# Protection of Digital Elevation Model--One Approach
## 1 Introduction  
Most human activities take place on the surface of the Earth. It is therefore reasonable that a person seeks to collect and analyze various information on the ground and the Earth's surface. On a topographic map, all features present on the field are projected orthogonally in the 2D projection. The use of such maps dates back to the 18 th century [1]. Many believe that the topographic map is one of the most important procedures in mapping history due to its convenience. With the development of technology and the human desire for a realistic representation of the earth's surface in three dimensions, the need for 3D data and 3D models arose. Digital terrain modeling involves a whole range of activities related to the collection and processing of terrain surface data, the creation of appropriate digital terrain models and analysis, as well as the use of acquired digital models in different areas [2]. Progress in the development of Earth observations allows the capturing of larger amounts of detailed data that are useful in many areas, such as geodesy, geography, geology, and others. LiDAR technology (light detection and ranging) allows for the capture of different layers of a surface with considerable precision [3]. The InSAR system used on the Shuttle Radar Topography Mission in February 2000, currently provides the most complete and robust source of elevation data at the global scale [4]. At the beginning of this century, with the cooperation of several US agencies, the so-called SRTM (Shuttle Radar Topography Mission) was carried out. With a special aircraft, radar stereo pairs were collected within a few days for almost the entire Earth's surface. By processing these stereo pairs, in the period from 2000 to 2003, an accurate and high-quality digital elevation model (spatial resolution 90 m) was created, and in it, these models were freely placed on the Internet. By further processing and supplementingprevious missions, on September 24, 2014, a 1-arc second global digital elevation model (spatial resolution 30 m) was published, which was used in the continuation of the work. Thanks to the SRTM mission, created by radar technology, it can be downloaded completely free on the Internet. To acquire topographic data, the SRTM payload was outfitted with two radar antennas. One antenna was located in the shuttle's payload bay, and the other allowed single-pass interferometry that was on the end of a 60 m mast that extended from the payload bay once the shuttle was in space. The technique employed is known as interferometric synthetic aperture radar or InSAR [5]. The ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) Global Digital Elevation Model (GDEM) determined elevation by two images at different angles using stereotopic pairs and photogrammetry. The measurement was subject to errors due to cloudiness over certain areas [6]. The Japan Aerospace Exploration Agency (JAXA) collected images using the ALOS satellite (Advanced Land Observing Satellite) with an integrated PRISM (Panchromatic Remote-sensing Instrument for Stereo Mapping) instrument. Based on those images, the ALOS Global Digital Surface Model (AW3D30) was generated [7]. Airborne Light Detecting and Ranging instruments provide high precision topographic mapping tools via discrete surface height samples instead of a continuous coverage across small areas [8]. Laser altimetry or LiDAR is a technology that has the most diverse application in many scientific disciplines. When it comes to laser scanning, it is generally an active laser system recording that records reflected signals from the Earth's surface. It can be said that LiDAR is a fully automated, active, optical-mechanical procedure for spatial data collection. This method provides a large amount of data, the so-called cloud of points or point cloud, characterized by extremely high density and speed of collecting points (10,000-80,000 points per second), based on which we can create DEMs with high spatial resolutions.