FIG workshop delves into Great Lakes, highlights GNSS techniques

AAGS Activities

• Promote a better understanding of geodesy as a science;
• Create a better appreciation of the value of geodetic surveys and thus encourage greater use of such surveys;
• Promote geodetic surveys by individuals, government, and private organizations;
• Foster the adoption of uniform standards and procedures for completing geodetic surveys;
• Promote the processing, publishing, and disseminating of geodetic survey data and information;
• Promote programs for testing, calibrating, and evaluating geodetic equipment;
• Further the development and implementation of the Global Navigation Satellite System (GNSS) for geodetic, land surveying, and land information system applications;
• Inform the membership of new technical developments by meetings of the association and publications in Surveying and Land Information Science (SaLIS);
• Promote educational programs in geodesy, geodetic surveying, and related fields;
• Cooperate with other similar organizations, both national and international, in support of the science of geodesy;
• Encourage the use of geodetic surveys and mathematical coordinate systems in establishing Public Land Survey System (PLSS) corners

FIG Commission 5

“FIG Commission 5 focuses on meeting the highest level of accuracy for positioning and measurement. It provides the tools, techniques and procedures to educate and train surveying professionals everywhere. Appropriate methodology for data collection and processing are required to be successful in an era of global, integrated geospatial data.”

Commission 4 and 5 Joint Session

Jacob Heck (U.S.) and Michael Craymer (Canada):

Updating the International Great Lakes Datum: Enabling the Integration of Water and Land Management in the Great Lakes Region (11046)
[abstract] [paper] [handouts] [video]

Summary from paper by Heck and Craymer

The International Great Lakes Datum provides a framework for water level management in the world’s foremost resource of surface freshwater. The current datum, IGLD (1985), is being updated and replaced by IGLD (2020). This updated datum will be fundamentally different in terms of definition and access to the datum. The datum will be identical to the new NAPGD2022 North American geopotential datum and will be compatible with the existing CGVD2013 (if not identical as well) at the reference epoch of 2020. IGLD (2020) is expected to be released in 2025 at about the same time as NAPGD2022. Access to both frames will be primarily through GNSS techniques. This will lead to more consistent heights across the entire Great Lakes region. Further information about the IGLD update can be found on the Coordinating Committee website.

For IGLD (2020), the geoid height, N, will be provided by GEOID2022 which will be used to define NAPGD2022 and the expected update to CGVD2013. IGLD (2020) dynamic heights will therefore be equivalent to dynamic heights in NAPGD2022 and CGVD2013 at the 2020 reference epoch. For IGLD (2020) heights of water levels, hydraulic correctors may also need to be applied.

An important advancement in the development of the new IGLD and North American datums will be the availability of an accurate crustal velocity model that can propagate ellipsoidal heights between different reference epochs. This will enable heights determined at any epoch to be propagated back to the adopted 2020 reference epoch used for IGLD (2020). This will effectively obviate the need to update the entire IGLD datum for the effects of GIA for a much longer period of time, except for incremental improvements to the velocity model and updates to the reference epoch.

Determining Heights in IGLD 2020

In previous realizations of IGLD, spirit leveling was used to determine geopotential numbers which were converted directly to orthometric heights that could then be converted to dynamics heights using equation 4 (𝐻𝐷 =𝐶/𝛾₄₅₎.

In the geoid-based IGLD (2020), heights will be primarily determined through GNSS techniques which provide a direct measure of ellipsoidal height. Although spirit leveling is more accurate over shorter distances, GNSS methods combined with an accurate geoid model are capable of providing more accurate heights over moderate to longer distances at a small fraction of the cost of leveling.

An orthometric height, H, above the geoid is obtained from a GNSS-derived ellipsoidal height, h, above the reference ellipsoid using the geoid height or undulation, N, of the geoid above the reference ellipsoid. This is represented by the simple equation:

𝐻 = ℎ − 𝑁   (5)

Using equations (2) – (5), the dynamic height can be obtained from the GNSS-derived ellipsoidal height using:

𝐻𝐷 =(𝑔̅ ∗ (ℎ − 𝑁))/𝛾₄₅   (6)

For IGLD (2020), the geoid height, N, will be provided by GEOID2022 which will be used to define NAPGD2022 and the expected update to CGVD2013. IGLD (2020) dynamic heights will therefore be equivalent to dynamic heights in NAPGD2022 and CGVD2013 at the 2020 reference epoch. For IGLD (2020) heights of water levels, hydraulic correctors may also need to be applied.

An important advancement in the development of the new IGLD and North American datums will be the availability of an accurate crustal velocity model that can propagate ellipsoidal heights between different reference epochs. This will enable heights determined at any epoch to be propagated back to the adopted 2020 reference epoch used for IGLD (2020). This will effectively obviate the need to update the entire IGLD datum for the effects of GIA for a much longer period of time, except for incremental improvements to the velocity model and updates to the reference epoch.

