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Jan Kryński - Monographic Series No 13

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Precise quasigeoid modelling in Poland – results and accuracy estimation

The paper presents a review and a summary of the results of investigations on modelling quasigeoid in Poland performed in the Polish research centres, in particular the results of research within the project PBZKBN- 081/T12/2002 supported by the Polish State Committee for Scientific Research in 2002-2005, and investigations conducted in the framework of the statutory project “The problems of geodesy and geodynamics” of the Institute of Geodesy and Cartography, Warsaw, in 2005-2007. Justification of initiating on a large scale an action towards the determination of centimetre geoid in Poland that results from the needs and availability of new data not applied earlier was preceded with a brief historical review of works conducted in that area in Poland.

All available data, i.e.

• terrestrial gravity data (pointgravity data and mean free-air anomalies) from Poland and neighbouringcountries;
• seaborne gravity data from the Baltic Sea and North Sea;
• airborne gravity data from southern Baltic, southern Sweden and Denmark;
• astrogeodetic and astro-gravimetric deflections of the vertical from Poland;
• levelling data from the vertical control network of Poland;
• GPS-derived coordinates and normal heights of points of the POLREF, EUVN and WSSG GPS/levelling networks;
• altimetry data from the Baltic and North Sea, tide gauge data from the stations on the Baltic Sea coast;
• data on the density of upper lithosphere in Poland;
• digital terrain models for Poland and neighbouring countries, were specified and described.

The results of quality and quantity analysis of that data were presented. Both standards and reference systems of the collected data were analysed. The data was then transformed to unified reference systems in accordance with recent standards, and then was archived in the respective databases. The characteristics of databases developed for each data type was given in the description of the System of Databases. Gravity, GPS and astronomic control surveys assigned for verification of gravity data, GPS/levelling data, and deflections of the vertical were designed and conducted. The analyses performed indicated global geopotential models recently most suitable for quasigeoid modelling in Poland with the use of the remove-compute-restore method. Methodology for computing terrain corrections with the use of available data was developed. It concerns in particular the determination of optimum maximum radius of integration of gravitational effects of topography represented in the form of prisms. Quality of terrain corrections calculated for 1 078 046 gravity points from the gravity database were analysed. An algorithm for computing mean gravity anomalies, adjusted to centimetre accuracy requirements as well as to height data available, was developed A new set of mean 1' × 1' Faye gravity anomalies was generated. The regional sea level model for the Baltic Sea was developed. Its regional and local characteristics as well as effective applicability of short tide gauge records for sea level modelling, monitoring of land uplift of Fennoscandia, and relation between Baltic Sea level variations and polar motion parameters were analysed. A design and survey of GPS/levelling control traverse established in the framework of the project for independent quality estimate of quasigeoid models developed was described. Quasigeoid heights of GPS/levelling control traverse points were compared with the respective ones determined from GUGiK 2001 quasigeoid model. Methodology of determination of quasigeoid models: astro-geodetic geoid/quasigeoid, gravimetric quasigeoid, GPS/levelling quasigeoid, integrated quasigeoid and gravimetric quasigeoid best fitted to the heights of the sites of the POLREF network, algorithms developed and the result obtained with specification of data used were presented. Quality of quasigeoid models developed was estimated with the use of internal accuracy analysis, including intercomparison of the models, as well as with the use of data from the control GPS/levelling traverse established within the project. Accuracy of the best of obtained quasigeoid models in Poland is at the level of 2 cm.