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Lecture 16 Terrain modelling: the basics. Outline introduction DEMs and DTMs derived variables example applications. Adding the third dimension. In high relief areas variables such as altitude, aspect and slope strongly influence both human and physical environments

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lecture 16 terrain modelling the basics

Lecture 16Terrain modelling:the basics

Outline

introduction

DEMs and DTMs

derived variables

example applications

GEOG2750 – Earth Observation and GIS of the Physical Environment

adding the third dimension
Adding the third dimension
  • 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
      • cut and fill calculations
      • etc.

GEOG2750 – Earth Observation and GIS of the Physical Environment

dems and dtms
DEMs and DTMs
  • Some definitions…
    • DEM (Digital Elevation Model)
      • set of regularly or irregularly spaced height values
      • no other information
    • DTM (Digital Terrain Model)
      • set of regularly or irregularly spaced height values
      • but, with other information about terrain surface
      • ridge lines, spot heights, troughs, coast/shore lines, drainage lines, faults, peaks, pits, passes, etc.

GEOG2750 – Earth Observation and GIS of the Physical Environment

uk dem data sources
UK DEM data sources
  • Ordnance Survey:
    • Landform Panorama
      • source scale: 1:50,000
      • resolution: 50m
      • vertical accuracy: ±3m
    • Landform Profile
      • source scale: 1:10,000
      • resolution: 10m
      • vertical accuracy: ±0.3m

GEOG2750 – Earth Observation and GIS of the Physical Environment

comparison
Comparison

Landform Panorama

Landform Profile

GEOG2750 – Earth Observation and GIS of the Physical Environment

lidar data light detection and ranging
LIDAR data (LIght Detection And Ranging)

Horizontal resolution: 2m

Vertical accuracy: ± 2cm

GEOG2750 – Earth Observation and GIS of the Physical Environment

modelling building and topological structures
Modelling building and topological structures
  • Two main approaches:
    • Digital Elevation Models (DEMs) based on data sampled on a regular grid (lattice)
    • Triangular Irregular Networks (TINs) based on irregular sampled data and Delaunay triangulation

GEOG2750 – Earth Observation and GIS of the Physical Environment

dems and tins
DEMs and TINs

DEM with sample points

TIN based on same sample points

GEOG2750 – Earth Observation and GIS of the Physical Environment

advantages disadvantages
Advantages/disadvantages
  • DEMs:
    • accept data direct from digital altitude matrices
    • must be resampled if irregular data used
    • may miss complex topographic features
    • may include redundant data in low relief areas
    • less complex and CPU intensive
  • TINs:
    • accept randomly sampled data without resampling
    • accept linear features such as contours and breaklines (ridges and troughs)
    • accept point features (spot heights and peaks)
    • vary density of sample points according to terrain complexity

GEOG2750 – Earth Observation and GIS of the Physical Environment

slide10
Task
  • Make you own TIN from a piece of paper

GEOG2750 – Earth Observation and GIS of the Physical Environment

derived variables
Derived variables
  • Primary use of DTMs is calculation of three main terrain variables:
    • height
      • altitude above datum
    • aspect
      • direction area of terrain is facing
    • slope
      • gradient or angle of terrain

GEOG2750 – Earth Observation and GIS of the Physical Environment

question
Question
  • What might slope and aspect maps be used for?

GEOG2750 – Earth Observation and GIS of the Physical Environment

calculating slope
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  • Slope = b2 + c2
Calculating slope
  • Inclination of the land surface measured in degrees or percent
    • 3 x 3 cell filter
    • find best fit tilted plane that minimises squared difference in height for each cell
    • determine slope of centre (target) cell

z = a + bx + cy

GEOG2750 – Earth Observation and GIS of the Physical Environment

calculating aspect
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Aspect = tan-1 c / b

Calculating aspect
  • Direction the land surface is facing measured in degrees or nominal classes (N, S, E, W, NE, SE, NW, SW, etc.)
    • use 3 x 3 filter and best fit tilted plane
    • determine aspect for target cell

GEOG2750 – Earth Observation and GIS of the Physical Environment

other derived variables
Other derived variables
  • Many other variables describing terrain features/characteristics
    • hillshading
    • profile and plan curvature
    • feature extraction
    • etc.

