## 2 Characteristics of the Research Area
The displacements were determined on the basis of levelling performed at intervals of several years. The oldest documented levelling measurements from 1879-1882 and 1948-1956 in this part of Poland showed large-scale vertical movements up to +1 mm/year [36]. Another historical study based on measurements from 1953-1958 and 1974-1975 [37] showed values from \(-\)2 mm/year to +0.5 mm/year, while the last vertical velocity map developed on the basis of levelling measurements from 1974-1975 and 1998-1999 shows values of approx. \(-\)3 mm/year [38]. In turn, the vertical movement velocity map of the Earth's crust in the area of Poland based on the GNSS data in the 2008-2012 period shows values of approx. +1 mm/year [39]. In Wroclaw, the levelling control network has been measured with the use of precise levelling method since the 1930s, at 30-year intervals. These measurements constituted a basis for establishing the ground surface displacements, which occurred in the area of Wroclaw between 1930 and 1998 [34; 40]. Vertical displacements in the area of the city were measured against the adjacent geological structures between 1956 and 1999 [41] on the basis of national precise levelling lines. The research revealed that the whole city area is subjected to subsiding movement in relation to the adjacent geological structures. The purpose of the current research is to evaluate ground surface displacements in the area of Wroclaw after 1999, to determine the time series. The first research on ground surface displacements in the Middle Oda Fault Zone investigated vertical displacements observable in the area of Wroclaw and covered the 1930-1968 [40] and 1968-1998 [34] periods. The results of these studies revealed that the relative vertical displacements in the area of the city reach \(-\)9 cm in the 1930-1998 period. During 68 years, the structure and area of the local levelling network underwent changes, so these studies were based on independent groups of points only partially overlapping. In the following research [42], an analysis of relative vertical displacements in the 1930-1998 period was conducted. It was based on a set of 319 common benchmarks. In the 1930-1968 period, the maximum displacement between points in the central, southern and northeastern part of the city compared to points in the southwestern part reached \(-\)34 mm (Figure 3). In the 1968-1998 period, this value reached \(-\)76 mm (Figure 4), and over the entire 68-year period \(-\)93 mm (Figure 5). Based on previous studies, it was assumed that the specified parts of the city undergo relative displacements with a mainly linear trend. Figure 4: Relative vertical displacements in the 1968–1998 period [42]. Figure 3: Relative vertical displacements in the 1930–1968 period [42]. Therefore, the scope of research was expanded to include the areas outside the city and outside the Middle Odra Fault Zone. Based on the changes of the elevation differences in the national precise levelling lines for the period of 1956-1999, a conclusion was made that the whole area of Wroclaw subsides in relation to the adjacent geological areas marked by the tectonic fault lines [43]. The borders of the subsiding trough are consistent with the north and south borders of the Middle Odra Fault Zone. Due to a relatively limited number of measurement points (benchmarks) and limited measurement frequency, determining a spatial model for vertical displacements was impossible. Between 2008 and 2012, another research project focused on horizontal displacements in the network of 12 GNSS points in the Middle Odra Fault Zone (Wroclaw is located in the center of a network of GNSS points). The estimated horizontal displacements in the area of Wroclaw are approximately \(-0.5\) mm/year/100 km in the SW-NE direction and \(+0.5\) mm/year/100 km in the perpendicular [43; 44; 45]. Preliminary research on the application of PSI to a fragment of the Wroclaw area was performed by [46] as part of the GEO-IN-SAR project, which covered selected areas in Poland.