## 1 Introduction  
The geology of the North European Plain is distinguished by the occurrence of numerous salt structures [1; 2], which are usually associated with tectonic deformations and sometimes the manifestation of active diapirism, which generates some problems for the stability of the ground surface [3; 4; 5; 6; 7; 8; 9; 10; 11]. Ground instability (subsidence, ground collapse, sinkholes) is caused by intense underground leaching (subrosion) and by the development of karst phenomena. These phenomena, in turn, are associated with salt tectonic sinkholes, which are serious geological hazards, especially when they occur in urban or built-up areas. Human-related actions (mining, construction of new buildings, groundwater pumping and infiltration, interventions in the hydrological regime, etc.) may lead to the formation or the re-activation of sinkholes at the surface [5; 6; 7].  
To avoid catastrophic effects, appropriate urban planning is crucial to reduce the environmental and economic impacts that accompany both natural and human-induced geohazardous events. Thus, local authorities and countries have developed proper policies and strategies which aim to effectively manage hazard risks, which are defined along with the other terms, in general urban ordinance plans.  
The other aspects deal with assessing the potential geohazards and with managing the risk. They are associated with the precise mapping of the areas affected by ground instability (especially the presence of active sinkholes) and the quantitative depiction of the ground instability (displacements, kinematic pattern, and spatial-temporal distributions).  
The first aspect is related to geophysical research, which enables the identification of underground sources of danger, and the second aspect is concerned with surveying methods for investigations of ground displacements.  
Detailed field surveys are essential for a more precise mapping of the sinkholes to confirm the existence of any buried sinkholes or underground caverns, and in fact they apply both the geophysical and the surveying methods. Such a multidisciplinary approach to shallow-geohazard research was applied in the town of Inowroclaw, central Poland.  
The key issue raised by the authors was the manner in which we could arrive at an assessment of displacements in the abandoned area of the former borehole exploitation. While geophysical research and historical documentation of old sinkholes and mining excavations seem to be sufficient for this area, there has been no recognition of surface deformations for decades. This has been the result of the loss of geodetic benchmarks many years ago, which was at the heart of the observation network established for the assessment of mining damage in the first half of the twentieth century.  
The task of evaluating surface movements for this region was performed based on data from archival maps, which included height spots and modern remote sensing measurements (LIDAR).  
Models of terrain surfaces and their differentiations are widely used in various fields aside from Earth sciences [11]. However, so far, there has been no description of the recent activity of salt dome movements in the European Zechstein salt formation with the use of high-resolution remote sensing data.  
The authors have determined the displacements between ground surfaces, which are estimated based on remote sensing data and historical data. This approach, which the authors have named the Elevation Difference Method, is not revealing in itself, but this term has a technical sense here. The authors formulate a terminological convention, i.e., they agree with the readers that they will replace the entire long description with this term of their choice in the form of a possibly short expression. This is why the term has been created.