user centered time geography for location-based...

USER CENTERED TIME GEOGRAPHY FOR LOCATION-BASED SERVICES

Geografiska Annaler · 86 B (2004) · 4

183


byMartin Raubal, Harvey J. Miller and Scott Bridwell

Raubal, M., Miller, H.J. and Bridwell

, S., 2004: User CenteredTime Geography for Location-Based Services.

Geogr. Ann

., B(4): 183–203.

ABSTRACT. Location-based services assist people in their deci-sion-making during the performance of tasks in space. They donot consider the user’s individual preferences, time constraintsand possible subtasks to be performed. In order to account forthese important aspects, a user-centred spatio-temporal theory oflocation-based services is required. We propose such a theory bycombining classical time geography with an extended theory ofaffordances. It assumes that affordances belong to three realms:physical, social-institutional, and mental. In addition to coveringthe capability, coupling and authority constraints from time geo-graphy, this allows for a user-centred perspective because af-fordances describe action possibilities with regard to a specificperson. Furthermore, the integration of mental affordances offersthe possibility to account for cognitive time constraints due to theduration of decision-making processes. This new theory for loca-tion-based services is closer to the individual user and more plau-sible with respect to their daily lives. A business traveller scenariois used as a case study to demonstrate this.

Key words

: Xxxxxx, xxxx, xxxxxx, xxxxxx

Introduction

Imagine the following situation. A business travel-ler arrives in a new city at 6 a.m. She is scheduledto have a meeting at 8 a.m. in a local office build-ing. On the way to the meeting – preferably by pub-lic transport – she would like to have breakfast –preferably an espresso and a bagel – read a news-paper, and make a phone call. Through her PersonalDigital Assistant (PDA) the business traveller con-nects to a location-based service (LBS), which sug-gests a way to do all of these tasks considering hertime constraints and personal preferences:

1. Take bus no 3 and get off after seven stops. Makeyour phone call while riding on the bus becausethe connection is good along this route.

2. Walk five minutes to Café X, where they haveespresso, bagels, and various newspapers. Youhave 45 minutes.

3. Walk three minutes to subway station Y. Takethe green line and get off after three stops.

4. Walk two minutes to the office building. Youwill be there at 7.55 a.m.

These instructions are complemented by additionalinformation, such as wayfinding maps and thenames of the bus and subway stops involved.

Receiving such information from LBS is yet avision. Current LBS neither consider the user’s per-sonal preferences nor can they integrate interlinkedtime constraints and subtasks. In the above scenariothe business traveller could only ask separately forthe ways to the office building, a café and a news-paper store. What is missing is a

user-centred-spa-tio-temporal theory

, which allows location-basedservices to assist users individually during multipleactivities within a specific period of time.

This paper presents a general framework forsuch a theory, which combines the ideas of classical

time geography

(Hägerstrand, 1970) with an ex-tended theory of

affordances

(Gibson, 1979). Timegeography has tried to define the time-space me-chanics of different constraints, i.e., the capability,coupling, and authority constraint. It does not in-clude cognitive constraints – although see (Kwanand Hong, 1998) – and does not integrate very wellthe possibility of telepresence and the ability toproject one’s manifestation beyond one’s physicallocation – although see Hägerstrand, 1970; Adams,2000. The concept of affordance has its roots inecological psychology. Affordances describe pos-sibilities for actions with reference to a user. In aneffort to extend the original concept with elementsof cognition, situational aspects and social con-straints, it has been proposed that affordances be-long to different realms – physical, social-institu-tional and mental (Raubal, 2001).

The integration of time geography and extendedaffordance theory allows for representing a user-specific level including time constraints and possi-ble subtasks to be performed. The capability con-straint is expressed through physical affordancesfor an agent, depending on its capabilities. Physicaland social-institutional affordances for agents atvarious places represent coupling and authorityconstraints. They also permit us to remove actionpossibilities from particular locations. In addition,we consider cognitive constraints by integrating

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mental affordances into the theory. These should beincluded in a plausible spatio-temporal theory be-cause using mental affordances (e.g. people engag-ing in decision-making processes) takes time andtherefore leaves less time for other actions. Timeconstraints and tasks are modelled by a hierarchy ofspace-time prisms. The main task is represented bya fixed space-time prism, which cannot be changed– in the above example the business traveller mustbe at the meeting at 8 a.m. Subtasks are more adapt-able and can therefore be represented by flexiblespace-time prisms with variable time constraints.

A user-centred theory for LBS must take into ac-count that different people have different preferenc-es for their various activities. Representing thesepreferences through affordances allows one to focuson the activities themselves instead of conventionalcategories of places, which are assumed to allow forcertain activities. The integration of social-institu-tional affordances into the preference model alsosupports the proposed classification of time-geo-graphic communication possibilities (based on agiven classification, which is extended by socialconstraints). This new classification seems to bemore plausible with regard to everyday life.

Section 1 gives an overview of location-basedservices. In section 2 the relevant principles andconcepts from time geography are introduced.Section 3 describes the original theory of af-fordances and explains the ideas behind the ex-tended theory. In section 4 the general frameworkof combining time geography and affordances isdemonstrated. We use a functional approach forrepresenting the extended theory of affordances tomodel time-geographic constraints and communi-cation modes. Section 5 describes additional ele-ments needed for the new theory of LBS: cognitivetime constraints in decision-making processes,user preferences, and hierarchies of space-timeprisms representing combinations of tasks. Sec-tion 6 shows how this integration leads to a newtheory for location-based services, which is dem-onstrated by using the case study. The final sectiongives conclusions and presents directions for a fu-ture research agenda.

1. Location-based services

Mutual advances in wireless communications andgeo-spatial technologies have spurred interest in de-veloping information services that are sensitive tothe location of a mobile user. These so-called

loca-tion-based services

(LBS) allow users to query their

location from a mobile terminal, such as a phone orPDA, and relate it to the surrounding environment.This facilitates the successful completion of taskssuch as navigation (Winter

et al.,

2001). Tremen-dous benefits may be achieved from the widespreadadoption of these services, providing large seg-ments of the population real-time decision supportfor purposes ranging from trivial (concierge servic-es, location-sensitive games) to critical (emergencyresponse). LBS may also serve as a mechanism forcollecting disaggregate activity-travel data from us-ers, providing researchers and planners with moredetailed information regarding spatio-temporal pat-terns of interaction in urban environments (Miller,forthcoming, d). In the longer term, many expect thetechnology to impact upon our lives in unpredicta-ble ways, similar to the initial development of theInternet (Jensen

et al.,

2002).