GEOG2750 – Earth Observation and GIS of the Physical Environment

examples
Examples

height

aspect

slope

hillshading

plan curvature

Feature extraction

GEOG2750 – Earth Observation and GIS of the Physical Environment

question1
Question
  • What other important variables can be derived from DEMs?

GEOG2750 – Earth Observation and GIS of the Physical Environment

problems with dems
Problems with DEMs
  • Issues worth considering when creating/using DTMs
    • quality of data used to generate DEM
    • interpolation technique
    • give rise to errors in surface such as:
      • sloping lakes and rivers flowing uphill
      • local minima
      • stepped appearance
      • etc.

GEOG2750 – Earth Observation and GIS of the Physical Environment

example applications
Example applications
  • Visualisation
    • terrain and other 3D surfaces
  • Visibility analysis
    • intervisibility matrices and viewsheds
  • Hydrological modelling
    • catchment modelling and flow models
  • Engineering
    • cut & fill, profiles, etc.

GEOG2750 – Earth Observation and GIS of the Physical Environment

terrain visualisation
Terrain visualisation
  • Analytical hillshading
  • Orthographic views
    • any azimuth, altitude, view distance/point
    • surface drapes (point, line and area data)
  • Animated ‘fly-through’
  • What if? modelling
    • photorealism
    • photomontage
    • CAD

GEOG2750 – Earth Observation and GIS of the Physical Environment

examples of hillshading and orthographic projection
Examples of hillshading and orthographic projection

Hillshading

Orthographic projection

DEM

GEOG2750 – Earth Observation and GIS of the Physical Environment

example surface drape
Example surface drape

Rainfall

Draped image

DEM

GEOG2750 – Earth Observation and GIS of the Physical Environment

example animated fly through
Example animated fly-through

GEOG2750 – Earth Observation and GIS of the Physical Environment

photorealism
Photorealism

GEOG2750 – Earth Observation and GIS of the Physical Environment

photo realism what if visualisation
Photo-realism “what if?” visualisation

Visualisation 1: before felling

Visualisation 3: strip felling

Visualisation 2: clear-cut

GEOG2750 – Earth Observation and GIS of the Physical Environment

wind farm photomontage
Wind farm – photomontage

before

after

wire-frame model

GEOG2750 – Earth Observation and GIS of the Physical Environment

conclusions
Conclusions
  • Need for third dimensional GIS
    • especially in environmental applications
    • new data models/structures
    • new opportunities for analysis
  • Basic uses and derived variables
  • Application areas
    • visualisation
    • visibility analysis
    • etc.

GEOG2750 – Earth Observation and GIS of the Physical Environment

practical
Practical
  • Using DEMs for hillslope geomorphology
  • Task: Derive key variables from DEM and relate to slope profiles
  • Data: The following datasets are provided for the Hohe Tauern Alps, Austria…
    • 25m resolution DEM
    • 10m interval contour data (derived from 25m resolution DEM)

GEOG2750 – Earth Observation and GIS of the Physical Environment

practical1
Practical
  • Steps:
  • Display DEM in ArcMap or GRID
  • Derive slope and aspect variables using slope and aspect functions in GRID
  • Derive valley cross and long profiles using the identity tool in ArcMap
  • Plot altitude, slope and aspect against distance along profile in Excel
  • Relate to physical form

GEOG2750 – Earth Observation and GIS of the Physical Environment

learning outcomes
Learning outcomes
  • Familiarity with TIN/DEM construction in Arc/Info
  • Experience with deriving surface variables
  • Experience with displaying surfaces in Arcplot

GEOG2750 – Earth Observation and GIS of the Physical Environment

useful web links
Useful web links
  • View global DEMs
    • http://www.ngdc.noaa.gov/mgg/image/images.html#relief
  • DEM derived operations
    • http://www.powerdata.com.au/derive.htm

GEOG2750 – Earth Observation and GIS of the Physical Environment

after reading week
After reading week…
  • Terrain modelling: applications
    • Access modelling
    • Landscape evaluation
    • Hazard mapping
  • Practical: Visibility assessment

GEOG2750 – Earth Observation and GIS of the Physical Environment

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