1. Statistical Surfaces, part II GEOG370 Instructor: Christine Erlien

2. 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)

3. 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

4. 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

5. 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.

6. Calculating slope: Raster From Demers (2005) Introduction to Geographic Information

7. Calculating slope: Applications Erosion analyses Landslide vulnerability Directing land development Updating soil surveys

8. 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

9. Calculating aspect: Applications Relating aspect to others layers such as soils, vegetation Building wind generators Land use planning As an input to moisture index

10. 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

11. From Demers (2005) Introduction to Geographic Information Characterizing surface shape

12. Calculating shape: Applications Watershed analyses –All areas that drain into a stream network –Uses: Ecology Hydrology Engineering Pollution control Flood control

13. 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

14. Visibility and Intervisibility From Demers (2005) Introduction to Geographic Information

15. 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

16. 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

17. 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

18. Wrapping up Non-linear interpolation methods –Weighting –Trend surfaces –Kriging (nugget, sill, range) Terrain analyses –Slope angle –Slope aspect –Shape/form –Visibility/intervisibility