LBS architectures

LBS are available in Japan and Europe, with morerudimentary services such as Vindigo (www.vindi-go.com) available in the USA. An importantemerging standard is the Open GIS Consortium(OGC) (http://www.opengis.org/) OpenLS initia-tive. It defines standards and interfaces to fosteropenness and interoperability in LBS developmentand deployment. These efforts seek to leverage ex-isting investments in geo-spatial data and process-ing resources with investments in communicationprotocols and infrastructures. This is conducted un-der the philosophy that the spatial processing anddata required to support LBS functions are alreadypresent, but fragmented among disconnected pro-prietary systems. Thus interoperable architecturesto support LBS may be achieved by defining thecore framework of services that can be linked to-gether to provide a functional LBS, and implement-ing a set of interfaces that wrap the functionality ofthe core services according to standard specifica-tions.

The core services defined by the OpenLS are: (1)directory services, (2) gateway services, (3) loca-tion utility services, (4) route services, and (5) pres-entation services (Bishr, 2002).

Directory services

provide users with online directories to assist infinding specific places, products or services, orranges of places defined by a distance threshold.

Gateway services

provide the interface to the loca-tion position server.

Location utility services

pro-vide geo-coding functions.

Route services

providea route between two given points, with options to



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include specific way-points within the route.Routes can be generated to minimize either dis-tance or time, and specified according to a particu-lar mode of travel. The results to route requests canoptionally include route geometry or textual de-scriptions.

Presentation services

provide the carto-graphic capabilities for the LBS; these may be tai-lored for different types of mobile devices.

The core services provide powerful functionali-ty, but are even more powerful when coupled to-gether. Consider the following request: ‘Give me amap with the route from here to the closest café.’This query would require each of the core servicesmentioned above. The service chain might beginwith a directory service to find the addresses ofcafés in that area. These addresses could then begeo-coded by a location utility service. The gate-way service would then query the location of theuser and provide this location, along with the cafélocations, to another directory service to determinewhich café is closest to the user. A route servicewould find the fastest route from the user to thecafé. All of this information would then be summa-rized into a map that is optimal with regard to theuser’s device.

In theory, each of the components to this querycould be completed by different entities, connectedthrough standard protocols. This is the goal of theOpenLS XML for Location Services (XLS) speci-fication. The XLS provides standard interfaces forrequests and responses to the core services dis-cussed above. These interfaces are implementedusing XML Schema, allowing for easy reuse of de-fined elements and attributes. Efforts have alsobeen developed to ‘harmonize’ the core serviceswith interfaces developed by other specifications.This means that any request or response created un-der either protocol will validate against the other.

LBS Limitations

Current LBS implementations in Japan and Europeas well as emerging architectures in the USA sup-port only basic locational queries such as location-sensitive maps, route finding and spatial searchingcapabilities (e.g. finding all cafés within 300 metersof my current location). These services provide sup-port for ‘first-order’ location queries, (i.e., ‘Whereshould I go from here and how do I get there?’). Thisrepresents only a limited scope of the broad spec-trum of services that could comprise LBS.

While interoperable architectures will likely pro-mote the widespread adoption of LBS, they fail to

account for some of the key properties of activitiesin space and time. First, the LBS inherit the GIS (Ge-ographic Information System) preoccupation withspace and fail to capture the temporal properties. Forinstance, the directory service considers proximityin space but not availability in time. One café may becloser than another, but it may not be open. The serv-ices discussed above would fail to consider this pos-sibility and could provide the user with misinforma-tion. Another limitation is lack of support for activityscheduling. More sophisticated LBS would support

n-order space-time activity queries

: the schedulingand execution of multiple, linked activities and sub-tasks over longer time frames (daily, weekly) and lo-cations rather than based only on a current location,independent of time.

The possibility of supporting sophisticated LBSqueries such as the situation presented at the startof this paper cannot be accomplished by chainingcore components together in an

ad-hoc

manner.Rather, services must be configured to reflect an ex-plicit theory about what is possible for an individ-ual in space and time. In addition, there should besome way to select from several possible activitylocations and schedules based on user preferences.This paper suggests that integrating time geogra-phy with an extended theory of affordances canprovide such theory.

Time geography

Time geography focuses on a necessary conditionat the core of human existence: ‘How does my lo-cation in space at a given time affect my ability tobe at other locations at other times?’ Since peopleand resources exist at a small number of locationsfor limited temporal durations, the ability to bepresent or telepresent at particular locations andtimes is required for almost every human activity.Conditioning these possibilities are transportationand communication services: they determine theability of a person to trade time for space (throughmovement or communication) in order to bepresent or telepresent at a particular location andtime (Hägerstrand 1970). This section reviews ma-jor time geographic concepts, particularly as theyrelate to LBS.

Space-time paths

The

space-time path

highlights the constraints im-posed by activities that are finite in space and timeas well as the need to trade time for space when



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moving among activities. Figure 1 illustrates aspace-time path representing a person’s movementand activity participation at three locations duringpart of a day.

Space-time stations

(depicted as tubes in Fig. 1)are locations containing resources required for ac-tivities such as eating, sleeping, work, shopping,obtaining medical services, and so on: in general,any activity that does not involve movement giventhe scale of analysis. If the path is vertical, the per-son is conducting a stationary activity. If the path isnot vertical, the person is moving between station-ary activities. A relatively shallow slope indicatesthat less time is required per unit space when mov-ing, (i.e. transportation services are more efficient).The path can never be horizontal: this would indi-cate a perfectly efficient transportation service(Lenntorp, 1976, 1978; Pred, 1981). Time geogra-phy traditionally considers movement at the geo-graphic scale, but the increasing spatio-temporalresolution allowed by positioning technologiescould push its domain to architectural scales suchas shopping in a city center or mall.

