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.
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.