1. May 18, 2016 Spring 2016 Institute of Space Technology
Digital Elevation Model
&
Terrain Analysis
May 18, 2016
Spring 2016
Institute of Space Technology

2. Chapter 11 of your Text Book

3. 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)
Terrain: Water quality through control of sediment entrainment and transport

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

6. Terrain Analysis Contour Lines Slope Aspect Catchment/watershed area
Viewshed
Flow path and direction

7. Contour Lines
Contour lines or topographic contours are connected lines of equal elevation
These are at right angles to the local slope
Typically created at fixed height interval (e.g. every 30m or every 500m, …)
The shape and density of contour lines provide detail information on terrain height and shape in 2-dimensional map
Density indicates terrain steepness

8. 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)
Slope is equal to _________ degrees when rise is equal to run.
Two commonly used terrain variables.
Slope is equal to 45 degrees when rise is equal to run.

9. Slope (continued)
Slope expressed as percent, calculated by 100 times the rise over the run ranges from 0 (flat) to infinite (vertical). A slope of 100% occurs when rise = run.
Slope direction at a point in the landscape is measured in the steepest direction of the elevation change

10. Fig: Direction of steepest slope
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

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

12. 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

13. Four nearest cells

15. 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
Derived from DEM

16. Aspect calculation . Alpha = aspect in degrees
Also varies with methods used to calculate dz/dx and dz/dy. Four nearest cell and 3rd order finite difference methods yield among most accurate results.

17. 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
Local topographic indices – derived from gridded elevation data.

18. Profile curvature Plan curvature
index of surface shape in the steepest downhill direction
Plan curvature
profile shape in the direction of the contour at right angle to the steepest direction

19. Hydrologic Functions Watershed Drainage network
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
May be small or large
•Identified from a flow direction surface

20. Drainage network and watersheds

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

22. Flow direction
Arrows and numbers reflecting azimuth degrees.

23. Eight Direction Pour Point Model

24. Flow Direction Grid 2 4 8 1 16

25. Flow Direction Grid 2 4 8 1 16

26. Grid Network

27. Hydrologic Functions

28. 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

29. Shaded relief surfaces
Depicts brightness of terrain reflections given a terrain surface and sun location
Also referred to as hillshade surface