Note that the person depicted in Fig. 1 left station1 but arrived early at station 2 – presumably she hadto wait until it was available. Consequently, thisperson arrived late at station 3 and had to leavewhen it was no longer available. She subsequentlyreturned to station 1 earlier than necessary. A betterchoice would have been to conduct the activity atstation 3 first: although station 3 is relatively distantfrom station 1, station 2 was available later and for

a longer duration. However, if transportation serv-ices were more efficient, the shallower slopes of thespace-time path during movement episodes couldhave made the original activity schedule feasible.

Constraints and the space-time prism

There are three major classes of constraints thatlimit an individual’s ability to participate in activi-ties in space and time.

Capability constraints

limitactivity participation through their inherent abili-ties and available resources. Having to be at homefor at least six to eight hours per day for sleep is afundamental physical limitation. Owning a car is aresource that allows more efficient trading of timefor space in movement. Having broadband Internetconnections allows more efficient communication.

Coupling constraints

require a person to occupy acertain location for a fixed duration in order to con-duct some activity. Attending a meeting, dinnerwith your family, having a coffee and surfing theweb at an Internet café all reflect coupling con-straints.

Authority constraints

are fiat restrictionson activities in space and time; these may includeprivate property restrictions such as a shoppingmall being open from only 9 a.m. to 9 p.m.

Coupling constraints lead to another fundamen-tal distinction in time geography: the partitioningof activities into fixed and flexible activities.

Fixedactivities

are those that cannot be easily relocatedand rescheduled in space and time, at least in theshort run. Examples include many home activities(particularly when children are involved), work,and scheduled meetings with other people.

Flexibleactivities

are those that are relatively easy to relo-cate or reschedule. Examples include shopping anddining. Although the boundary between fixed andflexible activities can be indistinct (e.g. a film ver-sus a live performance), this is a powerful conceptthat allows the analyst to link accessibility to indi-vidual activity schedules.

An individual’s physical reach in space and timehas a geometric expression; namely, the

space-timeprism

(STP). The STP delimits the possible loca-tions for the path based on the ability to trade timefor space when moving and participating in flexibleactivities in the limited durations between fixed ac-tivities during a given time horizon (hourly, daily,weekly and so on). Figure 2 illustrates a STP for thecase where two fixed activities occur at different lo-cations (say, home and work) and frame a flexibleactivity (say, shopping). The STP can be construct-ed if we know the times when the fixed activities

Fig. 1. A space-time path and stations.



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must occur (t1 and t2), the minimum time requiredfor the flexible activity (A) and the average maxi-mum travel velocity in the area (v). An activity orperson is accessible only if its station or path inter-sects the STP to a sufficient degree (i.e. a minimumtemporal duration, determined by the type of activ-ity). The projection of the STP to geo-space definesa

potential path area

(PPA): this shows all locationsin space that are accessible to the individual. Ignor-ing their temporal durations, an activity or personis accessible only if its location intersects the PPA(Miller, forthcoming, a).

Figure 2 shows only one type of STP. We canalso construct STPs for cases where the secondfixed activity is unspecified, the two fixed activitiesoccur at the same location, and the minimum re-quired flexible activity time is unspecified. See(Burns, 1979) and (Lenntorp, 1976) for examplesand analytical calculations. It is also possible toconstruct these entities within multi-modal trans-portation networks, accounting for spatial and tem-poral variations in travel velocities. This allowsmore realistic space-time prisms that are more use-ful directly in applications such as LBS (see Miller,1991, 1999; Miller and Wu, 2000; O’Sullivan,

etal.,

2000; Wu and Miller, 2001).

The STP can serve as a theoretical foundationfor space-time queries. Queries supported by aspace-time prism include (Miller and Shaw, 2001):

– What locations can I reach in 15 minutes?(What is the volume of the STP at my locationand time?)

– How long can I stay at this café? What about an-other café? (What is the degree of overlap be-tween a space-time station and my STP?)

– Where and when can I meet my friends thisevening? (Where and when do our STPs inter-sect?)

STP queries can only capture capability and cou-pling constraints; authority constraints do not fac-tor into the STP directly. Authority constraints canbe incorporated indirectly by eliminating the STPlocations that intersect with a restricted region inspace and time.

Cognitive constraints have received less atten-tion in time geography since the framework explic-itly avoids questions concerning individual prefer-ence and choice behaviour. However, incompleteinformation and locational preferences can limit aperson’s accessibility as well as the usefulness of

Fig. 2. A space-time prism and po-tential path area.



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activity possibilities obtained from a STP (Hall,1983; Kwan and Hong, 1998). This can be solvedin an indirect manner similar to incorporating au-thority constraints into the STP. The set of preferredlocations may be derived through behavioural anal-ysis of locational and activity attributes using mul-tidimensional projection and grouping techniques(Kwan and Hong, 1998). Intelligent agent and ma-chine learning techniques could also analyze the at-tributes of location queries versus those actuallyvisited by an LBS client.

Presence and telepresence

Classical time geography recognizes the possibili-ty of telepresence or the ability to project one’smanifestation using electronic communication.However, telepresence is greatly downplayed rela-tive to physical presence. For example, couplingconstraints traditionally require physical proximityin space and time. This leads to the emergence of

space-time bundles

, or clustering of space-timepaths in order to conduct a shared activity (usuallyat stations). Although Hägerstrand and others rec-ognize the possibility of sharing activities withoutphysical bundling, this has only recently receivedexplicit attention by researchers. Time geography’sfocus on time as a resource enabling activity par-ticipation fits naturally to emerging perspectivesthat view time as the major scarce resource in in-formation economies and accelerated modern life-styles (Miller, forthcoming, d).

Janelle (1995) classifies communication modesfrom a time-geographic perspective. Table 1 sum-

marizes classes based on their spatial and temporalconstraints. Spatial constraints are either physicalpresence or telepresence, while temporal con-straints are either synchronous or asynchronous.

Synchronous presence

(SP) is the time-honouredcommunication mode of face-to-face (F2F) inter-action. F2F requires coincidence both in time andspace.

Synchronous telepresence

(ST) requiresonly coincidence in time: telephones, radio andTV allow individuals to communicate among dif-ferent places at the same time.

Asynchronous pres-ence

(AP) requires coincidence in space but nottime: examples include Post-It® notes and hospi-tal charts.

Asynchronous telepresence

(AT) doesnot require coincidence in space and time. Printedmedia, e-mail and webpages are popular examplesof AT.

