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