Prospects for the unification of the PL-EVRF2007-NH frame with the International Height Reference System in Poland

In recent years, a significant challenge for geodesy has been the introduction of a global height reference system and the consideration of its regional or national applications. According to the Global Geodetic Observing System (GGOS) recommendations, providing and maintaining accurate and stable reference systems is highly desirable. Positioning of 3D spatial systems with regard to the reference ellipsoid is ensured by stable ITRF realisations and easily transformable local frames (e.g. ETRF). A more demanding task is the unification of vertical datums. The difficulties result from the multitude of height frames, their non-uniformity and usually missing elevation in-time change models. The uniform International Height Reference System aims to achieve an accuracy of 3 mm for heights and 0.3 mm/year for its velocities.

In this study, the authors focused on analysing the possible unification of the Polish national vertical datum (PL-EVRF2007-NH) with the IHRF. For this purpose, various global geopotential models (satellite and high-resolution GGMs) were tested. Their usefulness was checked in the context of the transition from the local system to the system related to the global geoid level recommended in IAG Resolution (No. 1). The impact of direct and indirect use of GGM to determine the normal heights of points in the IHRF frame in the national network was also examined. The first case included testing the possibility of obtaining normal heights based on height anomalies determined directly from the selected geopotential models. The second case involved the unification of systems on a national scale with a determined local ΔW₀ value ​​between the level of the PL-EVRF2007-NH and the W₀ of IHRS. To obtain reliable results, it was necessary to standardise input data with regard to the assumptions of the IHRS. In addition, a specific variant was tested in which the UELN (United European Leveling Network) points became the basis for a new realisation of the vertical datum in Poland.

The conducted analyses and numerical tests allowed for the formulation of recommendations regarding the methodology of unification of the PL-EVRF2007-NH and IHRF frames in Poland, in particular: evaluating and picking the optimal GGM model, selecting reference points for local height frame, collecting and assessing the quality of normal heights (or geopotential values) data.