# Protection of Digital Elevation Model--One Approach
## 1 Introduction
In a cloud of points obtained by scanning, a single point represents a footprint or electromagnetic wave reflection from a part of the surface of a scanned object [9]. Although there have been several studies analyzing the quality of the SRTM data in general [10; 11; 12], there has not been much research on the quality and usability of the SRTM data for a small surface area. Deviations from the SRTM data to the reference data set are in the following referred to as 'errors'. It shall be made clear, however, that the reference data set also has errors associated with it [13]. Digital elevation models (DEM) are the generally adopted data structures for storing topographic information [14; 15]. The Digital Surface Model (DSM) represents the surface of the earth and includes all objects on it [16]. In a raster-oriented GIS, the basic visual structure of data makes the correct (usually rectangular or square) network. This is a simplified view using square cells, i.e., pixels, and is determined by dimensions (spatial resolution). Each pixel has its value, and in the case of DTM representation, the pixel value refers to the height of the terrain [17]. There are two approaches to defining the height of the terrain surface using GRID [17]: surface grid and lattice. TIN (Triangular Irregular Network) is a digital data structure for terrain surface modeling that is a vector representation of the Earth's physical surfaces, and consists of irregularly arranged nodes and lines with three-dimensional coordinates that are assembled in a continuous triangle network [18]. The accuracy of digital data collected by terrain scanning directly depends on the technology and devices in use. However, the accuracy of data indirectly depends on the handling of it during creation [19], transmission, processing, and storage [20; 21; 22]. Like all other data, DEM data is at security risk primarily due to intentional or unintentional data changes during creation, transmission, and storage. Making decisions based on incorrect data can lead to material and financial damage, increased risk to people's safety, or even undesired consequences. This is characteristic of systems that use terrain shape data or data on what is on the terrain. DEM is important for military purposes, especially in command and information systems. It is therefore important that the data be secure and non-repudiated at the location of the reception and safe during transmission. Geospatial data is now widely distributed in government organizations, the military, municipal affairs, and disaster defense. Therefore, this data must be reliable and secure. One of the applied mechanisms in privacy protection models uses disguising the geospatial data location [23; 24]. Different security policies have been considered in different environments [25; 26], as well as the application of blockchain technology [27; 28; 29; 30] for geospatial data privacy and integrity preservation. In [31], the authors consider the protection of DEM information through encryption, hiding, and disguising. They analyze the difference between the mentioned protections to create theoretical support for DEM protection research. PKI (Public Key Infrastructure) has proven to be a good environment for preserving the integrity, authenticity, and protection of data, as well as the non-repudiation of parties in communication [32; 33; 34; 35; 36]. The purpose of this paper is to create an environment that will enable the authenticity, integrity, and non-repudiation of DEM data using PKI. The paper contains a comparative analysis of the digital elevation model data changing impact based on SRTM (Shuttle Radar Topography Mission) with a spatial resolution of 30 m and the LiDAR method of data collection with spatial resolutions of 1, 2, 5, 10, 20, and 30 m using suitable software and tools. When two or more DEMs for the same area are available, those with the highest resolution and best vertical accuracy should be considered as the reference basis for the representation of terrain features [37], which was LiDAR in this case.