SC 2.4: Gravity and Geoid

Chair: Hussein Abd-Elmotaal
Vice-Chair: none

Terms of Reference and Objectives

Sub-commission 2.4 is concerned with the following areas of investigation:

• Regional gravity and geoid sub-commissions: data sets, involved institutions, comparison of methods and results, data exchange, comparison with global models, connection of regional models.
• Gravimetric geoid modelling techniques and methods, available software, new alternative geoid determination techniques.
• GNSS/levelling geoid determination: methods, comparisons, treating and interpretation of residuals, common treatment of gravity and GNSS/levelling for geoid determination.
• Geoid applications: GNSS heights, sea surface topography, integration of geoid models in GNSS receivers, vertical datums.
• Other topics: topographic effects, downward and upward continuation of terrestrial, airborne, satellite data specifically as applied to geoid modelling.
  

Program of Activities 

Sub-Commission 2.4 is going to initiate and coordinate regional gravity and geoid sub-commissions. It will encourage and support the data exchange between agencies and will assist local, regional and national authorities in their projects of gravity field determination. It will help in organizing courses and symposia for gravity field determination.

SC 2.4a: Gravity and Geoid in Europe

Chair: TBD
Vice-chair: TBD

Terms of Reference

The primary objective of the Sub-commission is the development of improved regional geoid and quasigeoid models for Europe, which can be used for applications in geodesy, oceanography, geophysics and engineering, e.g., height determination with GNSS techniques, vertical datum definition and unification, dynamic ocean topography estimation, geophysical modelling, and navigation. Another emerging field is related tothe development of new optical clocks in physics with relative uncertainties at the levelof 10⁻¹⁸, as in accordance with the laws of general relativity, such clocks are sensitiveto the gravity potential at the level of 0.1m²/s², equivalent to 1 cm in height. The geoid and quasigeoid modelling will be based mainly on terrestrial gravity andterrain data in combination with state-of-the-art global geopotential models. In this context, upgraded terrestrial data sets as well as the utilization of GRACE and GOCE based global geopotential models led to significant improvements. The evaluation of the latest European gravimetric quasigeoid models by GNSS and levelling data indicates an accuracy potential of 1 – 2 cm on a national basis, and 2 – 4 cm at continental scales, provided that high quality and resolution input data are available within the area of interest. Further improvements can be expected from the inclusion of upgraded gravity field data sets, especially in areas with hither to insufficient input data.

Program of Activities 

• Utilization of state-of-the-art global geopotential models.
• Identification and acquisition of new terrestrial data sets, including gravity, terrain, and GPS/levelling data.
• Merging and validation of all data sets.
• Investigation of refined mathematical modelling techniques and numerical tests.
• Computation of new geoid and quasigeoid models.
• Evaluation of the results by GNSS/levelling data.
• Study of applications, such as vertical datum definition and unification, dynamic ocean topography estimation, ground truth for optical clocks, etc.
  

SC 2.4b: Gravity and Geoid in South America

Chair: Gabriel Guimarães (Brazil)
Vice-Chair: Ayelen Pereira (Argentina

Terms of Reference

This Sub-commission was established as an attempt to coordinate efforts to establish a new Absolute Gravity Network in South America, to carry out gravity densification surveys, to derive a geoid model for the continent as a height reference and to support local organizations in the computation of detailed geoid models in different countries.

Besides, a strong effort is being carried out in several countries in order to improve the distribution of gravity information, to organize the gravity measurements in the continent and to validate the available gravity measurements.

