GEOID Models

Official IAG-GGOS Product

The geoid is the shape that the surface of the oceans would take under the influence of Earth’s gravitation and rotation alone, in the absence of other influences such as winds and tides. As an equipotential surface of the Earth’s gravity field, the geoid has important implications in engineering for the definition of physical heights and for Earth system studies. The most precise technique for determining physical heights above sea level is the classical levelling, but this method is very time consuming and expensive. Today, GNSS-levelling provides a very efficient technology to obtain ellipsoidal heights, from which physical heights can be computed. However, an indispensable requirement for the application of this new methodology is the precise knowledge of the geoid. This underlines the importance of the geoid also for a broad spectrum of surveying and engineering tasks based on the availability of physical heights.

Geoid repository: (a) number of available geoid models covering the same area, where blank means missing data; (b) highest spatial resolution of the gridded geoid models available at ISG. [Reguzzoni et al., 2021]

Despite the remarkable improvements resulting from the satellite gravity missions CHAMP, GRACE, GRACE Follow-On, and GOCE, the derived global models are limited in terms of spatial resolution due to the dampening of the gravity signal at the satellites altitude. The use of ground, airborne and marine gravity data allows to refine and increase the spatial resolution of the geoid computed from global models. In general, geoid refinement is based on a variety of regional approaches solving a boundary value problem. Therefore, due also to historical reasons, many countries have their own regional or local geoid models.

The International Service for the GEOID

Already in 1992, IAG recognized the need for the establishment of a service to collect and redistribute regional/local geoid models at a worldwide scale, thus making them available to the user community. The International Service for the Geoid (ISG) was born as an IAG Service (formerly known as IGeS, International Geoid Service). ISG manages and preserves an openly accessible repository, aiming at storing and redistributing geoid models in standardized data format, providing also ancillary information useful for the geoid application to further analyses. Most models can be freely downloaded, some of them require the author’s permission to be accessible and a few are restricted and cannot be distributed. Since 2020, ISG also provides the assignment of digital object identifiers (DOIs) to the stored geoid models. This is done in collaboration with GFZ Data Services, allowing geoid models being cited in publications. Whereas ISG distributes from local to continental geoid models, ICGEM provides a service to synthesize the geoid (and other derived quantities) from global gravity field models.

The main products and tools of the ISG are:

  • Grids of local and regional geoid estimates, collected worldwide (geoid repository)
  • Software for local geoid estimation, terrain gravity effect calculation and global model handling
  • Organization of International schools on geoid determination and thematic courses
  • Publications on Newton’s Bulletin and the former IGeS Bulletin

Data Sources

  • ISG - Geoid Models Repository

    Repository of regional, national and continental geoid models at a worldwide scale at International Service for the Geoid (ISG)

Further Information

  • M. Reguzzoni, D. Carrion, C.I. De Gaetani, A. Albertella, L. Rossi, G. Sona, K. Batsukh, J.F. Toro Herrera, K. Elger, R. Barzaghi, and Fernando Sansó (2021). Open access to regional geoid models: the International Service for the Geoid. Earth System Science Data