week 21geog2750 – earth observation and gis of the physical environment1 lecture 17 terrain...

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