Roman Daniel (USA):
Determining an Optimal Geoid-Based Vertical Datum (10876)
[abstract] [paper] [handouts] [video]

Excerpt from FIG Paper by Dan Roman

2.3 International Height Reference System (IHRS)

The IHRS is relatively recent compared to the ITRS. Ihde et al. (2017) discussed plans for unification of heights globally, which were updated more recently in Sanchez et al (2021). Just as ITRF realizations are made within the ITRS, there will be IHRF realizations made within the IHRS. The key concept here is that positions will first be realized in the ITRS and then expressed in the IHRS. This means that GNSS-accessed geodetic coordinates will determine your position in a realization of the ITRF. Using those ITRF coordinates, geopotential values will be determined from an equivalent IHRF model based above a datum of W0 = 62,636,853.4 m² s^-2. This effectively gives your position in the Earth’s gravity field, which is a physical height. In adopting such a model then, all countries might provide consistent physical heights across their national boundaries and over the oceans.

Summary from FIG Paper by Dan Roman

There is a great deal of activity in modernizing how geospatial data are collected, processed and maintained globally. International agreements are in place to have everyone adopt the Global Geodetic Reference Frame to facilitate geospatial data transfer. The approach will be to realize coordinates in the International Terrestrial Reference Frame and then obtain physical heights from the International Height Reference Frame. Countries may adopt any realization of the ITRF but are restricted to a single geopotential value in the IHRF – W0 = 62,636,853.4 m² /s². If comparisons to local tide gauges demonstrate this is not optimum for national definitions of a vertical datum, then an alternate geopotential datum can be determined based on an approach that requires supplemental information.

GNSS-observations on multiple tide gauges will establish local Mean Sea Level and any variations due to Topography of the Sea Surface. A model of the TSS would be required to remove TSS effects at tide gauges to determine the geodetic coordinates of MSL. Use of a geopotential model enhanced by locally obtained gravity data would yield the geopotential number(s) at tide gauge(s). Assuming multiple tide gauges, then an average or some statistical analysis might be made to determine the optimal geopotential value to select as a geoid.

Excerpt from Strategy for the realisation of the International Height Reference System (IHRS)

Authors: Laura Sánchez, Jonas Ågren, Jianliang Huang, Yan Ming Wang, Jaakko Mäkinen, Roland Pail, Riccardo Barzaghi, Georgios S. Vergos, Kevin Ahlgren and Qing Liu1

Abstract

In 2015, the International Association of Geodesy defined the International Height Reference System (IHRS) as the conventional gravity field-related global height system. The IHRS is a geopotential reference system co-rotating with the Earth.

Coordinates of points or objects close to or on the Earth’s surface are given by geopotential numbers C(P) referring to an equipotential surface defined by the conventional value W₀ = 62,636,853.4 m² s⁻², and geocentric Cartesian coordinates X referring to the International Terrestrial Reference System (ITRS). Current efforts concentrate on an accurate, consistent, and well-defined realisation of the IHRS to provide an international standard for the precise determination of physical coordinates worldwide. Accordingly, this study focuses on the strategy for the realisation of the IHRS; i.e. the establishment of the International Height Reference Frame (IHRF). Four main aspects are considered: (1) methods for the determination of IHRF physical coordinates; (2) standards and conventions needed to ensure consistency between the definition and the realization of the reference system; (3) criteria for the IHRF reference network design and station selection; and (4) operational infrastructure to guarantee a reliable and long-term sustainability of the IHRF. A highlight of this work is the evaluation of different approaches for the determination and accuracy assessment of IHRF coordinates based on the existing resources, namely (1) global gravity models of high resolution, (2) precise regional gravity field modelling, and (3) vertical datum unification of the local height systems into the IHRF. After a detailed discussion of the advantages, current limitations, and possibilities of improvement in the coordinate determination using these options, we define a strategy for the establishment of the IHRF including data requirements, a set of minimum standards/conventions for the determination of potential coordinates, a first IHRF reference network configuration, and a proposal to create a component.

FIG Working Group 5.3

Vertical Reference Frames

Policy Issues

• • Educate FIG member agencies on current and future status of regional and global vertical reference frames and height systems
  • Educate FIG member agencies on practical aspects about the implementation of new geopotential datums including:
    • access using geoid height models and a geometric datum

• redefining heights on existing bench marks to serve as secondary control
• ties between height systems and local and global mean sea level
• Develop and expand relationships in IAG Commission 2, UN SCOG, and WG focused on implementing vertical control based on IHRF around the world.
  • IAG will develop an IHRF that will be a component of the UN GGRF.
  •  UN GGRF will encompass both ITRF and IHRF
  • Time varying aspects of the geoid, vertical control and the gravity field must be addressed.

Chair

David Avalos-Naranjo, Mexico
david.avalos@inegi.org.mx