1 gis tools for measuring individual accessibility in real and virtual spaces harvey j. miller...
TRANSCRIPT
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GIS Tools for Measuring Individual Accessibility in Real
and Virtual Spaces
Harvey J. MillerDepartment of Geography
University of UtahSalt Lake City, Utah USA
eSI Visitor Seminar, National e-Science Centre, Edinburgh, Scotland - 06 September, 2007
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Introduction
• What is accessibility?– A multi-faceted concept– Individual’s ability to conduct activities
• Shopping, education, health care, employment, recreation, socializing
– Fundamentally spatial• Ability to be “present” at an activity location
– Physical presence– Tele-presence
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Introduction
• Why is accessibility important?– Accessibility is central to
• Cities – compress lives in space & time• Transportation – physical access• Communication – info access
– Accessibility in theory and application• Theory - Central to urban, transportation, social,
economic theories• Application - Performance & social measures
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Introduction
• Renaissance in accessibility measurement– Policy
• Community livability (USA)• Social exclusion (Europe)• Resources, opportunities, social networks, social
capital– GIS and geospatial technologies
• Detailed geographic data• Spatial analysis, visualization
• We need better accessibility tools– Not just better maps of old measures
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Outline of talk
• Traditional accessibility measures – Place-based perspective– High mobility & connectivity
• Individual accessibility: Theories and tools – Time geography– Enabling geo-spatial technologies
• GIS tools for measuring individual accessibility – Network spaces– Multidimensional spaces– Virtual spaces
• Locational privacy
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Traditional accessibility measures
• Place-based methods– Distance
• Spatial or temporal separation
– Topological• Network connectivity
– Attraction-accessibility
• Spatial interaction & spatial choice
– Benefits• Consumer surplus
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Traditional accessibility measures
• People and place have become complex – A shrinking but shriveling world - Waldo Tobler
• Transport costs have collapsed
• But, relative differences are increasing – An accelerated world – James Gleick
• Increasing mobility at all geographic scales• Activity organization is more complex
– A fragmenting world - Helen Couclelis
• Information and communication technologies (ICTs)
• Activities are disconnecting from place and time
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Individual accessibility: Theories and tools
• Time geography– Torsten Hägerstrand (1960s)– Spatio-temporal constraints on
human activity
• Types of constraints– Capability – physical needs,
resources
– Coupling – need to be coincident with others
– Authority – fiat restrictions
Lund, Sweden
November 2001
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Theories and tools
• Time geographic concepts– Types of activities
• Fixed – e.g., home, work
• Flexible – e.g., shopping, recreation
– Stations• Locations and durations of
activities
– Space-time path• Individual movement with
respect to time
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Theories and tools
• Space-time prism– Accessibility to environment
• Spatio-temporal region• Activities & resources
within the region
– Determined by• Space-time anchors
– Fixed activities
• Time budget• Min. required activity time• Max. travel velocity
t
ija
ijt
jt
jx ix
it
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max velocity
stationaryactivity time
time budget
anchors
ijv
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Theories and tools
• Classical time geography - limitations– Uniform travel velocity
• Simplifying assumption for tractability
– Low-resolution• Lack of rigor in basic definitions, constructs• Cannot exploit new geospatial technologies &
data
– Physically-based theory• Does not handle information & communication
technologies well
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Enabling geo-spatial technologies
• Location-aware technologies (LATs)– Global Positioning System – Radiolocation– Inertial navigation
• Location-based services (LBS)– Wireless Internet’s “killer
app”– Information based on
location in real time
IBM Developerworks Library
www-106.ibm.com/developerworks
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Enabling geo-spatial technologies
• Space-time ecology – Where and when do people
spend time?– Sensitive to social factors
• Age/life cycle stage • Socio-economic status• Gender roles & household
organization• Culture
– LATs allow unprecedented, detailed analysis!
Space-time paths in Portland, Oregon Mei-po Kwan, Ohio State University
African-American womenAsian- American women
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Enabling geo-spatial technologies
• GIS– Mobile objects databases
• Geosimulation– Agent-based modeling
• High-resolution space-time data– Empirical and/or synthetic– Rethink theory and
analysis of human behavior
EpiSims: Individual-level simulation of disease propagation based on contacts in space and time
episims.lanl.gov
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Individual accessibility in real and virtual spaces
• Individual in space and time– Activity schedules and locations– Transportation resources and ICTs
• Leverages geospatial science & technology– GIS, LATs, mobile objects, simulation
• Accessibility in three spaces– Network – relax constant velocity assumption– Multidimensional – rigorous measurement theory– Virtual – relax physical space assumption
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Accessibility in network spaces
• Transportation networks – Realistic paths and travel
times – Linked to individual,
network referenced activity schedules
• Network time prism– Potential path tree (PPT)– Potential network area
(PNA)
PPT
PNA
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Accessibility in network spaces
• Dynamic networks– Travel velocity varies by
location & time• Congestion• Activity timing
• Other extensions– Multimodal networks
• O’Sullivan et al. (2000) IJGIS
– Cognitive/preference constraints
• Kwan and Hong (1998) JGS
Dynamic network PPT for SLC morning commute
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Accessibility in multidimensional space
• Problems with time geography– No analytical statements of basic entities &
relationships• Cannot support high resolution measurements• Query and analytical tool development
– Specific to two spatial dimensions• Cannot link 1D (networks) and 2D• Cannot extend to 3D (natural space)
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Accessibility in multidimensional space
