Comparisons and problem identification on local and global digital elevation models

Years of evolution have allowed the mankind to develop different tools and technologies that facilitate and improve several human activities, remarking that the technology cannot replace the human labor and that always this is needed to be supervised by an individual; it is the case of Satellite and Airbone acquisition systems which measure large areas in a matter of hours while a traditional method (theodolite or total station topographic survey) could take even months. Nevertheless, these new methods present a common dilemma, namely, the lost of accuracy. This fact depends not only on the equipment that performs the capture but the resolution of the acquisition and the area extension which references to an acquisition in a global or local scale. Based on these facts, it is originated the topic of this document, aimed to compared different global Digital Elevation Models (DEMs) such as ASTER, SRTM and GMTED2010 with local Digital Elevation Models such as Lidar, and then identified and analyze the problems local or a global model. Being useful that all these data are downloaded for free (OpenData), more quantity of tests can be executed. Moreover, from the Lidar page source it is possible to download either Digital Surface Model (DSM) and the Digital Terrain Model (DTM) which add to general scope of the study an extra scope. It consists in create a map of differences between them which likely shows the features present in the study area. The whole process is developed in 10 chapters that are briefly explained below Chapter 1. This chapter explains the general characteristics of the Digital Elevation Models, the way how they are acquire and the principal representations of these models. Chapter 2. This chapter will explain three of the most important and well-known Digital Elevation Models regarding to a global scale. They are clustered or categorized low resolution when compared with a LiDAR model. Nevertheless, these models (especially SRTM and ASTER) in a general point of view generate a high resolution database taking into account that the acquisition is done for almost the entire Earth surface and the purchase of these products is low cost or simply for free. Chapter 3. In the previous chapter global DEMs at a low resolution are mentioned, remarking that the acquisition is done for the entire Earth surface. However, when speaking locally, depending on the instruments that are used to make the measurements, this means high resolution as well accuracy. This chapter explains one of the most accurate methods to acquire information of local surfaces throughout remote sensing advantages. Chapter 4. Since it deals with all this Geographic Information it is important to concentrate it in a unique Geographic Platform as GRASS GIS, in such a way that this information can be read, visualized, edited and analyzed by means of FREE and Open Source software. That’s why in this chapter will be explained the main functions of GRASS. Chapter 5 This chapter will expose the general features of the study region which correspond to the Val di Sol area located in Trentino region. This general features correspond to climate, topographic, vegetation, etc. Chapter 6 After characterized the study area, four processes are followed to compare Digital Surface Model with the respective Digital Terrain Model of the Val di Sole area. First of all, the download process and some general characteristics of the data set are explained then algebraic operations between DSM and DTM have been done with the idea of analyze statistically the obtained differences which in this case represent the different features (trees, buildings, bridge…) present in the study area and finally a categorization process is carried out based on a official land use map. For this chapter two specific zones have been selected inside the whole Val di Sole area, namely, a small mountain sector and the main valley. This is done because one idea of the chapter is to compare the results between the Digital Models when the acquisition is performed in a mountain area and the plain. Chapter 7. This chapter explains step by step the process of download SRTM, ASTER and GMTED2010 Digital Elevation Models from its respective websites and also the way to import them into the GRASS platform. Chapter 8, Chapter 9 and Chapter 10 In this chapters are produce a final map that displays the differences between the global DEMs, namely, Aster, SRTM and GMTED with respect to the local DEM. Furthermore, calculate a correlation index between the height (h) and the variation of height between the two models (Δh).