Towards GNSS-related height system

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Timo Saari Name of the doctoral thesis Investigations of geoid models in Finland Publisher School of Engineering Unit Department of Built Environment Series Aalto University publication series DOCTORAL THESES 171/2022 Field of research Geoinformatics Manuscript submitted 22 August 2022 Date of the defence 12 December 2022 Permission for public defence granted (date) 27 October 2022 Language English Monograph Article thesis Essay thesis Abstract In Fennoscandia, heights and their relations between each other are in constant change due to the post-glacial rebound, so the national height system needs to be updated occasionally. Traditionally, the update has been done with a method known as precise levelling, which is accurate but considered slow, laborious, and expensive. This dissertation studied the modern, mainly Global Navigation Satellite System (GNSS) -based, height determination techniques that could replace precise levelling as the method for the next national height system of Finland. GNSS-based techniques provide the height component relative to a reference ellipsoid, which is a mathematical surface and therefore lacks a physical connection to the Earth – i.e. have no information on the direction of water flow. Here, we need a (quasi-)geoid model to tie the ellipsoidal heights to the surface of the Earth and to national height systems. The (quasi-)geoid model's accuracy is therefore crucial with GNSS/geoid techniques. The presented case study gave us knowledge from several techniques. The static GNSS proved the most promising, as the result was close to the one from precise levelling. However, due to the closeness of the evaluation points, the relative error of the geoid between them is negligible in practice, which is not the case in a nationwide network. The national quasi-geoid model of Finland, FIN2005N00, was created nearly two decades ago. The gravity satellites, Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and Gravity Recovery and Climate Experiment (GRACE), have since measured the Earth's gravitational field in unprecedented detail. At the time, we investigated all the published GOCE and GRACE global gravitational models (GGM) in Finland. We learned that the best models already performed at the same level or better than the pre-GOCE era high-resolution models. The most suitable model, DIR5, was chosen as a background model with the high-resolution EIGEN-6C4 in the quasi-geoid modelling of Finland. The new and present quasi-geoid models were evaluated on land and in sea areas. A significant improvement was achieved over the present models. A marine GNSS/gravity campaign was performed in the Gulf of Finland to improve the accuracy and validation of a quasi-geoid model in sea areas. Geoid height differences of up to 15 cm were found with the new gravity data included in the quasi-geoid modelling. The result was confirmed in the evaluation with marine GNSS measurements in combination with sea surface models. This dissertation's results will be important for producing the next national quasi-geoid model of Finland. Additionally, the knowledge obtained from the GNSS/geoid method will be beneficial for the decision making of the chosen method for the next national height system of Finland.