laura Sánchez, Deutsches Geodätisches Forschungsinstitut, Technische Universität München, Germany.
The objective of Focus Area 1 is the unification of the
existing vertical reference systems around the world. This
should be achieved through the definition and realization
of a global vertical reference system that
-> supports geometrical (ellipsoidal) and physical (normal,
orthometric, geoidal) heights world-wide with
centimetre precision in a global frame;
-> enables the unification of all existing physical height
systems (i.e., all geopotential differences shall be
referred to one and the same reference equipotential
surface with potential W0); and
-> provides high-accuracy and long-term stability of the
A first step towards the establishment of a worldwide
unified (standardized) height system was the release of an
IAG resolution for the definition and realization of an
International Height Reference System (IHRS) that was
issued during the 2015 IUGG General Assembly. This
resolution outlines the conventions for the definition of the
IHRS in terms of potential parameters: the vertical
coordinates are geopotential numbers referring to an
equipotential surface of the Earth's gravity field realized by
a conventional W0 value. 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, with known geopotential numbers referring
to the global IHRS. To guarantee a precise combination of
physical and geometric parameters and to support the
vertical datum unification worldwide, this reference
network should be collocated with fundamental geodetic
observatories, geometrical reference stations, reference tide
gauges, local levelling networks, and gravity reference
stations. For this purpose, it will use contributions from all
IAG Commissions, and the available databases, standards
and infrastructure of the IAG/GGOS Services.
-> Refinement of standards and conventions for the
definition and realization of the IHRS, including
unification of standards and conventions that are used
by the geometric and gravity Services of the IAG.
-> Develop GGOS products for realizing the IHRS.
-> Recommendation for a global vertical reference frame;
i.e. the IHRF.
-> Guidelines/procedures for height system unification.
-> Development of a registry (metadata) containing the
existing local/regional height systems and their
connections to the global one.
-> Strategies for the maintenance and use in practice of the
-> Determination and modelling of the temporal changes
of the IHRF.
-> Update the IHRS definition and realization as needed,
based on future improvements in geodetic theory and
-> Servicing the vertical datum needs of other geosciences
such as, e.g., hydrography and oceanography.
Efforts are currently underway to establish working groups
and processing centres that will focus on one or more of
the action items above. One such group is the already
established JWG 0.1.2, whose objectives are outlined