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

LIDAR (LIght Detection And Ranging) The Data… DEM TIN LIDAR (LIght Detection And Ranging)

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 Needs larger storage capacity Computationally difficult DEM TIN Needs larger storage capacity Computationally difficult Flexibility of data sources Can add points Better display More efficient Needs smaller storage capacity Computationally simpler Fixed with a given cell size Cannot add sample points Raster display 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 predicting what sites can see each other uses: providers of cellular phones service can determine areas served by transmission facilities and then locate new sites to serve blind spots. fig 8-1: Cross sectional view of a line of sight as it intersects the surface. Near objects block objects that fall below the previous angle. Obstructions on the surface can be modeled by increasing the height of the surface. ------------------------- Intervisibility Functions This GIS function is typified by the phrase LINE OF SIGHT. It is a graphic depiction of the area that can be seen from the specified target areas. Areas visible from a scenic lookout, or the required overlap of microwave transmission towers can be mapped using this procedure. Intervisibility functions rely on digital elevation data to define the surrounding topography. Applications such as landscape layouts, military planning, and the obvious communication utilization are best serviced. The output of this function is somewhat unique in that it is often displayed in a SIDE VIEW format. The vertical field of view and maximum viable distance are the component parameters. 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 3-D environmental impact analysis By building a 3-D model of a landscape it is possible to simulate the construction of a new feature which may have an impact on the natural beauty of an area. For example, planning a wind farm. By using accurate map data for the area, a realistic model can be created and viewed from all angles. This will help identify the location that the new wind farm will have the least impact upon. 3D landscape model impact on natural beauty

Application - Flood Risk Using 3-D height data and map data for river features it is possible to build a computer model of changing water levels; this can be used for predicting flood patterns and identifying areas in danger. By combining this model with address data, the likelihood of individual properties being flooded can be assessed. This is not just of environmental concern but of great value to insurance companies. 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 Source: http://www.ordnancesurvey.co.uk/gisfiles/section2/page14.html ©Crown copyright 2003

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.