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Department of Geography

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Presentation on theme: "Department of Geography"— Presentation transcript:

1 Department of Geography
GIS Tools for Measuring Individual Accessibility in Real and Virtual Spaces Harvey J. Miller Department of Geography University of Utah Salt Lake City, Utah USA eSI Visitor Seminar, National e-Science Centre, Edinburgh, Scotland - 06 September, 2007 1

2 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 2

3 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 3

4 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 4

5 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 5

6 Traditional accessibility measures
Place-based methods Distance Spatial or temporal separation Topological Network connectivity Attraction-accessibility Spatial interaction & spatial choice Benefits Consumer surplus 6

7 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 7

8 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 8

9 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 9

10 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 10

11 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 11

12 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 12

13 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! African-American women Asian- American women Space-time paths in Portland, Oregon Mei-po Kwan, Ohio State University 13

14 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 14

15 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 15

16 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 16

17 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 17

18 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) 18

19 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 19

20 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 20

21 21

22 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 Recall: Classic space-time prism 22

23 23

24 t 24

25 t 25

26 26

27 Past disc Disc intersection Future disc 27

28 Accessibility in multidimensional space
Disc Intersection 1D Line segment 2D Circle Lens-shaped region 3D Sphere Lens-shaped volume Simple geometric objects - easy to compute 28

29 “Potential path ellipse” (aka PPA)

30 Past disc Past disc & PPA PPA Future disc & PPA Future disc 30

31 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 Prism-prism intersection – Worse case in 2D 31

32 Accessibility in multidimensional space
Example Future and past discs based on network travel Phoenix, Arizona USA 32

33 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 SP Face-to-face ST Telephone TV Asynchronous AP Post-it notes AT Mail Webpages Donald Janelle (1995) 33

34 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) Spatial footprint of a portal A path and portals 34

35 Accessibility in virtual space
A path and portals 35

36 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 36

37 Accessibility in virtual space
Virtual interaction constraints Space Easy! - Presence or not Time More difficult Allen time predicates applied to message windows 37

38 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 38

39 Locational privacy Privacy protocols Spatio-temporal masking
Random perturbation mask Privacy protocols Notify Opt-in/out Security & authorization Build privacy into spatial representations Spatio-temporal masking Controlled noise into space-time trajectories Spatio-temporal weeding Scott Bridwell & Harvey Miller U of U Geography 39

40 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 40

41 Conclusion Future research Synoptic measures Imperfect measurement
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 41

42 Questions? Here and now? or asynchronous telepresence? Related papers (available at: 2007. “Place-based versus people-based geographic information science,” Geography Compass, 1, 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, 42

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