## 1 Introduction  
Gravity measurements in geodesy are used in many ways. One of the basics is their use in defining and determining the height of geoid and quasigeoid as a reference surface, as well as determining the height of points in a given height system in classical spirit levelling. For the purpose of developing geodetic levelling networks, gravity values are measured, interpolated, or determined from the appropriate gravity potential model [1, 2]. Note that the classic approach to the problem of geoid and quasigeoid modelling using gravity data, basically involves construction of the disturbing potential model in the external space [3, 4, 5]. Such a model could also be used to determine the gravity values. However, the modelling procedures themselves, which runs using global geopotential models, are usually complicated and require the introduction of various corrections and reductions to the data [6; 7], causing that gravity models that could be used, for example, to develop levelling networks, are not directly (during this procedure) determined. Obviously, techniques for building such gravity models e.g. [8; 9; 10], most often in the form of regular grids, used in classical geoid and quasigeoid modelling procedures, could be also used to determine the gravity values. Nevertheless, we can state that classical geoid and quasigeoid modelling techniques do not allow for the construction of models that allow determination of height anomalies, geoid undulations, or gravity directly from the same model. Individual mentioned characteristics of the gravity field are determined in separate, though often related, procedures.  
Models that have the ability to determine from the same model the values of different characteristics of the gravity field are global geopotential models. Their accuracy is currently very high, but particularly in mountainous areas, they are still much lower than the aforementioned classical solutions and requires introduction of the appropriate corrections. Thus, they are primarily used as reference models on the basis of which more precise, local, and regional models of the gravity field are built.  
A model that allows to determine various parameters of the gravity field is the GGI model (based on Geophysical Gravity Inversion) [11; 12]. The model is built on basis of a rare network of GNSS/levelling height anomalies and a dense network of gravity points. Based on this solution, height anomalies characterized by very high accuracy can be derived, at the level of accuracy of the classical solutions [13; 14]. Directly from the GGI model, can be also easily determined other characteristics of the gravity field, such as gravity values, differences between geoid undulations and quasigeoid heights, and complete Bouguer anomalies (CBA). Out of the three characteristics mentioned above, only preliminary analyses of the accuracy of the gravity values determined from the model were carried out [15]. Hence, this study analyzes in detail the accuracy of gravity values and is an attempt to assess the quality of the differences of the geoid undulations and quasigeoid heights as well as the CBA values determined on the basis of the GGI model.