3D and Surface/Terrain Analysis Prepared by: George McLeod With support from: NSF DUE-0903270 in partnership with: Geospatial Technician Education Through Virginia’s Community Colleges (GTEVCC) Digital Terrain Models • A digital terrain model is a model providing a representation of a terrain relief on the basis of a finite set of sampled data • Terrain data refers to measures of elevation at a set of points V of the domain plus possibly a set E of non-crossing line segments with endpoints in V D Data Sampling • Regular • Irregular Sampling effects resolution Our three Primary terrain Models • Digital Elevation Models (DEMS) – aka Regular Square Grids (RSGs) • Triangulated Irregular Networks (TINS) – aka Polyhedral terrain models • Contour Maps – aka “topo” maps The Data… DEM LIDAR (LIght Detection And Ranging) TIN Introduction to the Data • Terrain mapping • Land surface is 3-D • Elevation data or z-data is treated as a cell value or a point data attribute rather than as a coordinate. • Digital Elevation Model (DEM) – Gridded array of elevation points obtained from a variety sampling methods DEMs A constant function can be associated with each square (i.e., a constant elevation value). This is called a stepped model (it presents discontinuity steps along the edges of the squares) D • The function defined on each square can also be a bilinear function interpolating all four elevation points corresponding to the vertices of the square • Triangulated Irregular Network (TIN) – Series of non-overlapping triangles – Elevation values are stored at nodes – Sources: DEMs, surveyed elevation points, contour lines, and breaklines – Breaklines are line features that represent changes of the land surface such as streams, shorelines, ridges, and roads TINs • Example of a TIN based on irregularly distributed data Data for Terrain Mapping and Analysis • Triangulated Irregular Network (TIN) – Not every point in DEM is used – Only points most important – VIP (Very Important Points) algorithm – Maximum z-tolerance algorithm – Delaunay triangulation: all nodes are connected to their nearest neighbor to form triangles which are as equi-angular as possible. Tins vs. Grids DEM TIN • Needs larger storage capacity • Needs smaller storage capacity • Computationally difficult • Computationally simpler • Flexibility of data sources • Fixed with a given cell size • Can add points • Cannot add sample points • Better display • Raster display • More efficient • Less efficient Contour Mapping • Contouring is most common method for terrain mapping • Contour lines connect points of equal elevation (isolines) • Contour intervals represent the vertical distance between contour lines. • Arrangement of contour lines reflect topography Digital Contour Maps Contours are usually available as sequences of points A line interpolating points of a contour can be obtained in different ways Examples: polygonal chains, or lines described by higher order equations Digital Contour Maps: properties They are easily drawn on paper They are very intuitive for humans They are not good for complex automated terrain analysis Contour Profile Mapping • Vertical profile shows changes in elevation along a line, such as a hiking trail, road or stream. Cartographic Terrain Mapping • Hill shading is also known as a shaded relief or simply shading • Attempts to simulate how the terrain looks with the interaction between sunlight and surface features. • Helps viewers recognize the shape of land-form features on a map. Hillshading • Four factors control the visual effect of hill- shading – Sun’s azimuth is direction of incoming light (0 to 360°) – The sun’s altitude from horizon (0-90°) – Surface slope (0-90°) – Surface aspect (0 to 360°) Hypsometric Tinting • Hypsometric tinting – Applies different color symbols to represent elevation or depth zones. Methods of Analysis • Slope measures the rate of change of elevation at a surface location • Aspect is the directional measure of the slope (degrees- 4 or 8 directions) • Hillshade, refer to previous slides • Line of sight refers to the straight line visibility from an observer to a feature • Viewshed analysis refers to the areas of the land surface that are visible from an observation point or points. • Watershed analysis refers to an area that drains water and other substances to a common outlet. • Area and volume calculations Connectivity Function Example: Viewshed Analysis Image Source: Chrisman, Nicholas.(2002). 2nd Ed. Exploring Geographic Information Systems. p 198. fig. 8-14 . Line of Sight Analysis Slope Aspect Hillshade Setting a hypothetical light source and calculating the illumination values for each cell in relation to neighboring cells. It can greatly enhance the visualization of a surface for analysis or graphical display. Azimuth 315°, altitude 45° Viewshed • Viewshed identifies the cells in an input raster that can be seen from one or more observation points or lines. • It is useful for finding the visibility. For instance, finding a well-exposed places for communication towers hillshaded DEM as background Surface Area and Volume Application: Environmental Impact Analysis 3D landscape model impact on natural beauty Application - Flood Risk 3D height data changing water levels-danger areas The 3rd Dimension: Height Analysis – combining several methods together • Contours • Hill shading • Spot height symbols • Cliff & slope symbols • Viewpoint symbols 3D Terrain Analysis: Summation • GIS does not always provide exact answers to problems, but by identifying trends based on geography, GIS can reveal patterns that can help us make informed decisions. • A GIS can improve decision-making; it cannot make decisions for us.
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