Figure 3 characterizes the communicationmodes in Table 1 using the space-time path. Twopeople conduct a ST communication (say, a phonecall) early in the day and then conduct SP commu-nication at an agreed location (say, a café). Later,one person initiates an AP communication at an ap-propriate location (say, by leaving a note on an of-fice door). The other person receives the AP com-munication at that location later and then conductsan AT communication (say, by sending an e-mail).

Affordances

This section introduces Gibson’s theory of af-fordances and describes an extended theory, whichis more suitable for a spatio-temporal theory of lo-cation-based services.

Table 1. Spatial and temporal constraints on communications.

SpatialTemporal Physical presence Telepresence

Synchronous SP STFace to face (F2F) Telephone

Instant messagingTelevisionRadioTeleconferencing

Asynchronous AP ATRefrigerator notes MailHospital charts Email

Fax machinesPrinted mediaWebpages

Source: Based on Janelle (1995); Harvey and Macnab (2000).



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Gibson’s theory of affordances

The term

affordance

was coined by James J. Gib-son who investigated how people visually perceivetheir environment (Gibson, 1977, 1979). His theoryis based on

ecological psychology

, which suggeststhat knowing is a direct process and therefore theperceptual system extracts invariants embodyingthe ecologically significant properties of the per-ceiver’s world. An important point in Gibson’s the-ory is that animal and environment are an insepa-rable pair. This complementarity is implied by Gib-son’s use of

ecological physics

. Such physics con-siders functions of the environment at an ecologicalsize level in contrast to a description in terms ofspace, time, matter, and so on within classical phys-ics.

Affordances have to be described relative to theperson. For example, a chair’s affordance ‘to sit’ re-sults from a bundle of attributes, such as ‘flat andhard surface’ and ‘height’, many of which are rel-ative to the size of an individual. Later work withaffordances builds on this so-called

agent-environ-ment mutuality

(Gibson, 1979; Zaff, 1995). Ac-cording to Zaff (1995) affordances are measurableaspects of the environment, but to be measured onlyin relation to the individual. It is particularly im-portant to understand the

action-relevant

proper-ties of the environment in terms of values intrinsicto the agent. Warren (1995) shows that the‘climbability’ affordance of stairs is specified moreeffectively as a ratio of riser height to leg length.Experimentally, subjects of different heights per-ceived stairs as climbable depending on their ownleg length, as opposed to some extrinsically quan-tified value. In addition, dynamic or task-specificconditions must be considered (Warren, 1995).

Norman (1988) investigated affordances of eve-ryday items, such as doors, telephones, and radios,and argued that they provide strong clues to theiroperation. He recast affordances as the results fromthe mental interpretation of objects, based on peo-ple’s past knowledge and experiences, which areapplied to the perception of these objects. Gaver(1991) stated that a person’s culture, social setting,experience and intentions also determine her per-ception of affordances. Affordances therefore playa key role in an

experiential

view of space (Lakoff,1988; Kuhn, 1996), because they offer a user-cen-tered perspective. Similarly, Rasmussen andPejtersen (1995) pointed out that modelling thephysical aspects of the environment provides only

a part of the picture. ‘The framework must serve torepresent both the physical work environment andthe “situational” interpretation of this environmentby the actors involved, depending on their skillsand values’ (Rasmussen and Pejtersen 1995, p.122). This may be broken down into three relevantparts: the mental strategies and capabilities of theagents, the tasks involved, and the material proper-ties of the environment.

Extended theory of affordances

In this work we use an extended theory of af-fordances and integrate it with time geography inorder to develop a new theory for location-basedservices. It supplements Gibson’s theory of percep-tion with elements of cognition, situational aspectsand social constraints. This extended theory of af-fordances proposes that affordances belong to threedifferent realms: physical, social-institutional andmental (Raubal, 2001).

Physical affordances

require bundles of physi-cal substance properties that match the agent’s ca-pabilities and properties – and therefore its interac-

Fig. 3. Presence and telepresence in space-time paths.Source: Miller (forthcoming, c).



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tion possibilities. One can only place objects on sta-ble and horizontal surfaces, one can only drinkfrom objects that have a brim or orifice of an ap-propriate size and can be manipulated and so on.Common interaction possibilities are graspingthings of a certain size with one’s hands, walkingon different surfaces, and moving one’s eyes to per-ceive things. Physical affordances such as the ‘sit-tability’ affordance of a chair depend on body-scaled ratios, doorways afford going through if theagent fits through the opening, and monitors affordviewing depending on lighting conditions, surfaceproperties and the agent’s viewpoint.

Many times it is not sufficient to derive af-fordances from physical properties alone becausepeople act in environments and contexts with socialand institutional rules (Smith 1999). The use of per-ceived affordances, although physically possible,is often socially unacceptable or even illegal. Thephysical properties of an open entrance to a subwaystation afford for a person to move through. In thecontext of public transportation regulations it af-fords moving through only when the person has avalid ticket. The physical properties of a highwayafford for a person to drive a car as fast as possible.In the context of a specific traffic code it affordsdriving only as fast as allowed by the speed limit.Situations such as these include both physical con-straints and social forces. Furthermore, the wholerealm of social interaction between people is basedon social-institutional affordances: other people af-

ford talking to, asking, and behaving in a certainway. Many of these affordances are not tied to par-ticular locations (e.g. people can also talk to otherpeople over the phone).

Physical and social-institutional affordancesare the sources of

mental affordances

. During theperformance of a task a person finds herself in dif-ferent situation, where she perceives variousphysical and social-institutional affordances. Forexample, a public transportation terminal affordsfor a person to enter different buses and trains. Italso affords to buy tickets or to make a phone call.A path affords remembering and selecting, a de-cision point affords orienting and deciding and soon. In general, such situations offer to the personthe mental affordance of deciding which of theperceived affordances to use according to hergoal.

Combining time geography with affordances

This section describes how to represent elements oftime geography with affordances. By integratingtime geography and affordance theory we proposea conceptual framework, which serves as the basisfor a new theory of LBS. This new theory shouldfocus on the user and must explain what is possiblefor an individual in space and time. We first intro-duce a functional framework of representing theextended theory of affordances. In the following,the time-geographic constraints are modeled with

Fig. 4. Functional representation of affordances within activity process of an agent.