Objectives

• To re-measure existent absolute gravity stations and to encourage the establishment of new stations.
• To validate fundamental gravity network from different countries in order to establish a single and common gravity network for South America.
• To adjust national gravity networks and to link them together.
• To obtain and to maintain files with data necessary for the geoid computation like gravity anomalies, digital terrain models, geopotential models and satellite observations (GPS) on the levelling network of different countries.
• To provide a link between the different countries and the IGFS in order to assure access to proper software and geopotential models for local geoid computation.
• To compute a global geoid model for South and Central America using the availabledata. To encourage countries to cooperate by releasing data for this purpose.
• To encourage and eventually support local organizations in different countries endeavoring to increase the gravity data coverage, to improve the existing digital terrain models, to carry out GPS observations on the levelling network and to compute a high resolution geoid.
• To organize and/or encourage the organization of workshops, symposia or seminars on gravity and geoid determination in South America.
• To test and to use future geopotential models derived from the modern missions (GRACE and GOCE) as well as any new combined model.
• To support the IAG Sub-Commission 1.3b (Reference Frame for South and Central America, SIRGAS) in the activities related to the unification of the existing verticaldatums.
• Establish close connections with SC2.4c (Gravity and Geoid in North and CentralAmerica) to have a good overlap of data coverage in Central America and the Caribbean.

SC 2.4c: Gravity and Geoid in North and Central America

Chair: Jianliang Huang (Canada)
Vice-Chair: David Avalos (Mexico)

Terms of Reference and Objectives

The primary objective of this Sub-commission is the development of a regional gravity field and geoid model covering the region of North America and Central America by 2025 in order to achieve a common vertical datum. The region involved will encompass Iceland, Greenland, Canada, the U.S.A. (including Alaska and Hawaii), Mexico, countries forming Central America, the Caribbean Sea and the northern parts of South America. This model will serve as the official realization of the vertical datum for countries that want to adopt it. 

The intention is to ensure that a suitable North American Geoid is developed o serve as a common datum for everyone in the region. All countries in the region would be served by having access to a common model for translating oceanographic effects to terrestrial datums for various scientific, commercial, engineering and disaster preparedness applications. Likewise, it shall serve as the basis for the forthcoming International Great Lakes Datum in 2025 (IGLD 2020). 

The achievement of a geoid model for North and Central America will be accomplished by coordinating activities among agencies and universities with interest ingeoid theory, gravity, gravity collection, gravity field change, geophysical modelling, digital elevation models (DEM), digital density models (DDM), altimetry, dynamicocean topography, levelling and vertical datums. Of particular interest will be relating geoid and ocean topography models to ocean topography and tidal benchmarks, taking advantage of the recent satellite altimetry and geopotential field products.

The determination of a geoid model for North and Central America is not limited to a single agency, which will collect all necessary data from all countries. The Sub-commission encourages theoretical diversity in the determination of a geoid model among the agencies. Each agency takes responsibility or works in collaboration with neighboring countries in the development of a geoid model for their respective country with an overlap (as large as possible) over adjacent countries. Each solution will be compared, the discrepancies will be analyzed, and the conclusions will be used to improve on the next model.

Program of Activities 

The Sub-commission will support geoid activities in countries where geoid expertise is limited by encouraging more advanced members to contribute their own expertise and software. The Sub-commission will encourage training and education initiative of its delegates (e.g., ISG geoid school, graduate studies and technical cooperation projects). Starting in 2024 the Sub-commission will organize regular meetings with representatives of Central American and Caribbean countries to promote an increase of expertise as well as to create a wide network of specialists. The chair (or a delegaterepresentative) of the Sub-commission will meet with the equivalent European and South American projects to discuss overlap regions and to work towards agreements to exchange data. The delegates of the Sub-commission will keep close contact with all related Study Groups of the IAG. The Sub-commission is open to all geodetic agencies and universities across North and Central America with an interest in the development of a geoid model for the region. The meetings of the Sub-commission 2.4c are open to everyone with interests in geodesy, geophysics, oceanography and other related topics.The members will communicate primarily using e-mail. In addition, starting in 2023, Canada (CGS), USA (NGS) and Mexico (INEGI) have been organizing audio/videoconferences every four weeks to discuss activity plans and present results. The Sub-commission also plans to organize annual meetings if enough delegates can be present. Preferably, these meetings will be held during international conferences. Minutes ofmeetings will be prepared and sent to all delegates of the Sub-commission.

