## 6 Discussion  
In accordance with the discussed approach (the Elevation Difference Method), the different morphologies of the terrain surface from the analyzed years were compared. The only information about the morphologies from the past is the data (topographic pickets) recorded on the master maps (which had to be produced due to the regulations for territorial mapping). Considering the large number of pickets, it was possible to develop a reliable model of the terrain surface. Based on the obtained points and using the Surfer v.23 software (Golden Software Inc., Golden, CO, USA), interpolation grids with a mesh size of 5 \(\times\) 5 m were calculated using kriging, and as a result, terrain surface models were determined for the individual years. They took the form of regular grids, with geometries consistent with those of the LIDAR data (2018). The interpolation error was computed as: error = interpolated value \(-\) observed value. The obtained interpolation error is characterized by small values of single millimeters, so these values are two orders of magnitude smaller than the error of the determined height changes and do not affect the analysis result. Thanks to this approach, we were able to determine the changes in the terrain surface for the time intervals 1978-1998, 1998-2018, and for the long interval of the period of time from 1978 to 2018. Figures 6 and 7 show spatial distributions, respectively, for the longer (with more significant changes) and the shorter time intervals. Extremely high values of height changes on the western border of the area under study, in its northern and southwestern parts, are the result of the reconstruction of communication routes. Figure 6: The map of the height changes in the time interval of 1978–2018 and locations of abandoned mine workings in the area. The legend presents the signatures used on the map and their numerical designations/ Their meaning is as follows: 1—boundary of the area under study; 2—workings of the shallow mines from the beginning of the twentieth century; 3—benchmarks of leveling measurements; 4—sinkholes that existed before 1861, according to [18]; 5—sinkholes that occurred after 1861, according to [18]; 6—boundary of the salt dome at the depth of 470 m; 7—historic, currently non-existent railway lines; 8—abandoned mine shafts; 9—boreholes; 10 —profile lines. The statistical analysis has provided valuable insights into the characteristics of the height change distributions. For a short time interval (1978-1998), and at a level of height change above the margin of error value (\(>\) 1.0141  m), 52.0% of the set of all points were found to show height changes above the margin of error value (10.6% of them demonstrated subsidence, and 41.4% of them demonstrated uplift). However, for the longer time interval (1978-2018), and at a level of the changes above the margin of error value (\(>\) 1.080 \(\,\) m), 68.9% of the set of all points showed height changes above the margin of error value (13.9% demonstrated subsidence, and 55% demonstrated uplift). Similar results were obtained using a more rigorous approach in the statistical analysis that examined changes in the topographic picket heights since 1978 (in these cases using just the measurement points, not points produced from the interpolation algorithm). Therefore, for the points with the coordinates of these pickets (1589), their heights were estimated on subsequent DTM models, and as a result, height changes were determined for the points. These points could be considered as the corresponding points of the topographic pickets (projections of these points) on the other terrain surfaces, working from results gained from the base (reference) terrain surface of 1978 on the other terrain surfaces. These were projections including that which was determined from LIDAR data (2018). The distributions outlined as histogram plots presented in Figure 8 clearly demonstrated that  
Figure 7: The map of the height changes in the time interval of 1978-1998 and locations of abandoned mine workings in the area.