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affordances. Affordances are further used to repre-sent an enriched model of spatio-temporal commu-nication.

Representing affordances

The proposed formal framework of affordancesuses an adjusted version of the HIPE theory, offunction, which explains how functional know-ledge is represented and processed (Barsalou

et al.,

forthcoming). According to the HIPE theory func-tion representations integrate four types of concep-tual knowledge: history, intentional perspective,physical environment, and events. This theoryseems to be well suited for the formalization of af-fordances because of their functional character.Similar to functions, affordances are complex rela-tional constructs, which depend on the agent, itsgoal and personal history, and the setting. TheHIPE theory allows for representing what causesan affordance and therefore supports reasoningabout affordances. More specifically, it is possibleto specify which components are necessary andsufficient to produce a specific affordance for a spe-cific agent.

Figure 4 shows the abstract functional represen-tation of the relation between the three affordancecategories during the process of an agent perform-ing a task. The agent is represented through a phys-ical structure (

PS

), spatial and cognitive capabili-ties (

Cap

), and a goal (

G

). Physical affordances(

Paff

) for the agent result from invariant com-pounds (

Comp

) – unique combinations of physical,chemical and geometrical properties, which to-gether form a physical structure – and the physicalstructure of the agent. This essentially representsGibson’s concept of affordance: a specific combi-nation of (physical) properties of an environmenttaken with reference to an observer.

Social-institutional affordances (

SIaff

) are creat-ed through the imposition of social and institutionalconstraints on physical affordances (i.e. whenphysical affordances are perceived in a social-insti-tutional context

Cont (SI))

. While performing atask the agent perceives various physical and so-cial-institutional affordances in a spatio-temporalenvironment represented through

Env (S,T)

. Thisallows for localizing the perception of affordancesin space and time. Otherwise it would be impossi-ble to determine where and when the agent per-ceives a specific affordance.

Mental affordances (

Maff

) arise for the agentwhen perceiving a set of physical and social-insti-

tutional affordances in an environment at a specificlocation and time. Affordances offer possibilitiesfor action as well as possibilities for the agent tothink about them and decide whether to use them ornot (i.e. mental affordances). The agent needs toperform an internal operation

Op (Int)

to use a men-tal affordance. Internal operations are carried outon the agent’s beliefs and lead to an internal out-come

O (Int)

. In order to transfer such an outcometo the world, the agent has to perform an externaloperation

Op (Ext)

, which then leads to an externaloutcome

O (Ext)

(i.e. some change of the externalworld). This external change, in turn, leads to newphysical affordances, situated in social-institution-al and spatio-temporal contexts. The following sce-nario from a navigation task illustrates the func-tional framework.

Imagine a person finds herself in the negotiatingsituation while following a route. The person wantsto cross a river by car and has two options to do so,either driving over a bridge or using a ferry. Trans-lated to the formal model this means that a

drivingagent

perceives the physical affordances

drive over

from the compound

bridge

and

drive on to

from thecompound

ferry

. Notice the importance of integrat-ing spatio-temporal constraints: the bridge alwaysaffords driving over whereas the ferry affords theagent to drive on to it only when it is there (i.e. con-nected to the road network). The social-institution-al contexts of the applicable traffic code and the fer-ry business create social-institutional affordancesfor the agent. For example, while driving over abridge the agent is not allowed to exceed a certainspeed limit and driving on to a ferry is allowed onlyfor paying passengers. These social-institutionalaffordances are imposed on the physical affordanc-es. The mental affordance for the agent is then todecide which of the perceived affordances to useaccording to the goal of crossing the river. Theagent performs an internal operation, (e.g. a utilityfunction), which might result in the outcome thatthe agent wants to drive over the bridge because itis cheaper. The external operation is

driving overthe bridge

, which leads to the external outcome thatthe river has been crossed. At this point, new af-fordances may appear in the environment, such asa restaurant affording to eat.

Modelling constraints with affordances

The three classes of constraints in time geographylimit a person’s ability to participate in spatio-tem-poral activities. Positively formulated, they offer a



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specific set of possible actions for an individual.Affordances are such action possibilities; thereforethey may be used to represent the time-geographicconstraints.

Capability constraints

result from an individ-ual’s biological and physical structure, its variousabilities, and the environment’s resources. Thisrelates strongly to the roles of the physical agentstructure and its surrounding environmentalstructure represented in the functional frameworkof affordances. Capability constraints lead to spe-cific sets of

Paff

for a person. For example, a bedat home affords sleeping for an individual; a caraffords driving for particular individuals only,(i.e. when the physical structure of the car can beused by the person). Figure 5 gives an example ofthe corresponding functional activity process.The

Paff

A

‘car affords moving around for personA’ offers to the person to think about this action

possibility (

Maff

A

). The person then performs aninternal operation, deciding whether to drive thecar or not. The outcome of this operation could bethat the person wants to drive the car. Driving thecar is an external operation and after some timethe person could reach a certain location. Given afixed activity at this location, the final two stepsmay be ideally represented geometrically throughthe corresponding STP

A

. It is the spatio-temporalconsequence of using

Paff

A

with regard to personA’s physical reach. Figure 6 demonstrates theconsequences of two different

Paff

for a giventime interval (t1, t2) by showing the correspond-ing STPs. STP

A

results from the

Paff

A

‘car affordsmoving around for person A’ whereas STP

B

re-sults from the

Paff

B

‘public bus affords movingaround for handicapped person B’ (who cannotdrive a car).

Coupling constraints

fall into two categories,

Fig. 5. Functional activity process for person A.

Fig. 6. Space-time prisms result-ing from different affordances.