• Time geographic measurement theory– Paths, prisms etc under perfect information
• Finite but perfect instruments • Real world instruments are finite but imperfect
– Theory properties• Information assumptions are explicit• Multidimensional space and time• Supports
– Space-time query design– High-resolution measurement– Analysis of error & uncertainty propagation
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Accessibility in multidimensional space
• Space-time path– Two major
components• Control points -
measured• Segments -
unobserved
– Perfect info assumption
• Control points determine segments perfectly
Recall: Classic space-time path
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jij ts xxi 1
ij
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tt
tt
X
t
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Accessibility in multidimensional space
• Space-time prism– Temporally adjacent
control points– Maximum velocity:
Assumed or measured
• Temporally disaggregate prism– Prism at time t– Intersection of simple
objects in n - dimensional space
t
ija
ijt
jt
jx ix
it
x
max velocity
stationaryactivity time
time budget
anchors
ijv
Recall: Classic space-time prism
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jx
t
jt
ix
tijZ
)(tZij : A time “slice” of the prism
Case 1: No activity time
it
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jx
it
ix
ijiii vtttf xxx
“Future disc” of ci
t
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jx
jt
ix
ijjjj vtttp xxx
“Past disc” of cj
t
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jx
t
jt
ix
)()( tptftZ jiij
Prism at time t: Intersection of two discs
it
tijZ
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jx ix
Evaluating the prism using time boundaries
tt
jt
it
Future disc
Disc intersection
Past disc
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Accessibility in multidimensional space
Disc Intersection
1D Line segment Line segment
2D Circle Lens-shaped region
3D Sphere Lens-shaped volume
Simple geometric objects - easy to compute
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jx ix
Case 2: Stationary activity time
ijijijjiij vatg xxxxx
“Potential path ellipse” (aka PPA)
ijjiij gtptftZ x
tZij
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jx ix
it
0t
t
t
t
jt
Future disc & PPA
Future disc
PPA
Past disc & PPA
Past disc
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Accessibility in multidimensional space
• Intersections– Path-prism intersections
• Is a path or station within a prism at time t ?
– Point in disc and/or ellipse problem
– Prism-prism intersections• Do two prisms intersect at
time t ?– Intersection of discs and
or ellipses– n-disc case: Helly’s
theorem
rkc
qic
rlc
qjc
Prism-prism intersection – Worse case in 2D
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Accessibility in multidimensional space
• Example– Future and past
discs based on network travel
– Phoenix, Arizona USA
ij
Past disc(t’)
PPAij
Future disc(t’’)
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Accessibility in virtual space
• Virtual interaction– Accessibility to
information and people using ICTs
– ICT modes • Spatial constraints
– Presence– Telepresence
• Temporal constraints– Synchronous– Asynchronous
Temporal Spatial
Presence Telepresence
Synchronous SPFace-to-face
STTelephoneTV
Asynchronous APPost-it notes
ATMailEmailWebpages
Donald Janelle (1995)
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Accessibility in virtual space
• New time geographic objects– Portal
• A station that allows virtual interaction
– A point location– A service radius
• Examples:– Internet connection (point w/
zero radius)– WAP (point w/ positive radius)– Cell phone base station (point
w/ positive radius)
A path and portals
Spatial footprint of a portal
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Accessibility in virtual space
A path and portals
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Accessibility in virtual space
– Message windows• Communication events• Defined by a portal and
a time span• Send and receive
windows
– Two types• General: An actor
interacting with a portal• Strict: An actual
message
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Accessibility in virtual space
• Virtual interaction constraints– Space
• Easy! - Presence or not
– Time• More difficult• Allen time predicates applied to message windows
Predicate Definition
st BEFORE rt rk
sj tt
st BEFORE-1 rt rl
si tt
st MEETS rt rk
sj tt
st MEETS-1 rt rl
si tt
st EQUAL rt rl
sj
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st OVERLAP rt rl
sj
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st OVERLAP-1 rt rl
sj
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si tttt
st DURING rt rl
sj
rk
si tttt
st DURING-1 rt rl
sj
rk
si tttt
st STARTS rt rl
sj
rk
si tttt
st STARTS-1 rt rl
sj
rk
si tttt
st FINISHES rt rl
sj
rk
si tttt
st FINISHES-1 rt rl
sj
rk
si tttt
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Accessibility in virtual space
• Example: Who can receive a given message?– Synchronous
• “Actor must interact with a portal during the entire message”
– Asynchronous• “Actor must interact with a portal anytime after the
message is sent”• Side conditions (not shown):- There must be enough
time
Sent message
Possible receive window
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Locational privacy
• Privacy protocols– Notify– Opt-in/out– Security & authorization– Build privacy into spatial
representations
• Spatio-temporal masking– Controlled noise into space-
time trajectories
Random perturbation
mask
Spatio-temporal weeding
Scott Bridwell & Harvey
Miller
U of U Geography
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Conclusion
• Traditional accessibility measures– Still important, but incomplete– High mobility and ICTs
• Complex relationships between person, place & activities
• Individual accessibility measures– Activities in space and time
• Transportation networks• High-resolution measurement using LATs• Virtual interaction using ICTs
– GIS tool development for accessibility analysis• Space-time activity queries, toolkits
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Conclusion
• Future research– Synoptic measures
• Aggregate accessibility patterns • Make sense of large space-time activity datasets
– Imperfect measurement • Error propagation in time geographic queries
– Applications: Theory and models• Accessibility-related phenomena
– Travel demand, urban dynamics, social networks, social exclusion, epidemiology
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Questions?
• Here and now? – or asynchronous telepresence?
• Related papers (available at: www.geog.utah.edu/~hmiller)
– 2007. “Place-based versus people-based geographic information science,” Geography Compass, 1, 503-535.
– 2005. "A measurement theory for time geography," Geographical Analysis, 37, 17-45
– 2005. "Necessary space-time conditions for human interaction," Environment and Planning B: Planning and Design, 32, 381-401 .