Statistical Surfaces, part II GEOG370 Instructor: Christine Erlien GIS Terrain Surface & Topographic Analyses Operations on terrain data terrain data frequently produced using spatial interpolation and/or stereoscopic interpretation of aerial photography Terrain data models usually represented using a DEM (digital elevation model) also sometimes as a TIN (triangulated irregular network) GIS Terrain Surface & Topographic Analyses Basic terrain surface properties, e.g.: Slope angle (gradient, steepness) Slope aspect (direction/orientation) Shape or form (convexity/concavity) Intervisibility Combine basic properties to achieve more complex analyses or create models Derived Properties: Slope Angle Slope angle: Change in elevation per unit horizontal change i.e., how steep is the slope?, what is its gradient? units generally are degrees or percent Steepness of Slope Slope 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: Raster From Demers (2005) Introduction to Geographic Information Calculating slope: Applications Erosion analyses Landslide vulnerability Directing land development Updating soil surveys Derived Properties: Slope Aspect Slope aspect: Orientation of the line of steepest slope i.e., what direction does the slope face units generally degrees from cardinal north Calculating aspect: Applications Relating aspect to others layers such as soils, vegetation Building wind generators Land use planning As an input to moisture index Derived properties: shape/form Visualize by producing cross-sectional profile of the surface Software 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 & neighbors From Demers (2005) Introduction to Geographic Information Characterizing surface shape Calculating shape: Applications Watershed analyses All areas that drain into a stream network Uses: Ecology Hydrology Engineering Pollution control Flood control Derived properties: visibility/intervisibility Viewshed analyses Determine what areas on a terrain surface can be seen from a given point Viewshed: The portion of the terrain you can see Visibility and Intervisibility From Demers (2005) Introduction to Geographic Information Performing viewshed analyses Vector (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 areas Raster method operates similarly but is more computationally intensive Calculating visibility/intervisibility: Applications Applications 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 towers Wrapping up Surfaces X, Y, Z values Representing phenomena (points, rasters, contours, TINs) Isometric vs. Isoplethic DEMs Interpolation Exact vs. inexact Global vs. local Linear vs. non-linear Wrapping up Non-linear interpolation methods Weighting Trend surfaces Kriging (nugget, sill, range) Terrain analyses Slope angle Slope aspect Shape/form Visibility/intervisibility