1. Terrain modelling: the basics

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
Hillshade
VIewshed

3. Digital Elevation Models are:
Data files that contain the elevation of the terrain over a specified area, usually at a fixed grid interval
The intervals between each of the grid points will always be referenced to some geographical coordinate system. (e.g. latitude-longitude, UTM, SP).
The closer together the grid points are located, the more detailed the information will be in the file.
The details of the peaks and valleys in the terrain will be better modeled with a small grid spacing than when the grid intervals are very large.

4. Digital Elevation Models are:
Elevations other than at the specific grid point locations are not contained in the file.
As a result peak points and valley points not coincident with the grid will not be recorded in the file.
The file also does not contain civil information such as roads or buildings.
It is not a scanned image of the paper map (graphic).
It is not a bitmap.
The does not contain elevation contours, only the specific elevation values at specific grid point locations.

5. DEMs and DTMs Some definitions… DEM (digital elevation model)
Set of regularly or irregularly spaced height values
No other information
DTM (digital terrain model)
But, with other information about terrain surface
Ridge lines, spot heights, troughs, coast/shore lines, drainage lines, faults, peaks, pits, passes, etc.

6. Digital Elevation Model: What it looks like67 56 49 46 50 12 11 53 44 37 38 48 58 55 22 31 24 61 47 21 16 19 34
Lay a grid over some part of the world and find the average elevation in each cell

7. Cell Definition cell size (cell value) cell67 56 49 46 50 12 11 53 44 37 38 48 58 55 22 31 24 61 47 21 16 19 34
cell size
cell
(cell value)
The size of a cell is in either: meters, feet, degrees, or arc seconds

8. Data Sources Primary Data Sources:
Shuttle Radar Topography Mission (SRTM) or other airborne sensors
Secondary Sources from existing maps:
30ms from 1:24,000 scale map
1” (arc second ) National Elevation Dataset
3" (100m) s from 1:250,000 scale maps
30" of the earth (GTOPO30)

9. Data Source Generation: Space Shuttle

10. Shuttle Radar Topography Mission (SRTM)
1 arc-second elevation data for the United States, 3 arc-second data for the globe
Produced by radar measurements from a Shuttle mission, Feb 11-22, 2000
data.html

12. Santa Barbara, California

13. Interferometry used by SRTM
In interferometry, two images are taken from different vantage points of the same area.
The slight difference in the two images allows scientists to determine the height of the surface.

14. Secondary Data Source Generation: Using USGS Topographic Maps

15. 200 Meter Mesh (Universal Transverse Mercator Coordinates)
1km
1km

16. 100 Meter Mesh (UTM Coordinates)
1km
100m
1km

17. 30 Meter Mesh Standard for 1:24,000 Scale Maps

18. Lattice Points

19. Cell Stores Elevation at Lattice Point

20. Elevations
Contours
720
700
680
740

21. Comparison
Cell Size
30m
100m

22. Ordnance Survey Ordnance Survey: Landform Panorama Landform Profile
source scale: 1:50,000
resolution: 50m
vertical accuracy: ±3m
Landform Profile
source scale: 1:10,000
resolution: 10m
vertical accuracy: ±0.3m

23. Comparison
Landform Panorama
Landform Profile

24. LIDAR data (LIght Detection And Ranging)
Horizontal resolution: 2m
Vertical accuracy: ± 2cm

25. Modelling building and topological structures
Two main approaches:
Digital Elevation Models (s) based on data sampled on a regular grid (lattice)
Triangular Irregular Networks (TINs) based on irregular sampled data and Delaunay triangulation

26. TIN based on same sample points
DEMs and TINs
with sample points
TIN based on same sample points

27. 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

28. 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

29. 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
Slope = b2 + c2 10 8 9 7 6 5

30. 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
Aspect = tan-1 c / b 10 8 9 7 6 5

31. Other derived variables
Many other variables describing terrain features/characteristics
Hillshading
Profile and plan curvature
Feature extraction
Etc.

32. Examples height aspect slope hillshading plan curvature
Feature extraction

33. Problems with DEMs Issues worth considering when creating/using DTMs
Quality of data used to generate
Interpolation technique
Give rise to errors in surface such as:
Sloping lakes and rivers flowing uphill
Local minima
Stepped appearance
Etc.

34. Example applications Visualisation Visibility analysis
terrain and other 3D surfaces
Visibility analysis
intervisibility matrices and viewsheds
Hydrological modelling
catchment modelling and flow models
Engineering
cut & fill, profiles, etc.

35. Terrain visualisation
Analytical hillshading
Orthographic views
any azimuth, altitude, view distance/point
surface drapes (point, line and area data)
Animated ‘fly-through’

36. THANKS…