week 21geog2750 – earth observation and gis of the physical environment1 lecture 17 terrain...
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Week 21 GEOG2750 – Earth Observation and GIS of the Physical Environment
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Lecture 17Terrain modelling: applications
•Outline– introduction– access modelling – landscape evaluation
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Introduction
• Many applications of terrain models– visualisation covered already:
hillshading and orthographic viewsanimation and photorealism
– others:access modellingvisibility analysis and landscape evaluationslope and hazard mappinghydrological modelling
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Access modelling
• 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 factorBarrier features
– additional layer in GIS access models
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Distance 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
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Example distance model output
Buffer zones Distance surface
Anisotropic surface
Residuals
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Routing models
• Cost or “friction” surfaces can be used to calculate shortest path between two points– Euclidean model takes only distance into
account result is straight line or “as the crow flies”
– anisotropic model takes cost or friction surface into accountmay be positive (push) or negative (pull) uses “cost” of traversing a cell in a particular
direction to identify least accumulative cost route result is unlikely to be a straight line
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Example routing output
Actual route
Predicted route
Minimum distance/time surface
Crianlarich-Benmore circular walk
Check-points
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Case study: modelling remoteness
• 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
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Question
• What other cost factors might we include in a model of off-road accessibility?
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Remoteness model
• 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°
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Results
• Naismith's/Dijkstra's model used to model relative remoteness of Cairngorms area under different scenarios– with and without mountain-bike access along trails
– before and after proposed ski funicular
• ArcGIS alternative Costpath– calculates the least-accumulative-cost distance over
cost surface from source cell(s) accounting for surface distance and horizontal/vertical cost factors.
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“What if?” modelling of Mountain bike restrictions Mar Lodge estate
With mountain bike use along track from Linn of
Dee
Without mountain bike use along track from Linn
of Dee
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Effects of the Cairngorm Ski Funicular
Without parking restrictions at the Day Lodge or along access road
With parking restrictions at the Day Lodge and along
access road
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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?
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Calculating viewsheds
• Uses line of sight from observer point to terrain surface to calculate intervisibility matrix:– visible parts of terrain surface– non-visible areas (i.e. ‘dead’ areas)
• Use of observation point and terrain offsets– e.g. height of person or observation tower– e.g. height of wind turbine or other feature
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Calculating an inter-visibility matrix
Offset bOffset a
vv v nvnvnv
visible not visible
without offset b
with offset b
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Example viewsheds
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Uses of visibility analysis
• Many different uses…– visual impact analysis– landscape evaluation– siting of observation towers and cellular
communications masts– modelling coverage of cellular communications– military applications– virtual GIS
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Wind farm impact assessment
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Landscape evaluation of Scotland
Litton’s 1968 scenic assessment
50m DEM Intervisibility matrix
(After Miller)
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Landscape evaluation of Britain
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Visual impact of human features
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Cell phone coverage
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Military applications
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Virtual GIS
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Conclusions
• Many uses for DEMs in environmental applications of GIS– key variable determining accessibility– important landscape variable– controlling factor in “gravity” hazards
including flooding, avalanches, landslides, etc.
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Practical• Visibility assessment• Task: Calculate viewshed of a wind farm• Data: The following datasets are
provided…– Digital elevation model (50m resolution
1:50,000 OS Panorama data)– Wind farm turbine location(s) – ITE LCM90 data
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Practical
• Steps:
1. Display DEM and turbine locations in ArcMap or GRID
2. Calculate viewshed of wind turbines using both 1 and 16 turbines assuming a turbine height of 30m using visibility
3. Display results in ArcMap or GRID
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Learning outcomes
• Familiarity with the VISIBILITY command in Arc/Info
• Experience with developing impact assessments based on environmental variables
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Useful web links
• Access modelling– http://www.geogr.ku.dk/dkgs/image/pub_pdf/artikler/2
002/GT2002_05tb.pdf
• Archaeology and viewshed analysis– http://www.casa.arizona.edu/MPP/viewshed/vspaper.ht
ml
• Scenic highway designation– http://crssa.rutgers.edu/projects/highway/highway.html
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Next week…
• Hydrological modelling– Basics of hydrology– Creating hydrologically correct DEMs– Modelling catchment variables
• Practical:– Derive stream network from DEM