1. 3D and Surface/Terrain Analysis
Prepared by:
George McLeod
With support from:
NSF DUE
in partnership with:
Geospatial Technician Education Through Virginia’s Community Colleges (GTEVCC)

2. 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 

3. Data Sampling
Regular
Irregular

4. Sampling effects resolution

5. 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

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

7. 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.

8. Digital Elevation Model (DEM)
Gridded array of elevation points obtained from a variety sampling methods

9. 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

10. The function defined on each square can also be a bilinear function interpolating all four elevation points corresponding to the vertices of the square

11. 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

12. TINs
Example of a TIN based on irregularly distributed data

13. 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.

14. 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

15. 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

16. 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

17. 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

18. Contour Profile Mapping
Vertical profile shows changes in elevation along a line, such as a hiking trail, road or stream.

19. 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.

20. 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°)

21. Hypsometric Tinting Hypsometric tinting
Applies different color symbols to represent elevation or depth zones.

22. 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

23. 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 fig

24. Line of Sight Analysis

25. Slope

26. Aspect

27. 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°

28. 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

29. Surface Area and Volume

30. 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

31. 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

32. The 3rd Dimension: Height Analysis – combining several methods together
Contours
Hill shading
Spot height symbols
Cliff & slope symbols
Viewpoint symbols
Source: ©Crown copyright 2003

33. 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.