## 5 The Elevation Difference Method  
While a significant part of the town area has been examined in terms of geohazards (resulting from old mining activities or geological phenomena and processes) via combined geophysical and geodetic methods, for the area in the western part of the town where the town's development was stopped 100 years ago, there has been virtually no knowledge of ground surface displacements for decades. This is due to the lack of observation points. The old ground benchmarks were destroyed years ago, which is typical for urban green space. Due to the lack of buildings in the area, it was not possible to mount wall benchmarks, which usually have a long durability (the oldest wall benchmarks in the town are over 120 years old and no ground benchmarks were saved). The expected causes of these displacements could be the natural uplift of the salt dome (well recognized in the other part of the town), the effects of hydrological processes (suffosion, karst phenomena), post-mining effects (space enlarging of old excavations resulting for its leaching of their walls with unsaturated brine, etc.). As already mentioned, the location of sinkholes marked on old mining maps has not been confirmed by gravimetric measurements (unlike in other parts of the town), but it was possible to ascertain the gravitational effect of old mining excavations from the beginning of the twentieth century. While there are no geodetic measurements that provide us directly with information on displacements, data of another type are available. They are records of the heights of topographical pickets included in old master maps of the town. The reason for their occurrence in land information systems was because of cartographic regulations regarding master map requirements [33]. Special geodetic instructions, and then ministerial ordinances, ensureddetailed regulation of the rather high density of establishing such non-embedded control points, for which the height was determined, and thus were able to help to determine the characteristics of the relief. Figure 5 shows a part of a section of the master map representing the analyzed area in the year 1978 (digitally remasetered). This cartographic projection of the north-eastern part of the area analyzed here at that moment in time includes archival data, amongst which are the height of pickets. They are presented with an accuracy of 0.1 m and in some cases even up to 0.01 m. On the basis of master maps from 1978, topographic pickets were digitized, and in this way, information on 1589 points (height pickets) was obtained, which made it possible to assess the Digital Elevation Model (DEM) for the state of 1978. The same was also possible for the year 1998, but height pickets were entered in this case for geodetic and cartographic resources of the Inowroclaw district. The latest data on the distribution of topographic heights for the area come from satellite measurements, which are made public and are given in the Kronstadt elevation system (the formal name of this datum is PL-KRON86-NH). This datum is no longer valid; it was replaced by the PL-EVRF2007-NH just as PL-KRON86-NH had replaced \"Kronszatd 60\" before it. In the case of the Kronstadt60, heights were expressed up to the 1980s. The average differences between these systems in Poland range from 2 to 8.5 cm [33]. This value is clearly lower than the accuracy of the topographic picket heights shown on the 1978 map. Nevertheless, research on height changes should take this fact into account. It should be noted here that LIDAR (Light Detection and Ranging) measurement data from airborne laser scanning ALS (Airborne Laser Scanning) represent the terrain surface in the form of a cloud of measurement points with determined XYZ coordinates. The files are saved in the LAS format and, in addition to the coordinates of the points, contain information about the class of a given point and the intensity of the signal reflection, among other information.