# Protection of Digital Elevation Model--One Approach
## 4 Discussion
### Discussion on the Impact of Changing DEM Data  
Within the analysis of the impact of intentional or unintentional data changes (errors) in LiDAR digital models, the areas of the Center and Petrovaradin Fortress will be treated, because they are characters in themselves. The first area belongs to the plain part of the terrain whereas the other area belongs to the hilly part. First of all, it is important to emphasize that unauthorized data change may appear to be an accidental error in the whole process of creating DEMs. The error can be described as a recess that otherwise does not exist on that part of the terrain (Figure 9). Since the unified graph of the movement of height values does not show certain anomalies, it is necessary to divide the graphics (Figure 10). By dividing graphs, it can be noticed that the impact of this type of error is present most in digital models of higher spatial resolutions, such as 1, 2, and 5 m. While in digital models of lower spatial resolutions, such as 10, 20, and 30 m, the impact of this error is eliminated. Figure 10: Divided graphs of the movement of height values. Figure 9: Movement of height values of the DEM of the Center area. Based on the graphic representation of height movement for the Center area, it can be concluded that it is more convenient to use a digital elevation model created by lower spatial resolution because it is a flat terrain that does not require a high degree of detail that is obtained by the use of high-resolution digital models. As seen from the graphs in this way, it is possible to avoid accidental errors in the input data. This means that a malicious person can cause such a change in data at the source, during transmission, or stored data so the change is not visible in low resolution, whereas in high resolution it can represent an advantage for the attacker. After establishing the error and extracting certain conclusions from this case, the error is resolved. To correct the error, after performing the classification of points, it is necessary to perform reclassification and remove these kinds of anomalies by manual processing. When the reclassification is performed, one can notice that the result is much more reliable than the previous one (Figure 11). However, this result is based on the data changed during the attack and is not reliable because there is no information on which DEM data was changed during the attack. It is also necessary to analyze the results of other areas to make a conclusion regarding the use of digital elevation models created by the LiDAR data acquisition method. The next area of interest is the Petrovaradin Fortress. Unlike the previous area, this area lies on hilly terrain and has very characteristic contours of the terrain. With a graphical presentation of height values movement, it can be seen that there is a large degree of terrain unevenness, but what can also be seen is, as in the previous corrected example, the congruence of the terrain lines (Figure 12). By comparing Figures 11 and 12, it is concluded that digital elevation models created by the LiDAR method of data collection give very consistent results when talking about different spatial resolutions. Also, based on data changes made during the attack it was noted that it is more advantageous to use DEMs created with lower spatial resolution since the impact of such errors is neglected. However, the changes made in high resolution will significantly change the terrain and affect decision making. The choice of spatial resolution in the process of DEM creation depends largely on the continued use of created DEMs. If a further flow of use requires a higher degree of detail in  
The next step in the analysis of the results is the comparison of the difference in digital elevation models created by the LiDAR method, as well as the difference between LiDAR and SRTM models.