## 5 Discussion  
The presented results of analyses spanning the years 1995 to 2019 show 4-5 year periods of negative displacements and 2-3 year period of stabilization or positive displacements. After periods of relative stability, there are longer periods with relatively higher values of negative displacements. Therefore, analysis of levelling newtork epoch measurements performed every few decades presented in [PERSON] [40], [PERSON] [42], and [PERSON] et al. [41] show mainly subsidence in the city area. Due to the possible oscillatory nature of these movements, mean values of LOS displacement velocities may change in particular periods of observation. Periodic oscillations are  
Figure 11: LOS displacements in the 2014–2019 period—Sentinel-1A. Figure 10: LOS displacements in the 2002–2008 period—Envisat. Figure 9: LOS displacements in the 1995–2003 period—ERS-2. mainly visible in the development of ERS-2 SAR images, while the data from the other sensors give only partial information, as they cover relatively short periods of 4 and 6 years. In addition, there is a gap of six years between Envisat and Sentinel-1A observations. The values of displacements in the overlapping observation periods of ERS-2 and Envisat show some differences, which are the result of different reference systems. The observations of these sensor were developed independently, so each of the InSAR stacks has a different spatial and temporal reference. Vertical displacements are assumed to have the greatest influence on the obtained results. However, the signal is deflected from surfaces inclined at various angles (e.g., squares, building roofs, chimneys, etc.) and, as a result, displacements of different directions have been obtained for the selected surrounding area. Some studies described in literature have been aimed at identifying PS points with regard to spatial information, which includes data on buildings, road infrastructure, etc., as well as at classifying and interpreting these points with regard to the types of objects identified as PS points [52]. Interpreting the velocities of PS points is problematic due to various types of scatterers and deformations. The determined displacement of PS points may be the consequence of the displacement of the subsoil and movement of the building structures, which are the sources of reflection [53]. Proposal for detailed interpretation of the PSI data in combination with geospatial data was presented by [PERSON] et al. [52]. This approach requires access to detailed GIS databases. Additional problems may occur when attempting to unambiguously identify the PS points and to find their precise location [27]. Potential factors causing deformations in the studied area include: slow, large-area deformations associated with tectonic movements covering the region of southwestern Poland, compaction of Cenozoic deposits, and local factors related to development of large constructions projects. The geodynamic research in the region of south-western Poland [45] indicates intraplate horizontal displacements causing compression in the SW-NE direction, tension in the NW-SE direction and negative vertical displacements on the most part of the area. The GNSS epoch observations were carried out in the network consisting of several dozen of points, while geometric levelling measurements were done in the National Levelling Network at intervals of at least 25 years. The relatively small number of these points on the research area and its surroundings made it impossible to accurately assess the surface deformation process of individual parts of the city with results of geodetic measurements. Until now, a constant rate of horizontal intra-plate displacements and deformations at the 0.5 mm/year/100 km and a mean vertical displacements rate of approx. \(-2\) mm/year were assumed on the city area. The presented current measurements of displacements using InSAR technique made with higher temporal and spatial resolutions indicate predominant periods of ground surface subsidence with periodic 2-3 year stabilization or uplift.