Members

Jianliang Huang (Canada);Chair
David Avalos (Mexico); Vice-Chair
Rene Forsberg (Denmark)
Xiaopeng Li (USA)
Dan Roman (USA)
Laramie Potts (USA)
Yan Min Wang (USA)
John Crowley (Canada)
Ismael Foroughi (Canada)
Guido Gonzalez (Mexico)
Vinicio Robles (Guatemala)
Anthony Watts (Cayman Islands)
Alvaro Alvarez (Costa Rica)
Wilmer Medrano (Nicaragua)
Christopher Ballesteros (Panama)

SC 2.4d: Gravity and Geoid in Africa

Chair: Hussein Abd-Elmotaal (Egypt)
Vice-Chair: S.A. Benahmed Daho (Algeria)

Terms of Reference

The main goal of the African Gravity and Geoid Sub-commission is to determine the most complete and precise geoid model for Africa that can be obtained from the avail-ble data sets. Secondary goals are to foster cooperation between African geodesists and to provide high-level training in geoid computation to African geodesists.

Objectives and Activities

The objectives and activities of the sub-commission are summarized as follows:

• Identifying and acquiring data sets - gravity anomalies, DTMs, GPS/levelling, seismic Moho.
• Training of African geodesists in geoid computation.
• Merging and validating gravity data sets.
• Computing African geoid models.
• Evaluating the computed geoid models using GPS/levelling data.
• Updating the geoid models using new data/strategies to obtain better geoidaccuracy (dynamic process).

SC 2.4e: Gravity and Geoid in the Asia-Pacific

Chair: Cheinway Hwang (China-Taipei)
Vice-Chair: Wenbin Shen (China)

Context

The Asia-Pacific region, encompassing approximately 40-50 countries, has seen significant investments in the development of improved geoid models. Recent advancements in gravimetric technologies and satellite altimetry have substantially increased the accuracies and densities of land and marine gravity measurements. The use of satellite remote sensing and LiDAR data for the creation of digital elevation models (DEMs) has been integral to geoid modeling. The increasing frequency of GNSS/leveling observation campaigns across the region is instrumental in collecting data essential for the assessment and quality controls of geoid models. These collective efforts are pivotal in refining geoid models throughout the Asia Pacific.

Terms of Reference and Objectives

As a continuation of its predecessor, this Sub-commission is dedicated to advancing the science and practice of geoid modeling in the Asia Pacific region. This objective is achieved by fostering the collection of gravity data, enhancing geoid processing and evaluation techniques, and broadening the application of geoid knowledge within the Asia-Pacific. A special focus will be placed on densifying gravity measurements using the latest gravity measurement technologies. The Sub-commission will facilitate work-shops for the exchange of data, methodologies, and best practices in geoid modeling and assessment.

Program of Activities

1. Gravity and Related Data
• Facilitate the sharing of available gravity data and DEMs, especially along common borders.
• Collaborate on terrestrial gravity surveys in border regions.
• Advocate for regional airborne/shipborne gravity surveys.
• Develop methodologies to derive improved coastal gravity anomalies from satellite altimeter measurements, including those from the scanning altimeter SWOT.
  
2. Gravimetric geoid and hybrid geoid quality control
   
• Share GNSS/leveling and vertical deflection data for enhanced geoid quality control.
• Encourage and coordinate regional GNSS/leveling and vertical deflection campaigns.
• Work towards the unification of regional vertical datums using geoid models.
  
3. Education & Research
• Organize conferences, meetings, and workshops to advance the modeling and evaluation of geoid models, and promote their applications in height modernization and vertical datum unification
• Propose technical sessions at scientific and professional conferences.
• Identify and address matters of mutual concern and interest within the field

Members