Lecture 16 Terrain modelling: the basics

1. Lecture 16Terrain modelling:the basics Outline introduction DEMs and DTMs derived variables example applications GEOG2750 – Earth Observation and GIS of the Physical Environment

2. Adding the third dimension • In high relief areas variables such as altitude, aspect and slope strongly influence both human and physical environments • a 3D data model is therefore essential • use a Digital Terrain Model (DTM) • derive information on: • height (altitude), aspect and slope (gradient) • watersheds (catchments) • solar radiation and hill shading • cut and fill calculations • etc. GEOG2750 – Earth Observation and GIS of the Physical Environment

3. DEMs and DTMs • Some definitions… • DEM (Digital Elevation Model) • set of regularly or irregularly spaced height values • no other information • DTM (Digital Terrain Model) • set of regularly or irregularly spaced height values • but, with other information about terrain surface • ridge lines, spot heights, troughs, coast/shore lines, drainage lines, faults, peaks, pits, passes, etc. GEOG2750 – Earth Observation and GIS of the Physical Environment

4. UK DEM data sources • Ordnance Survey: • Landform Panorama • source scale: 1:50,000 • resolution: 50m • vertical accuracy: ±3m • Landform Profile • source scale: 1:10,000 • resolution: 10m • vertical accuracy: ±0.3m GEOG2750 – Earth Observation and GIS of the Physical Environment

5. Comparison Landform Panorama Landform Profile GEOG2750 – Earth Observation and GIS of the Physical Environment

6. LIDAR data (LIght Detection And Ranging) Horizontal resolution: 2m Vertical accuracy: ± 2cm GEOG2750 – Earth Observation and GIS of the Physical Environment

7. Modelling building and topological structures • Two main approaches: • Digital Elevation Models (DEMs) based on data sampled on a regular grid (lattice) • Triangular Irregular Networks (TINs) based on irregular sampled data and Delaunay triangulation GEOG2750 – Earth Observation and GIS of the Physical Environment

8. DEMs and TINs DEM with sample points TIN based on same sample points GEOG2750 – Earth Observation and GIS of the Physical Environment

9. Advantages/disadvantages • DEMs: • accept data direct from digital altitude matrices • must be resampled if irregular data used • may miss complex topographic features • may include redundant data in low relief areas • less complex and CPU intensive • TINs: • accept randomly sampled data without resampling • accept linear features such as contours and breaklines (ridges and troughs) • accept point features (spot heights and peaks) • vary density of sample points according to terrain complexity GEOG2750 – Earth Observation and GIS of the Physical Environment

10. Task • Make you own TIN from a piece of paper GEOG2750 – Earth Observation and GIS of the Physical Environment

11. Derived variables • Primary use of DTMs is calculation of three main terrain variables: • height • altitude above datum • aspect • direction area of terrain is facing • slope • gradient or angle of terrain GEOG2750 – Earth Observation and GIS of the Physical Environment

12. Question • What might slope and aspect maps be used for? GEOG2750 – Earth Observation and GIS of the Physical Environment

13. 10 9 8 8 8 7 7 6 5 • Slope = b2 + c2 Calculating slope • Inclination of the land surface measured in degrees or percent • 3 x 3 cell filter • find best fit tilted plane that minimises squared difference in height for each cell • determine slope of centre (target) cell z = a + bx + cy GEOG2750 – Earth Observation and GIS of the Physical Environment

14. 10 9 8 8 8 7 7 6 5 Aspect = tan-1 c / b Calculating aspect • Direction the land surface is facing measured in degrees or nominal classes (N, S, E, W, NE, SE, NW, SW, etc.) • use 3 x 3 filter and best fit tilted plane • determine aspect for target cell GEOG2750 – Earth Observation and GIS of the Physical Environment

15. Other derived variables • Many other variables describing terrain features/characteristics • hillshading • profile and plan curvature • feature extraction • etc. GEOG2750 – Earth Observation and GIS of the Physical Environment

16. Examples height aspect slope hillshading plan curvature Feature extraction GEOG2750 – Earth Observation and GIS of the Physical Environment

17. Question • What other important variables can be derived from DEMs? GEOG2750 – Earth Observation and GIS of the Physical Environment

18. Problems with DEMs • Issues worth considering when creating/using DTMs • quality of data used to generate DEM • interpolation technique • give rise to errors in surface such as: • sloping lakes and rivers flowing uphill • local minima • stepped appearance • etc. GEOG2750 – Earth Observation and GIS of the Physical Environment

19. Example applications • Visualisation • terrain and other 3D surfaces • Visibility analysis • intervisibility matrices and viewsheds • Hydrological modelling • catchment modelling and flow models • Engineering • cut & fill, profiles, etc. GEOG2750 – Earth Observation and GIS of the Physical Environment

20. Terrain visualisation • Analytical hillshading • Orthographic views • any azimuth, altitude, view distance/point • surface drapes (point, line and area data) • Animated ‘fly-through’ • What if? modelling • photorealism • photomontage • CAD GEOG2750 – Earth Observation and GIS of the Physical Environment

21. Examples of hillshading and orthographic projection Hillshading Orthographic projection DEM GEOG2750 – Earth Observation and GIS of the Physical Environment

22. Example surface drape Rainfall Draped image DEM GEOG2750 – Earth Observation and GIS of the Physical Environment

23. Example animated fly-through GEOG2750 – Earth Observation and GIS of the Physical Environment

24. Photorealism GEOG2750 – Earth Observation and GIS of the Physical Environment

25. Photo-realism “what if?” visualisation Visualisation 1: before felling Visualisation 3: strip felling Visualisation 2: clear-cut GEOG2750 – Earth Observation and GIS of the Physical Environment

26. Wind farm – photomontage before after wire-frame model GEOG2750 – Earth Observation and GIS of the Physical Environment

27. Conclusions • Need for third dimensional GIS • especially in environmental applications • new data models/structures • new opportunities for analysis • Basic uses and derived variables • Application areas • visualisation • visibility analysis • etc. GEOG2750 – Earth Observation and GIS of the Physical Environment

28. Practical • Using DEMs for hillslope geomorphology • Task: Derive key variables from DEM and relate to slope profiles • Data: The following datasets are provided for the Hohe Tauern Alps, Austria… • 25m resolution DEM • 10m interval contour data (derived from 25m resolution DEM) GEOG2750 – Earth Observation and GIS of the Physical Environment

29. Practical • Steps: • Display DEM in ArcMap or GRID • Derive slope and aspect variables using slope and aspect functions in GRID • Derive valley cross and long profiles using the identity tool in ArcMap • Plot altitude, slope and aspect against distance along profile in Excel • Relate to physical form GEOG2750 – Earth Observation and GIS of the Physical Environment

30. Learning outcomes • Familiarity with TIN/DEM construction in Arc/Info • Experience with deriving surface variables • Experience with displaying surfaces in Arcplot GEOG2750 – Earth Observation and GIS of the Physical Environment

31. Useful web links • View global DEMs • http://www.ngdc.noaa.gov/mgg/image/images.html#relief • DEM derived operations • http://www.powerdata.com.au/derive.htm GEOG2750 – Earth Observation and GIS of the Physical Environment

32. After reading week… • Terrain modelling: applications • Access modelling • Landscape evaluation • Hazard mapping • Practical: Visibility assessment GEOG2750 – Earth Observation and GIS of the Physical Environment