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Digital elevation models. Day 2: digital elevation models. Principles Derived variables Visibility analysis Accessibility models Catchment modelling 3D visualisation. Principles.

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day 2 digital elevation models
Day 2: digital elevation models
  • Principles
  • Derived variables
  • Visibility analysis
  • Accessibility models
  • Catchment modelling
  • 3D visualisation
  • In high relief areas variables such as altitude, aspect and slope strongly influence both human and physical environments
      • a 3D data model is therefore essential
      • use a Digital Terrain Model (DTM)
      • derive information on:
        • height (altitude), aspect and slope (gradient)
        • watersheds (catchments)
        • solar radiation and hill shading
  • Many applications of terrain models
      • Visualisation:
        • hillshading and orthographic views
        • animation and photorealism
      • others:
        • access modelling
        • visibility analysis and landscape evaluation
        • hazard mapping
        • hydrological modelling
  • Six main data sources
    • ground survey using traditional methods of theodolite or EDM survey
    • maps via digitising contours
    • satellite data via stereoscopic techniques
    • aerial photographs via stereoscopic techniques
    • LIDAR (LIght Detection And Ranging) and SAR (Synthetic Aperture Radar)
    • GPS data

Comparison of DEM sources

Landform Panorama

Landform Profile

  • LiDAR data
  • More LiDAR data...
derived variables
Derived variables





plan curvature

Feature extraction

visibility analysis
Visibility analysis
  • Use of DTM to calculate “viewshed” of particular point
      • where can point X be seen from on surface Y?
      • what part of surface Y can be seen from point X?
  • Multiple point viewsheds combined to calculate viewshed of line and area features
      • where and part of feature X be seen on surface Y?
      • what part of surface Y can be seen from which point on feature X?

not visible


Visibility analysis

  • Uses line of sight from observer point to terrain surface to calculate intervisibility matrix: visible/non-visible parts
  • Use of observation point and terrain offsets

Offset a

Offset b







without offset b

with offset b

visibility analysis1
Visibility analysis
  • Many different uses…
      • visual impact analysis and landscape evaluation
      • siting and modelling of observation towers and cellular communications masts
      • military applications
      • virtual GIS
wind farm impact assessment
Wind farm impact assessment

Visibility analysis

accessibility models
Accessibility models
  • Terrain is a vital element for realistic access models
      • flat, boundless plains of Weberian industrial location analysis just don’t exist!
      • need to take terrain-based costs into account
        • Slope as push/pull factor
        • Barrier features
      • additional layer in GIS access models
accessibility models1
Accessibility models
  • Isotropic distance models
      • don’t take cost factors into account
      • e.g. eucdistance in GRID or buffer in Arc/Info
    • Anisotropic distance models
      • take cost factors into account
      • e.g. costdistance in GRID
accessibility models2
Accessibility models

Buffer zones

Distance surface

Anisotropic surface


accessibility models3
Accessibility models
  • Off-road accessibility is function of:
      • distance from nearest road
      • slope relative to direction of travel
      • ground conditions (trafficability)
      • barrier features (rivers, lakes, cliffs, etc.)
  • Combine within anisotropic access model as cost or friction surfaces
accessibility models4
Accessibility models
  • Combined model integrating:
      • Dijkstra’s Shortest Path Algorithm
        • calculate shortest path from origin to any destination based on relative costs of movement through set of cells between origin and destination
      • Naismith’s Rule (1892)
        • “an hour for every three miles on the map, with an additional hour for every 2,000 feet of ascent”
        • -10 minutes/300 m descent for slopes 5°>12°; +10 minutes/300 m descent for slopes >12°
accessibility models5
Accessibility models

With mountain bike use along track from Linn of Dee

Without mountain bike use along track from Linn of Dee

catchment modelling
Catchment modelling
  • The “Golden Rule” of hydrology.....
  • “water flows down hill”
    • under force of gravity
    • BUT, may move up through system via:
      • capillary action in soil
      • hydraulic pressure in groundwater aquifers
      • evapotranspiration
catchment modelling1
Catchment modelling
  • Surface shape determines water behaviour
      • characterise surface using DTM
        • slope
        • aspect
        • (altitude)
      • delineate drainage system:
        • catchment boundary (watershed)
        • sub-catchments
        • stream network
      • quantify catchment variables
        • soil moisture, etc.
        • flow times... catchment response
catchment modelling2
Catchment modelling

GEOG5060 - GIS and Environment

practical exercise
Practical exercise
  • Hands-on Exercise #4
  • Terrain analysis in ArcMap