## 4 Results  
For all the analyzed sets of SAR images, the number of PS points is proportional to the urban density. PS density is the highest in the city center and gradually decreases with increasing distance from the city center. The highest density of PS points on the research area was obtained for the Sentinel-1A stack, while the smallest for the ERS-2. This is due to the lower quality of ERS-2 SAR images, as well as the long observation period of nine years. With the increase in the length of the observation period, there are factors affecting the decorrelation of radar signal, which lead to a reduction in the number of PS. Mean displacements velocities (LOS) calculated for ERS-2 stack for the period 1995-2004 ranged from \(-\)3.5 to +1.3 mm/year (Figure 6). For most of the area, negative displacements prevailed, while positive displacements occurred in a compact sub-area in the southern and central parts of the city and are dispersed in the northeastern part. In the next period of 2002-2008, displacements velocities (LOS) determined from the Envisat InSAR stack ranged from \(-5.5\) mm/year to \(+0.9\) mm/year (Figure 7). Positive displacements were recorded in several places in the northeastern part, while negative displacements occurred on the remaining area. For the Sentinel-1A SAR stack in the period 2014-2018, the average LOS displacement velocities ranged from \(-2.9\) mm/year to \(+2.4\) mm/year (Figure 8). Positive movements occurred in the southern and southwestern part, while the remaining area was subjected to negative movements. Figure 6: LOS velocities determined on the basis of ERS-2 data for the period 1995–2004. Figure 7: LOS velocities determined on the basis of Envisat data for the period 2002–2008. The presented maps of displacement velocities indicate subsidence of the prevalent city area and periodic uplift in the southern part. The results confirm the continuation of the subsidence process documented by classical measurements [41]. In order to describe the ground surface deformation process in time for each InSAR stack, LOS displacements were determined for every epoch of data acquisition. Based on historical maps of displacements (Figures 3-5) and taking into account the coverage with PS points, several characteristic places marked from P1 to P10 (in Figures 6-8) were selected. In these places, the average value of LOS displacements for PS points inside a circle with a radius of 250 m were calculated and presented in Figures 9-11. Points P1-P10 have been selected in representative locations in the city, the selection criteria included: the history of displacements in the period 1930-1998 (Figures 3-5), the occurrence of benchmarks of the levelling network, and the presence of PS points for all the analyzed periods and sensors. The radius of 250 m is connected with the distance between benchmarks in Wroclaw, which is in the range of 150-250 m. It was assumed that at least three benchmarks should be included in the reference area, which, after the next levelling network measurement, will allow comparison of results obtained from SAR. Calculating average values for persistent scatterers located in the surface units has been suggested by [PERSON] et al. [26] for development of displacement map on the basis of data obtained from the multisensor InSAR for the area of Bratislava. The processing of ERS-2 observations in the period 1995-2003 revealed increasing negative LOS displacements in the period 1995-2000 for the most of analyzed elementary fields and change to positive in 2000-2002 period (Figure 9). After 2002, LOS displacements are varied. Figure 8: LOS velocities determined on the basis of Sentinel-1A data for the period 2014–2018. In the next period, 2002-2006 results of Envisat data analysis indicated increased negative LOS displacements and after 2006 displacements probably stabilized or changed trend to positive (Figure 10). There is no possibility of unambiguous assessment due to the end of observation of this area carried out by the Envisat satellite in 2008.