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In the process of establishing the National Leveling Network (NLN), the individual cycles are associated with the use of different technologies, instruments, and methods for data structuring and processing. The study focuses on the territory of Bulgaria, where in the last three leveling cycles, various methodological and technological approaches for data organization, processing, and analysis have been applied. In this context, taking into consideration the large volume of accumulated data from the different reali...

The maintenance of a modern vertical reference system in Bulgaria, aligned with the European Vertical Reference System (EVRS) standards, is directly dependent on the precise knowledge of the Earth's gravity field. The research presented in this paper is motivated by the 5th cycle of the 1st order National Leveling Network (NLN) re-measurement, initiated in 2021, which involves complex observations across 55 leveling lines. The primary objective of this study is to evaluate the extent to which existing gravity reso...

This paper is focused on the verification of two quasi-geoid models for the territory of Bulgaria (the model integrated into the Geodesy, Cartography and Cadastre Agency (GCCA) official product for the transformation of geodetic data and a model derived using the GNSS/levelling method) through direct geodetic measurements. The study was developed in the framework of the current 5th cycle of the 1st Order National Levelling Network (NLN), conducted in accordance with the requirements of the new 2021 Instruction [1]...

In order to study and analyze the state of the State Gravimetric Network for the territory of the Republic of Bulgaria, relative gravimetric measurements were carried out for part of the country. The studied territory covers 9 points of the reference gravimetric network. The gravimetric measurements were designed and implemented according to the realized gravimetric lines between them and gravity differences along the gravimetric lines were calculated. The obtained gravity differences are compared with the same fr...

In addition to its main purpose: establishing and maintaining the parameters of the height system for the territory of a given state (country), state levelling networks also serve to establish (register) the contemporary (recent) vertical movements of the Earth's crust. The detection of such movements, besides in a purely research sense, is of great practical importance. The displacement of the benchmarks over time plays an essential role in seismic forecasting in the short and long term. Sometimes, not very often...

The calculation of geodetic coordinates ??,??,?? of points on the Earth's surface or in near-Earth space using the spatial geocentric coordinates ??,??,?? obtained by GNSS determinations, has wide application in geodetic practice. The direct problem consisting in determination of ??,??,?? with given ??,??,?? is solved with the established Helmert formulas. The inverse problem (determination of ??,??,?? from ??,??,??) is more complicated, the difficulty is related to the determination of ?? and ??. For the determin...

Bulgaria was integrated with the European Vertical Reference System (EVRS) in 2003 through the connection of the State Levelling Network (SLN) of Bulgaria to the United European Levelling Network (UELN) and Bulgaria's participation in the European Unified Vertical Network Densification Action project. The SLN of the Republic of Bulgaria is connected to the UELN and consequently to the EVRS through the last completed IIIrd realization of the SLN Class I, which was completed in 1984. In 2010, the Ordinance No. 2/30....

Three cross-border levelling lines between state levelling networks of Republic of Bulgaria and Republic of North Macedonia are made. The purpose of the realization of these lines is connecting of the state levelling network of Republic of North Macedonia in European Vertical Reference System. That could be made directly through the Bulgarian state levelling network ? part of UELN95/98 from 2003. Precise levelling measurements are initialized from the state geodetic agencies - Agency for Real Estate Cadastre of th...