193

depending on whether other individuals are in-volved or not. For example, surfing the web at anInternet café does not require another person andtherefore the activity’s possibility may be repre-sented by a

Paff

only, (i.e. ‘computer at Internetcafé affords surfing the web for person A’). In caseswhere other individuals are involved, combina-tions of

Paff

and SIaff are required for representingthe coupling constraints because the Paff are em-bedded in a social-institutional context. Take, forexample, the situation of a person A making aphone call to a person B. Figure 7 illustrates partsof the functional activity processes, assuming thatperson B uses a mobile phone. The space-time pathof person A (Fig. 8) shows two space-time stationscontaining the resources of making a phone call(i.e. a telephone) and therefore offering the Paff‘telephone affords calling person B for person A’.On the other hand, the space-time path of person

B shows a continuous offering of the Paff ‘mobilephone affords calling and being called for personB’. In addition, we need to consider the SIaff be-cause although physically possible, person B maynot want to talk to person A, or the two individualsmay speak a different language, which makescommunication impossible. The geometric repre-sentation of Fig. 8 is based on the functionalframework, but in addition we can now identify thetwo time intervals where communication is possi-ble, (i.e. T1 and T2). It is important to note that cou-pling constraints also involve capability con-straints (in the sense of Paff) because the couplinghas to be both physically possible as well as so-cially.

Certain domains in everyday life are controlled,leading to authority constraints. In some cases,such as a private property restriction of a shoppingmall, these constraints can be represented by neg-

Fig. 7. Functional activity processes for persons A and B.

Fig. 8. Coupling constraints forperson A calling person B.



ative physical affordances, (e.g. ‘shopping mall’sentrance is locked and does not afford entering forperson A’). By representing authority constraintswith affordances it is also possible to model activ-ities that are physically possible but not alloweddue to social-institutional rules, (e.g. legal regula-tions). For instance, at certain parking spots alongbusy streets parking is allowed only during nighthours. Figure 9 shows the geometric representationof such a situation: Although parking is physicallyafforded for a car (driver) C between t1 and t4, thisPaff is restricted by the negative SIaff ‘parking spotdoes not afford parking for car (driver) C’ betweent2 and t3.

In general, space-time stations are representedby sets of affordances for specific locations andtime intervals. Such sets may be used to analyse theaction possibilities for an individual. Again, thereare consequences for the respective STPs. The STPfor a person following the speed limit with her caris smaller (STPB in Fig. 6) than the STP for a personexceeding the speed limit and therefore violatingthe SIaff (STPA in Fig. 6).

Modelling communication modes with affordancesSection 2 highlighted the importance of communi-cation-related time geography. The extended theo-ry of affordances allows for representing the differ-ent modes of spatio-temporal communication in a

plausible way. For user-centered time geographyhowever it is necessary to extend the given classi-fication (Janelle 1995) based on spatial and tempo-ral constraints by a third dimension, (i.e. socialconstraint). Figure 10 shows the possibilities forcommunication. The social constraints are therebyrepresented through SIaff. It is through these af-fordances that communication finally becomespossible or not. Spatial and temporal coupling isnot sufficient if one of the individuals cannot ordoes not want to communicate for social reasons(e.g. speaking a different language).

Communication between two persons throughsynchronous physical presence or synchronous tel-epresence is achieved when the relevant Paff andSIaff for both of them match. The first case requiresthat the Paff ‘place X affords being there for personA’ and ‘place X affords being there for person B’both exist for a point in time t. Furthermore, the re-spective SIaff need to correspond: ‘person A af-fords talking to person B’ and ‘person B affordstalking to person A’ (see also Fig. 8 for the case oftelepresence).

Communication based on asynchronous phys-ical presence is made possible through the crea-tion of a new Paff, such as ‘note on office door af-fords picking up for person A’. Creating a new af-fordance in our framework means adding it to theexisting set of affordances at a space-time station.Again, we need to consider the correspondingSIaff for person A to evaluate whether actual com-

Fig. 9. Space-time stations are rep-resented through sets of affordanc-es.


Geografiska Annaler · 86 B (2004) · 4 195

munication is possible. Illegibility of the notemight be an obstacle.

In the case of asynchronous telepresence a newPaff is created at different places (i.e. space-timestations). For example, by sending an email onecreates the same new Paff ‘e-mail affords receivingfor person A’ at different places with access to theInternet at the same time. In addition, these placesmust be accessible (afford being there) for personA at some later date.

Decision-making and user preferencesIn this section we describe the additional elementsneeded for a user-centered spatio-temporal theoryof location-based services. Individual decision-making processes take time, which can be takeninto account by considering mental affordances.The modelling of user preferences through af-fordances allows for representing an individual’spreferred activities. Finally, we demonstrate themodelling of tasks and subtasks through hierar-chies of STPs.

Decision-making processesSpatial reasoning involves a variety of decision-making methods and choice behavior. Decisiontheory covers a wide range of models with differentfoci on describing how decisions could or shouldbe made and on specifying decisions that are made(Golledge and Stimson, 1997). Mathematically, adecision rule is a function that assigns a value toeach alternative, showing what will happen when aparticular strategy is adopted. Decision-makingcriteria are a set of procedural rules that oversee theevaluation of the outcome when decision rules areapplied to a task situation. A strategy contains de-cision rules that seek a result from all possible waysof making a relevant decision.Classical decision-making theories may be classi-fied into the categories of riskless decision-making,risky decision-making, transitivity in decision-making, and game theory and statistical decisionfunctions. Golledge and Stimson (1997) argue thatin many cases human decision-making is not strict-ly optimizing in an economical and mathematicalsense – such as proposed by the algorithms of clas-sical decision-making theories – and therefore em-phasize behavioral decision theory. In this respectthey refer to Timmermans’ (1991) typology of de-cision-making according to spatial choice. It in-cludes models accounting for:

– variety-seeking behavior such as in recreationalchoice;

– uncomplicated choice among limited alterna-tives such as choice of travel mode;

– complex choice situations including preferenceand attitude;

– temporal choice involving stochastic models;– simulation of complicated choice outcomes.

The decision-making process of an LBS user, suchas the business traveller in our case study, typicallyinvolves uncomplicated choice among limited al-ternatives (e.g. going by car or taking the bus),complex choice situations involving a preference(e.g. having an espresso and bagel for breakfast),and temporal choice (e.g. be at the meeting at 8a.m.).

