Statistical Surfaces, part IIGIS Terrain Surface & Topographic AnalysesSlide 3Derived Properties: Slope AngleSteepness of SlopeCalculating slope: RasterCalculating slope: ApplicationsDerived Properties: Slope AspectCalculating aspect: ApplicationsDerived properties: shape/formCharacterizing surface shapeCalculating shape: ApplicationsDerived properties: visibility/intervisibilityVisibility and IntervisibilityPerforming viewshed analysesCalculating visibility/intervisibility: ApplicationsWrapping upSlide 18Statistical Surfaces, part IIGEOG370Instructor: Christine ErlienGIS Terrain Surface & Topographic AnalysesOperations on terrain data–terrain data frequently produced using spatial interpolation and/or stereoscopic interpretation of aerial photographyTerrain data models–usually represented using a DEM (digital elevation model)–also sometimes as a TIN (triangulated irregular network)GIS Terrain Surface & Topographic AnalysesBasic terrain surface properties, e.g.:–Slope angle (gradient, steepness)–Slope aspect (direction/orientation)–Shape or form (convexity/concavity)–IntervisibilityCombine basic properties to achieve more complex analyses or create modelsDerived Properties: Slope AngleSlope angle: Change in elevation per unit horizontal change –i.e., how steep is the slope?, what is its gradient?–units generally are degrees or percentSteepness of SlopeSlope is a measure of the steepness of a surface and may be expressed in either degrees or percent of slope. In this example, the red cells show steep areas and the green cells show flat areas.Calculating slope: RasterFrom Demers (2005) Introduction to Geographic InformationCalculating slope: ApplicationsErosion analysesLandslide vulnerabilityDirecting land developmentUpdating soil surveysDerived Properties: Slope AspectSlope aspect: Orientation of the line of steepest slope–i.e., what direction does the slope face–units generally degrees from cardinal northCalculating aspect: ApplicationsRelating aspect to others layers such as soils, vegetationBuilding wind generatorsLand use planningAs an input to moisture indexDerived properties: shape/form Visualize by producing cross-sectional profile of the surfaceSoftware examines a line & generates a profile of how elevation changes over the distance of the line–Vector generates cross-sectional profile–Raster generates coverage showing relationship between target cell & neighborsFrom Demers (2005) Introduction to Geographic InformationCharacterizing surface shapeCalculating shape: ApplicationsWatershed analyses–All areas that drain into a stream network–Uses:•Ecology•Hydrology•Engineering•Pollution control•Flood controlDerived properties: visibility/intervisibilityViewshed analyses–Determine what areas on a terrain surface can be seen from a given point–Viewshed: The portion of the terrain you can seeVisibility and IntervisibilityFrom Demers (2005) Introduction to Geographic InformationPerforming viewshed analysesVector (using TIN)–Select a viewing location–Ray tracing: Follow a line from each target point to the viewing location, looking for higher elevations•All higher areas classified as non-visible•Coverage will show visible and non-visible areasRaster method operates similarly but is more computationally intensiveCalculating visibility/intervisibility: ApplicationsApplications–Forestry: Determine where logging can take place out of sight from roads & nearby populated areas–Locating towers for observing forest fires–Help site scenic overlooks or communication repeater towersWrapping upSurfaces –X, Y, Z values–Representing phenomena (points, rasters, contours, TINs)–Isometric vs. Isoplethic–DEMsInterpolation–Exact vs. inexact–Global vs. local–Linear vs. non-linearWrapping upNon-linear interpolation methods–Weighting–Trend surfaces–Kriging (nugget, sill, range)Terrain analyses–Slope angle–Slope
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