DEM, Visibility, and Intervisibility

DEM (Elevation, Slope, and Aspects)

Elevation data is available in both raster and vector format

Elevation information can be interpolated to calculate different forms of surfaces:

  • Steepness of slopes
  • Azimuth or orientation (aspects)
  • Visibility and intervisibility

The term digital elevation model (DEM) is a generic term used to describe a continuous digital elevation surface (raster data) – could be referred to as DSM or as DTM

  • A digital surface model (DSM) includes the elevation of all features in the terrain above sea level such as bare grounds, buildings, trees, powerlines, etc.
  • A digital terrain model (DTM) contains only elevations of the bare ground above sea level, without any buildings, trees, etc.

A DTM has the following features

  • A variety of representation forms
    • In digital form, various forms of representations can be easily produced, such as topographic maps, vertical, and cross sections, and 3-D animation
  • No accuracy loss of data over time
    • As time goes by, paper maps may become deformed, but the DTM can keep its precision owing to the use of digital medium
  • Greater feasibility of automation and real-time processing
    • In digital form, data integration and updating are more flexibly than in analog form
  • Easier multi-scale representation
    • DTM can be arranged in different resolutions, corresponding at different scales

Other alternatives to DTM have been used:

  • Digital elevation models (DEMs), America
  • Digital height models (DHMs), Germany
  • Digital ground models (DGMs), UK
  • Digital terrain elevation models (DTEMs), USGS

Morphological terrain parameters:

  • Slope is the first derivative of a surface and has both magnitude and direction (aspect)
    • Slope is a vector consisting of gradient and aspect
  • Aspect is defined as the direction of the biggest slope vector on the tangent plane projected onto the horizontal plane
    • Aspect is the bearing (or azimuth) of the slope direction, whose angles range from 0 to 360
    • In this context, the term slope is still used to refer to the gradient

Slope and aspect can be based on:

  • The algorithm used
  • The local topography
  • The resolution of the original data

Slope calculation is based on the relationship between horizontal distance and the vertical change in elevation (rise/run (%)) – basic Pythagoras theorem.

Method to calculate slope in raster uses a 3x3 moving window (kernel) to create a trend surface

  • Kernel passes over every pixel of the grid to calculate the maximum rate of change of value (slope) based on its neighbouring cells
  • Either use 4 closest cells or all 8 cells

Method to estimate slope compared the 4 adjacent cells to determine the rise values of the central cell (2 axes)

  • Keep the highest run/rise value
  • Calculate the slope in percentage -> Rise/Run * 100 (%)

Orientation (aspect) of a pixel is derived from the slope

  • Aspect identifies the down slope direction of the maximum rate of change in value from each cell to its neighbours

Estimation of the aspect by comparing the 4 rise values (4 axes)

  • Keep the highest rise/run value
  • Estimate the direction of the slope (north, east, south, or west)

Surface Analysis in GIS using slope and aspect:

Type of AnalysisSurface tool(s)Application
Creating ContoursContour, Contour list, Contour with barriersContours can be useful for finding areas of the same value. You could be interested in obtaining elevation values for specific locations and examining the overall gradation of the land
Terrain Relief and VisualizationHillshadeAnalyze how the landscape is illuminated at various times of the day by lowering and raising the sun angle. Provide an attractive and realistic backdrop
Volumetric AnalysisCut FillYou may be leveling a site for building construction and want to demine the volume of material that needs to be excavated and dumped

We can produce the hillshape by using the slope and aspect

  • Shows the intensity of lighting on a surface given a light source at a particular location
  • It can model which parts of a surface would be shadowed by other parts
  • Conventions:
    • Sun azimuth, and direction: 315* (NW)
    • Sun altitude in degrees: 45*
Visibility and Inter-visibility (View-shed)

The viewshed is the portion of the land surface that is visible from one or more viewpoints.

Viewshed analysis defines the regions that are visible from a particular point based on the terrain. A viewshed analysis requires 2 input data sets:

  • A point layer (viewpoints)
  • An elevation layer – can be in raster (DEM) or vector (TIN)

Line-of-sight (sight-line) operation: the basis for viewshed analysis

  • Connects the viewpoint to the target point
  • Interpolates the elevation of the intermediate cells
  • Determine if the target is visible or not based on elevations of intermediate cell
  • Expands this operation to every possible cell to determine which one are visible or invisible

Viewshed applications:

  • Site selection of facilities (e.g. wireless telephone based station)
  • Housing and resort area development (e.g. visual intrusion of new facilities)
  • Resource management (e.g. reducing the visual impact of clear-cuts)
    • Maximizing of billboards & other views
    • Reducing detection of troops (military)
    • Maximizing crowd surveillance (event security )
  • Advertisement, scenic path