The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) promotes through its Focus Area 1 Unified Height System the definition and realization of a global vertical reference system with homogeneous consistency and long-term stability. For the term 2011-2015 DGFI-TUM coordinated the Working Group Vertical Datum Standardization, which main purpose was to determine an updated value for the gravity potential W0 of the geoid to be introduced as the conventional reference level for the realization of a global height system.
The derived value was officially adopted by the IAG in its Resolution No. 1, July 2015, as the conventional W0 value for the definition and realization of the International Height Reference System (IHRS). A detailed description about the DGFI-TUM computation strategy of W0, applied models, conventions and standards, as well as results is presented in the recent publication A conventional value for the geoid reference potential W0 (Journal of Geodesy, 2016, DOI: 10.1007/s00190-016-0913-x).
The geoid is understood to be the equipotential surface of the Earth's gravity field that coincides with the worldwide mean sea surface. The value of the gravity potential of the geoid is denoted by W0.
A usual approximation of W0 is the averaged potential value WS at the mean sea surface. In this way, the value of W0 depends not only on the Earth's gravity field modelling, but also on the conventions defining the mean sea surface. W0 computations performed since 2005 demonstrated that current published estimations differ by up to -2.6 m2/s2 (corresponding to a level difference of about 27 cm), which could be caused by differences in the treatment of the input data.
According to this, DGFI-TUM concentrated on performing a new W0 estimation relying on the newest gravity field and sea surface models and applying standardized data and procedures. Main conclusions indicate that the satellite-only component (degree n = 200) of a static (quasi-stationary) global gravity model is sufficient for the computation of W0. This model should however be based on a combination of at least SLR, GRACE and GOCE data. The mean sea surface modelling should be based on mean sea surface heights referring to a certain epoch and derived from a standardized multi-mission cross-calibration of several satellite altimeters. Uncertainties caused by geographically correlated errors, including shallow waters in coastal areas and sea water ice content at polar regions should be considered in the computation of W0 by means of a weighed adjustment using the inverse of the input data variances as a weighting factor. The input data in this case correspond to the sea surface heights provided by the DGFI-TUM Open Altimeter Database (OpenADB).
As a reference parameter, W0 should be time-independent (i.e., quasi-stationary) and it should remain fixed for a long-term period (e.g., 20 years). However, it should have a clear relationship with the mean sea surface level (as this is the convention for the realisation of the geoid). According to this, a suitable recommendation is to adopt a potential value obtained for a certain epoch as the reference value W0 and to monitor the changes of the mean potential value at the sea surface WS. When large differences appear between W0 and WS (e.g., > 2 m2/s2), the adopted W0 may be replaced by an updated (best estimate) value. The potential value obtained for the epoch 2010.0 (62 636 853.4 m2/s2) is recommended as the present best estimate for the W0 value.