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1 GIS Tools for Measuring Individual Accessibility in Real and Virtual Spaces Harvey J. Miller Department of Geography University of Utah Salt Lake City,

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Presentation on theme: "1 GIS Tools for Measuring Individual Accessibility in Real and Virtual Spaces Harvey J. Miller Department of Geography University of Utah Salt Lake City,"— Presentation transcript:

1 1 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

2 2 Introduction What is accessibility? –A multi-faceted concept –Individuals 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

3 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

4 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

5 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

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

7 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

8 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

9 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

10 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

11 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

12 12 Enabling geo-spatial technologies Location-aware technologies (LATs) –Global Positioning System –Radiolocation –Inertial navigation Location-based services (LBS) –Wireless Internets killer app –Information based on location in real time IBM Developerworks Library

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

14 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

15 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

16 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

17 17 Accessibility in network spaces Dynamic networks –Travel velocity varies by location & time Congestion Activity timing Other extensions –Multimodal networks OSullivan et al. (2000) IJGIS –Cognitive/preference constraints Kwan and Hong (1998) JGS Dynamic network PPT for SLC morning commute

18 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)

19 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

20 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

21 21

22 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

23 23

24 24 t

25 25 t

26 26

27 27 Future disc Disc intersection Past disc

28 28 Accessibility in multidimensional space DiscIntersection 1D Line segment 2D CircleLens-shaped region 3D SphereLens-shaped volume Simple geometric objects - easy to compute

29 29 Potential path ellipse (aka PPA)

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

31 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: Hellys theorem Prism-prism intersection – Worse case in 2D

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

33 33 Accessibility in virtual space Virtual interaction –Accessibility to information and people using ICTs –ICT modes Spatial constraints –Presence –Telepresence Temporal constraints –Synchronous –Asynchronous TemporalSpatial PresenceTelepresence SynchronousSP Face-to-face ST Telephone TV AsynchronousAP Post-it notes AT Mail Webpages Donald Janelle (1995)

34 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) A path and portals Spatial footprint of a portal

35 35 Accessibility in virtual space A path and portals

36 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

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

38 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

39 39 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

40 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

41 41 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

42 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, –2005. "Necessary space-time conditions for human interaction," Environment and Planning B: Planning and Design, 32,

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