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

Chapter 11 of your Text Book

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

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

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

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

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.

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

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

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.

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

Four 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 Derived from DEM

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.

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.

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

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

Drainage network and watersheds

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

Flow direction Arrows and numbers reflecting azimuth degrees.

Eight Direction Pour Point Model

Flow Direction Grid 2 4 8 1 16

Flow Direction Grid 2 4 8 1 16

Grid Network

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 Also referred to as hillshade surface