Lecture 03 CE797 F24.pdf

Faculty of Engineering Lecture (3) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani...

Faculty of Engineering Lecture (3) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (1 of 3) GIS Applications in Civil Eng. 2.1 DIGITAL EEVATION MODEL (DEM) Digital Elevation Modeling is the science of quantitative modeling and analysis of the topographic surface and relationships between topography and other natural and artificial components of geosystems. Digital Elevation Modeling comprises the process of representing the elevation characteristics of the terrain in discreet form in a three-dimensional space of the land surface. Digital Elevation Modeling is the digital representation of the land surface elevation with respect to any reference datum. DEM is frequently used to refer to any digital representation of a topographic surface. DEM is the simplest form of digital representation of topography. 2.2 TYPES OF DIGITAL ELEVATION MODELS A digital elevation model (DEM) is one of the most important spatial datasets in many geographical information systems (GIS). It is defined as an Digital Elevation Model (Regular grid) ordered or unordered digital set of ground elevation (spot height) for terrain representation. There are three commonly used terms related to this, namely, digital elevation model (DEM), Digital Terrain Model (DTM), and Digital Surface Model (DSM). A DEM (Digital Elevation Model) is a bare land surface Digital Elevation Model model, which is supposedly free of trees, buildings or other “nonground” objects. A DEM is a bare-earth regular or irregular grid elevation points referenced to a vertical datum. When you filter out non-ground points (such as bridges, buildings, towers and trees), you get a smooth digital elevation model. These non-ground objects aren’t included in a DEM. A DSM (Digital Surface Model) is an elevation model that includes the tops of everything, including buildings, treetops, towers plus the bare ground where there is nothing else on top of it. A DSM captures both the natural and built/artificial features of the on the ground surface. ▪ DSMs are ideal for runway approach zone encroachment in aviation, and urban planning to check how a proposed building may affect views. Beyond that, DSMs can be used for visualization, disaster management, navigation, vegetation management, A DTM is a more generic term referring to a DEM with one or more types of terrain information, such as terrain morphological features, drainage patterns, breaklines, peaks, spot heights, ridges and troughs. DTM DTM (left) versus DSM (right) typically augments a DEM, by including vector features of the natural terrain, such as rivers and ridges. A DTM may be interpolated to generate a DEM, but not vice versa. Obviously, a DEM is a subset of DTM. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 1 of 4 Faculty of Engineering Lecture (3) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (1 of 3) GIS Applications in Civil Eng. 2.3 STRUCTURE OF DEM We mean by DEM structure is the distribution of the DEM points. According to the curvature of earth, there are two main types of DEM grids: plane grids and spheroidal grids. In this course we will be working on plane grids in the XY plane. There are three main types of DEM structures: 1. Gridded DEM: Regular grids based on squares, rectangles, equilateral triangles, hexagons, and so on. The square grid is the most popular one. ▪ Gridded DEM is referred to as secondary (computed) DEM because it is usually interpolated Regular Grid from irregular grid TIN DEM or Contour-based DEM. ▪ Gridded DEM is considered as a raster image composed of a matrix of pixels of similar size Regular grid (rows Irregular Grid arranged in rows and columns. & columns) ▪ Gridded DEM is the most common type of DEM data because of its simple structure and ease of manipulation. 2. Triangulated Irregular Network (TIN): This DEM structure is based on irregular grid points. These points are connected by non-overlapping, nonintersecting triangles. vertices of the triangles match with the surface elevation of the sampling point. The triangles TIN (based on irregular grid) represent the planes connecting the points. ▪ TIN DEM is referred to as primary or measured DEM since it is usually generated by direct measurement of elevation. ▪ TIN DEM is a vector-based data, in contrary to TIN (based on irregular grid) Gridded DEM which is a raster-based data. ▪ TIN DEM usually gives more accurate representation of terrain surface compared to other Grid structures; however, it involves much greater number of computations. 3. Contour-Based DEM: This is an irregular DEM structure; however, the points are located along contour lines. A contour line is a polygon or a polyline connecting points of equal elevation with respect to a vertical datum like Contour-Based DEM mean sea level. ▪ Contours that are very close together represent steep slopes. Widely spaced contours or an absence of contours means that the ground slope is relatively Contour lines level ▪ Contour maps are the most common type of topographic maps. ▪ The elevation difference between adjacent contour lines, called the contour interval, is selected to best show the general shape of the terrain. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 2 of 4 Faculty of Engineering Lecture (3) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (1 of 3) GIS Applications in Civil Eng. 2.4 GENERATION OF DEM There are three main elevation data collection methods used to generate DEM. These are field methods, remote sensing methods, and laboratory methods. Field methods: (1) Conventional topographic survey, (2) Differential GPS survey (DGPS) Laboratory methods: (1) Digitizing contours, (2) Scanning and vectorizing contours. Remote sensing methods: (1) Satellite remote sensing, (2) Airborne remote sensing, (3) Drones The techniques used to generate DEM from these methods are numerous, however, we will list only eight of the main techniques used: Topographic surveys 1) Conventional topographic surveys 2) Kinematic global navigation satellite system (GNSS) surveys 3) Digitizing of contours 4) Stereophotogrammetry 5) Synthetic aperture radar (SAR) 6) Laser altimetry or Lidar (Light Detection and Ranging) 7) Satellite radar altimetry 8) Airborne optical sensing of bathymetry 2.4.1 Field Methods Field methods are used to generate very high-resolution DEM, however they are only suitable for large-scale mapping of small areas because of their high cost and require long time. (1) Conventional topographic surveys Topographic survey is one of the field techniques for DEM generation. Surveying instruments used are mainly Total Stations (optical or laser) and leveling instruments. Topographic surveys are suitable for large-scale mapping of small areas. ▪ Topographic surveys generate high resolution DEM with high accuracy compared to other techniques, but, at a higher cost and time. ▪ Differential leveling and traversing surveys are conducted to determine elevation and horizontal coordinates of stations. ▪ Topographic survey is then executed at each traverse station. (2) Kinematic GNSS surveys Global positioning satellite system with GPS receivers mounted on an all-terrain vehicle or moved by an operator. Kinematic GPS surveys offer fast and cost-effective DTM-based solutions for many applications that require detailed and accurate large-scale maps. ▪ Two GNSS receivers are used: a base stationary receiver, and a rover receiver mounted on an all-terrain vehicle or moved by an operator (see the figure). The base station receiver samples data from satellites and transmits them, together with its known position, via radio to the roving receiver. The rover receives the transmitted data and processes the GNSS measurements obtained at both the base and rover receivers to determine the position of the rover. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 3 of 4 Faculty of Engineering Lecture (3) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (1 of 3) GIS Applications in Civil Eng. 2.4.2 Laboratory methods Laboratory methods include converting old topographic paper maps to DEM by manual digitizing or automated vectorizing of contours. Although the digitizing of contours per se is a laboratory approach, contours on traditional topographic paper maps are, in most cases, results of conventional topographic surveys and photogrammetric processing of aerial stereo pairs. (1) Manual Digitizing of Contours Topographic and geological maps of various scales are digitized to produce DEMs of the land surface and surfaces of stratigraphic horizons, respectively. Digitizing of a contour consists of the transformation of the contour lines into a discrete sequence of georeferenced points located along the contour. In digitizing, one may use ancillary cartographic information, such as elevation/depth values for summits and bottoms, as well as structural lines Manual digitizing can be done by a computer mouse and a paper map placed on plane table, or a scanned map on the monitor screen. Each digitized contour is tagged with its contour elevation. Map-based DEMs are widely used to produce medium- and small-scale mapping depending on the scale of the original digitized topographic maps. Manual digitizing is time consuming and requires lot of efforts. Isobaths on hydrographic paper maps are usually the result of echo sounding. Contours of stratigraphic surfaces on geological maps are, as a rule, based on the results of drilling. (2) Scanning and vectorizing contours In this lab method, the contour lines in the raster scanned topographic map are vectorized by a fully automatic process called “Raster to vector conversion”. The computer traces all contour lines on the map and convert them to polylines. Each contour line is then tagged with its elevation. Further manipulating and editing might be necessary to eliminate some errors in the vectorization process. The end product is a contour-based DEM suitable for medium to small scale maps. This method gives higher accuracy with low efforts and less time. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 4 of 4