Digital Elevation Model &

Terrain Analysis

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)

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

Terrain Analysis

• Slope• Aspect• Catchment/watershed area• Viewshed• Flow path and direction

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 (continued)

Slope direction at a point in the landscape is measured in the steepest direction of the elevation change

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

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 ases• dz/dx = rise (change

in z) over run in x-direction

• dz/dx = rise (change in z) over run in y-direction

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

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

Aspect calculation

• .

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

• 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

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

Drainage network and watersheds

Watershed delineation

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

Flow direction

Hydrologic Functions

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

Shaded relief surfaces

• Depicts brightness of terrain reflections given a terrain surface and sun location