It takes time for an individual to make a deci-sion about what to do next – think of a tourist onher way through an unfamiliar city. These timeconstraints are essentially cognitive constraints,which differ from person to person. The availabil-ity of Paff and SIaff at space-time stations leads toMaff for an individual. The time used for utilizingthe mental affordances may be represented andhas an influence on how much time is left for otheractivities, (i.e. the longer people need to make de-cisions, the less time they have for doing otherthings). Figure 11 illustrates the situation: At timet1 a person faces a Maff whose utilization takes un-til t2. By then the person has decided on which of

Fig. 10. Communication possibilities from a user-centered time-geographic perspective.



the available Paff and SIaff to use, (i.e. the choiceact). One needs to distinguish between the choiceact – the outcome of a decision-making process –and a preference – an activity within the decision-making process expressing what is desirable. Notethat due to the time loss, the size of the originalspace-time prism at this decision point (STP1)shrinks in size (STP2), therefore leading to a re-duction of accessible places considering a futurefixed activity at t3.

Modelling of user preferencesRepresenting what is desirable for an individual isa major aspect for a user-centered time geography.One benefit lies in the support of spatio-temporalqueries for a particular person (Miller, forthcom-ing, d): A general query such as ‘Which locationscan I reach in 15 minutes?’ results in a different an-swer whether the person prefers to walk or go bypublic transport. User preferences are stronglylinked to capability, coupling and authority con-straints (and also cognitive constraints) becausewhat people desire is not always achievable. Theconstraints are generally intervening between pref-erence and choice (Golledge and Stimson 1997).

Our model of user preferences is strongly fo-

cused on activities (i.e. what people want to do incertain situations and places). This has the advan-tage of correlating with the action potential of theagent–environment duality represented through theextended theory of affordances and also of search-ing for places where an individual can engage in aparticular activity (Jordan et al., 1998). It allows forquestions such as ‘find me a place where I can eatpizza for lunch’ instead of asking for a restaurantwith ‘type = pizzeria’. The latter would result in alist of pizzerias whereas the answer to the firstquestion might also include a café with pizza on themenu.

User preferences are therefore desired activities,which have the effect of reducing the sets of af-fordances at various space-time stations to subsetsof these (see Fig. 12). Figure 13 gives an example:The capability, coupling, and authority constraintsfor a café and person A within a particular spatio-temporal context result in a set of four Paff (drinkcoffee, smoke cigarette, eat pizza, eat bagel) andtwo SIaff (talk to person B, talk to person C). All ofthese represent action possibilities for person A atthe given space-time station. By considering thegeneral (e.g. never talk to person C) and time-spe-cific (e.g. drink coffee only before noon) preferenc-es of person A this set is reduced to the subset of

Fig. 11. Cognitive time constraintdue to decision-making.



two Paff and one SIaff. In general, user preferencesneed to be specified for various domains and spatio-temporal contexts in order to be applied to differentsituations.

Tasks and subtasksPeople often combine different tasks and dividecomplex tasks into smaller subtasks. Such combi-nations and divisions of tasks can be representedthrough hierarchies of space-time prisms. Thesehierarchies form the basis for analyzing people’s

performance of various tasks in a spatio-temporalenvironment and also for finding optimal solu-tions to their efficient spatio-temporal combina-tion.

An optimal solution under time constraints isrequired when a person needs to participate in a fu-ture fixed activity and has time beforehand to en-gage in some flexible activity. The main task ofmoving to the place of the fixed activity is therebyrepresented by a fixed STP. The respective STPsconcerning the flexible activity are contained

Fig. 12. Preferences lead to a sub-set of Paff and SIaff.

Fig. 13. Example for reducing setof affordances through user pref-erences.

Fig. 14. Optimal solution for flex-ible activity before fixed activity.



within the fixed STP. Figure 14 illustrates thiscase: The fixed activity (e.g. participating in ameeting) starts at t6 at space-time station C (rep-resented through a set of Paff and SIaff). STP1 rep-resents the spatio-temporal possibilities for an in-dividual to move there. Engagement in the flexibleactivity (e.g. spending time at a café) must there-fore occur within STP1 but there are two places tochoose from. Notice that the original STP0 shrinksin size due to the time spent on using the Maff –which comprises the three sets of Paff and SIaff –(i.e. (t1-t0)). In the case of conducting the activityat space-time station A and then moving to space-time station C (dashed space-time path), the timeloss due to movement would be (t3-t1) + (t6-t4),leaving (t4-t3) for the activity. Engaging in the ac-tivity at space-time station B would minimize thetime loss and leave more time for the activity, (i.e.(t5-t2)). The latter is therefore optimal with regardto the preference of spending time on the activity.The respective space-time prisms (STP2 andSTP3) limit the amount of time which may bespent on the flexible activity, keeping in mind thefuture fixed activity.

Every subtask performed within a fixed STPleads to a new smaller fixed STP, which can againbe analyzed for action possibilities at space-timestations. The optimal solution for the combinationof various tasks and subtasks may be found bysearching through possible STP hierarchies.

Towards a user centered theory for location-based servicesThis section presents an overview of the conceptualframework developed above, and explains why itwill lead towards a more plausible and effectivetheory for LBS. The application to our case studydemonstrates how this theory should help in solv-ing complex problems.

OverviewCurrent location-based services do not support amajority of tasks, which people perform in theirdaily lives (see also section 1). In particular, theyare not tuned to the individual users, therefore ig-noring the users’ preferences. The conceptualframework of integrating time geography and af-fordance theory presented in the above sections isintended to serve as the basis for a user-centeredtheory of LBS, which can explain user-specificpossibilities in space and time. By taking into ac-count not only spatial but also temporal, social andcognitive aspects (Fig. 15), this theory allows forassisting people in tasks, such as activity schedul-ing based on individual preferences and time con-straints, which current services fail to support.

