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Lecture 16 Terrain modelling: the basics
Outline introduction DEMs and DTMs derived variables example applications Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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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. Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
DEMs and DTMs Some definitions… DEM (Digital Elevation Model) set of regularly or irregularly spaced height values no other information DTM (Digital Terrain Model) but, with other information about terrain surface ridge lines, spot heights, troughs, coast/shore lines, drainage lines, faults, peaks, pits, passes, etc. Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Comparison Landform Panorama Landform Profile Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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LIDAR data (LIght Detection And Ranging)
Horizontal resolution: 2m Vertical accuracy: ± 2cm Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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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 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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TIN based on same sample points
DEMs and TINs DEM with sample points TIN based on same sample points Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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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 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Task Make you own TIN from a piece of paper Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Question What might slope and aspect maps be used for? Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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 Slope = b2 + c2 10 8 9 7 6 5 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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 Aspect = tan-1 c / b 10 8 9 7 6 5 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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Other derived variables
Many other variables describing terrain features/characteristics hillshading profile and plan curvature feature extraction etc. Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Examples height aspect slope hillshading plan curvature Feature extraction Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Question What other important variables can be derived from DEMs? Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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. Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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. Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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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 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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Examples of hillshading and orthographic projection
DEM Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Example surface drape Rainfall Draped image DEM Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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Example animated fly-through
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Photorealism Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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Photo-realism “what if?” visualisation
Visualisation 1: before felling Visualisation 2: clear-cut Visualisation 3: strip felling Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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Wind farm – photomontage
before after wire-frame model Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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. Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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) Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
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 Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Learning outcomes Familiarity with TIN/DEM construction in Arc/Info Experience with deriving surface variables Experience with displaying surfaces in Arcplot Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
Useful web links View global DEMs DEM derived operations Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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GEOG2750 – Earth Observation and GIS of the Physical Environment
After reading week… Terrain modelling: applications Access modelling Landscape evaluation Hazard mapping Practical: Visibility assessment Week 19 GEOG2750 – Earth Observation and GIS of the Physical Environment
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