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The paper analyzes normal corrections for the determination of the normal height differences along the First Order Levelling lines in Bulgaria. Normal corrections are calculated in two ways – with measured gravity accelerations and with gravity data from Global Geopotential Models (GGM). The study uses the last contemporary GGMs developed up to 2160-2190 degree and ultra-high resolution GGMPlus model. Calculations for Bouguer anomalies are made by the calculation service in the International Centre for Global Earth Models (ICGEM). The method for determination of the interpolated values of the gravity anomalies is based on that considered in [15] for all GGMs, with the exception of the interpolated data from GGMPlus. Gravity anomalies from GGMPlus are calculated by using known and modelled data for the height anomalies. In particular, the paper studies the accumulation of systematic errors along the levelling line due to modelled gravity data. Systematic and random error in the differences between the modelled and measured normal corrections are calculated by the Lallemand’s formulas. Although the main part of the value of the gravity anomaly is formed by the development up to degree 2160, the contribution of the omission part of the development can have a significant impact in the conditions of anomalous gravitational fields and in mountain areas. Calculated systematic and random errors are negligible in a smooth gravity field. Application of the present GGM data for each separate levelling distance is possible only for flat areas with a weak anomaly field. The determination of geopotential numbers and geopotential differences can be performed in two ways leading to identical results. The first is the classic one – by using the reduced gravity acceleration values and measured levelling differences. The second one is direct – by using the direct relationship between geopotential differences and normal height differences. [ABSTRACT FROM AUTHOR]

Correlation studies have been made between the gravitational field (Bouguer anomalies -G) and the magnetic field (common vector (T) and the vertical component dZ) for theterritory of Bulgaria. The map data is in digital format, which makes it easier tocalculate. The correlation itself is estimated from -1 to 0 and from 0 to +1, which meansthat a positive correlation is possible in the case of increased values of both parametersand vice versa -decreasing values are associated with reduced values. This forms apositive correlation. A negative correlation is observed when both parameters haveminimum values. Zero correlation means -NO correlation (both parameters are min-max or max-min). The calculations from the maps give a clear picture of the positiveand negative correlations and the ability to calculate areas with significant positive and /or negative correlationsThe results might be used for the interpretation of the studied potential fields in case of structural, regional and / or local changes in the gravity and magnetic fields, and their respective sources. [ABSTRACT FROM AUTHOR]

The possibilities of using gravimetric data obtained from global geopotential models (GGM) for the territory of Bulgaria in the processing of precise levelling measurements for the determination of normal differences and geopotential differences are presented in the article. The researches were carried out on first order levelling lines of the State Levelling Network of the Republic of Bulgaria, situated in different topographic conditions, as well as in conditions of different anomalous gravitational field. The data used for the calculation of the second normal correction and geopotential differences on the levelling lines are data from gravimetric measurements and interpolated gravimetric data for the Bouguer anomalies of EGM2008. Normal differences and geopotential differences were determined using different gravimetric data as well as data for benchmark heights from previous levelling measurements. The differences between the two types of height determinations based on conventional gravimetric data and GGM data are analyzed. The dependence of the series from the different physiographic conditions has been studied. The obtained results for differences on the levelling lines are compared with the regulatory requirements for accuracy in Bulgaria concerning the precise levelling determinations. The applications of gravimetric data from GGM are outlined both for precise height determinations and for detection of gross errors in precise levelling processing. The study for the territory of Bulgaria is based on an idea presented in the publication by Popadyev V. V., Gulyyev D. A. (2017) “Application of global geopotential models to high- precision levelling processing”, where such studies were made to determine a normal correction on first-order levelling lines. [ABSTRACT FROM AUTHOR]