Digital Elevation Model & Terrain Analysis

1. Digital Elevation Model & Terrain Analysis

2. Terrain Analysis • Terrain - an integral part of • determining the natural availability and location of surface water, soil moisture and drainage • determining transportation networks • site suitability for a variety of applications (cost and methods of house and road construction)

3. Elevation and slope: • flood zones and watershed boundaries delineation • hydrologic networks

5. Terrain Analysis • Slope • Aspect • Catchment/watershed area • Viewshed • Flow path and direction

6. Slope and Aspect • Slope • change is elevation (a rise) with a change in horizontal position (a run) • Slope is often reported in degrees (0° is flat, 90° is vertical)

7. Slope (continued) Slope direction at a point in the landscape is measured in the steepest direction of the elevation change

8. Slope (continued) • slope direction often does not point parallel to raster rows and columns • which cells to use? • How to obtain the values for rise/run? • there are different methods • Examples: • Four nearest cells • 3rd Order Finite Difference Fig: Direction of steepest slope

9. Slope (continued) Where: s = slope Z = elevation x, y coordinate ases dz/dx = rise (change in z) over run in x-direction dz/dx = rise (change in z) over run in y-direction • Elevation expressed as Z • Calculated with a symmetrical moving window (3x3, 5x5, other odd numbered windows) • Slope at each center cell is calculated from:

10. Slope (continued) • Many different formulas proposed for calculating dz/dx and dz/dy • Most simplest – based on cells adjacent to the center cell • Four nearest = uses 4 nearest cells

12. Aspect • Azimuth angle, measured clockwise in degrees from north (0 – 360 degrees) • Aspect at a point is the steepest downhill direction • Flat areas have no aspect (no downhill direction) • Used to define: • water flow direction • Amount of sunlight at a location • Portion of landscape visible from viewing point

13. Aspect calculation • .

14. Profile curvature and plan curvature • Derived form DEM • Helpful in measuring and predicting • Soil water content • Overland flow • Rainfall-runoff response in small catchment • Distribution of vegetation

16. Profile curvature • Index of surface shape in the steepest downhill direction • Pane curvature • Profile shape in the direction of the contour – at right angle to the steepest direction

17. Hydrologic Functions • Watershed • An area that contributes flow to a point on the landscape • Water falling anywhere in the upstream area of a watershed will pass through that point • Drainage network • A set of cells through which surface water flows • Based on the flow direction surface

18. Drainage network and watersheds

19. Watershed delineation • Condition DEM • Fill DEM • Flow Direction • Flow Accumulation • Stream Definition • Outlet Identification • Watershed Delineation

20. Flow direction

21. Hydrologic Functions

22. Viewshed • Is the collection of areas visible from a specific point • Non flat areas block the view • Elevation will hide points if the elevations are higher than the line of sight between viewing point and target point

23. Shaded relief surfaces • Depicts brightness of terrain reflections given a terrain surface and sun location