The main objective of the Doctoral Thesis is to develop a new solution for the Earth gravity field determination based on spherical-cap-harmonic modelling, using both digital-zenith camera and gravimetric measurements’ hybrid data. In first instance digital zenith cameras provide astronomic coordinates as information to the true gravity potential W of the Earth, in analogy to gravity values. Astronomical coordinates give the direction vector in the observation point P of the true potential, and often called the geometrical information, while g is called physical information on W. Digital zenith camera is a new kind of astrogeodetic instruments, employing recent advancements in several technology areas (GNSS positioning, digital imaging, extensive and accurate astrometric reference star catalogues, and high resolution electronic tiltmeter technology) to obtain direct measurements of vertical direction (Φ, Λ) and derived vertical deflection (ξ, η) values. Over several years Institute of Geodesy and Geoinformatics (GGI) was engaged in design of the digital zenith camera. Presently the camera and data acquisition control and processing software are finished, and field measurements are actively done now. The intention is to use vertical deflection measurements along with GNSS/levelling and gravity data to improve a local gravity field and thus a local quasi-geoid model computation including both physical and geometrical data. The method of Digital Finite Element Height Reference Surface software (DFHRS) is applied for this purpose, allowing the use of both physical observations and geometrical observation types. Main aspect is concerning the implementation of Spherical Cap Harmonics (SCH) modelling to local model of the potential, developing a general parameterization of gravity field related coefficients for a least squares’ estimation. Terrestrial gravity measurements, deflections of vertical, height fitting points with known ellipsoidal and normal heights, and the use of the available global gravity models as additional observations will be used for quasi-geoid model determination. One of the aims is regarding the computation of a high precise local potential model W with the ability to derive all components related to the potential W. These observation components are gravity g, quasi-geoid height, the geoid height N, deflections of the vertical in the east and north direction, the fitting points and apriori information in terms of coefficients of a local potential model derived from the developed methods of a mapping of a global one. There are different types of Spherical Cap Harmonics, such as: Adjusted Spherical Cap Harmonics (ASCH), Translated-Origin Spherical Cap Harmonics (TOSCH) and the Revised Spherical Cap Harmonics (RSCH) and the ASCH is chosen for the local gravitational potential modelling. The Spherical Cap Harmonics modelling encounters problems on the boundary, requiring application of the solution using an oversizing of the cap area with respect to the area of interest, but due to a lack of data near the borders, this is complicated task that still exist and should be investigated and solved. The first chapter provides the fundamental theory of Earth gravity potential and quasi-geoid modelling, including the types of gravity data and methods of its processing and adjustment using both standard Least Squares and Robust estimation techniques. The height systems are described, and its physical differences are explained. The second part introduces the development of Digital Zenith Camera at the Institute of Geodesy and Geoinformatics (GGI), the basic principle of vertical deflection determination, as well as its construction, measurement technique and data postprocessing is widely described in this chapter including all stages of data processing. In the third chapter the principles of quasi-geoid determination are introduced, starting from DFHRS method and its stages of development and finishing with collocation method. The fourth chapter concerns spherical harmonics and global gravity modelling, introducing spherical cap harmonic modelling and one of its methods – Adjusted Spherical Cap Harmonics modelling. The next part of this chapter concerns the basic principle of development of global gravity models and its techniques. The fifth chapter introduces the results and analysis of quasi-geoid of Latvia based on new solution, where both vertical deflection data and gravity data are used. The summary results and conclusions of the fulfilled thesis are also discussed in this chapter. The Thesis includes 39 figures, 127 formulas, and 7 tables. The total amount of PhD Thesis is 104 pages.