Current LBS focus primarily on the spatial as-pect, which is manifested through geo-coding func-tions, spatial search capabilities and route services.The latter calculate an optimal route and provide asequence of instructions for this route. These in-structions are always the same for the same route –they are not tailored to the user. For example, manypeople want route instructions to include landmarks(Raubal and Winter, 2002), while others prefer sur-vey information. Considering the user’s preferences(section 5) allows for making LBS more adaptive tothe individual in various contexts. Spatial activitiesoccur in time but the temporal aspects are mainlyneglected by existing implementations of LBS – es-timations of how long it takes to follow a calculatedroute being an exception to the rule. The proposedframework allows for a theory of LBS, which ex-plicitly captures the temporal properties. Af-fordances correspond to spatio-temporal sets, (i.e.they are available at certain locations and for giventime intervals (section 4)). The model considerstemporal availability of possible activities, whichmakes the scheduling of multiple tasks over longertime frames achievable (section 5). Social aspects,including institutional and legal characteristics, af-fect our daily lives and need to be integrated into a

Fig. 15. User-centered theory of LBS.



plausible user-centered theory for LBS. So far, onlya few social services, such as friends finder services,are supported. The new theory allows not only forintegrating institutional and legal facts, but also forcalculating their spatio-temporal consequences(section 4). This is a direct result of representing so-cial-institutional affordances in a time-geographiccontext. Furthermore, it becomes possible to modelthe various communication modes, including telep-resence, in a socially plausible way, i.e., by takinginto account an individual’s willingness or unwill-ingness to communicate with other individuals (sec-tion 4). A user-centered LBS theory must includecognitive aspects regarding its users. In combina-tion with user preferences, the new theory accountsfor individual decision-making processes throughthe integration of mental affordances and their spa-tio-temporal consequences, (i.e. spatial accessibili-ty and temporal availability (section 5)).

Application to the case studyLBS that are based on the new user-centered theorysupport sophisticated queries, including taskscheduling and time constraints. As an example, weshow its application to our case study, described atthe beginning of this paper. The case study coversmost of the concepts presented, such as time-geo-graphic constraints in the form of affordances, tel-epresence, user preferences and subtasks.

Figure 16 demonstrates the LBS reasoning proc-ess based on a user-centered spatio-temporal theo-ry, which allows for solving the business traveller’s task scheduling problem. The numbers refer tothe affordances in Table 2, which also shows the us-er’s fixed and flexible activities and preferences. Inorder to keep the figure simple, the specific instruc-tions for using public transport and wayfinding arenot considered. Answering the user’s query may bevisualized through geometric intersections of spa-tio-temporal sets. The following steps are applied:

1. mark fixed activities;2. create STPs for possible transportation modes;3. mark affordance sets at space-time stations;4. intersect STPs and affordance sets;5. intersect previous results with set of user pref-

erences;6. calculate space-time paths and analyze them;7. show result.

This approach is similar to Kwan and Hong’s(1998) GIS-based method of restrictive spatial

choice set formation, which results in the cognitivefeasible opportunity set (CFOS). There are impor-tant differences however: First, we consider all fea-sible activity locations for the individual, whereasa CFOS does not consider unknown locations as al-ternatives but only those present in the user’s cog-nitive map. Such an approach does not work for us-ers of LBS in unfamiliar environments, however.Second, a CFOS is based on locational preference,whereas here we focus on activities, which allowsfor a more comprehensive search (see section 5).

For the case study, the method works as follows.The business traveller has two fixed activities: ar-riving in the city at 6 a.m. (at train station TS) andbeing at a meeting, which starts at 8 a.m. (at officebuilding OB). She has two possibilities of going tothe meeting, either by car or by public transport(represented through Paffs 7 and 8). The LBS nowderives two different STPs depending on the modeof travel – STPcar and STPpub. Next, the spatio-tem-poral affordance sets for relevant space-time sta-tions are being marked (i.e. for locations A, B, C,D, and E). These sets are then intersected with theSTPs, reducing the number of relevant space-timestations to four (B is eliminated because it does notfit into the time constraints). The result of this in-tersection are sets of possible activities for the busi-ness traveller within her available time interval.These sets are then intersected with the set of theuser’s preferences (Table 2) with the result of elim-inating A because it falls completely into STPcar(which is eliminated because of the preference ‘goby public transport’). The resulting three sets C, Dand E are highlighted in dark grey. Based on thesesets, two possible space-time paths are calculatedby the service. The first path (dashed line) leadsfrom TS to newspaper store E, where the businesstraveller would arrive early. After waiting for 20minutes (the store opens at 7 a.m) she could buy anewspaper and go by public transport to café C. AtC the traveller would have 15 minutes to enjoybreakfast (espresso and bagel) and make a phonecall before heading off to the office building. Thesecond path (solid line) leads from TS directly tobistro D. During this journey the business travelleris telepresent (represented through {6}pub) andcould therefore make her phone call. At D shewould have 45 minutes for breakfast (again, espres-so and bagel) and reading a newspaper (variousnewspapers are available at this place). Based onthe user’s preference of ‘spending time for break-fast’, the LBS suggests the latter result to the busi-ness traveller for scheduling her desired spatio-



temporal activities. This example does not involvecognitive time constraints due to decision-makingbecause the user is presented with only one result.Cognitive time constraints need to be representedwhen more than one possible space-time path issuggested to the user, leaving the final decision toher (i.e. creating a mental affordance for her). SIaffsuch as for communication can be taken into ac-count only when the person to be called is also rep-resented by the LBS.

Conclusions and research agendaIn this paper we proposed a conceptual frameworkbased on the integration of time geography and af-fordance theory for a user-centered theory of LBS.This integration allows space-time mechanics andhuman interactions to be expressed as user-specificaction possibilities. A case study was presented todemonstrate the benefits of this approach in an-swering sophisticated spatio-temporal queries in-

Table 2. Activities, preferences, and affordances for the case study (depending on the level ofgranularity – which is being ignored here, but see research agenda – activities comprise one ormore actions; preferences are desired activities/actions; and affordances are possibilities for ac-tions).

User Affordances

Fixed activities Arrive in city 1 Drink espressoBe at meeting 2 Eat bagel

Flexible activities Go to meeting 3 Eat sandwichHave breakfast 4 Read newspaperRead newspaper 5 Buy newspaperMake phone call 6 Make phone call

Preferences Go by public transport 7 Go by carDrink espresso (for breakfast) 8 Go by public transportEat bagel (for breakfast)Spend time for breakfast

Fig. 16. LBS theory applied to thecase study.



volving time constraints, subtasks and user prefer-ences. However, implementing this framework as atheoretical foundation for LBS development willrequire coordination among several research direc-tions.

Enhanced knowledge regarding the decision-making process of individuals is one potentially vitalresearch direction. Particularly intriguing questionsinvolve the time constraints imposed by necessarydecision-making activities. Future research shouldattempt to gain insights into how these constraintsare constructed and how variations occur among dif-ferent people and cultures, in different geographicaland temporal contexts, and for different types of de-sired activities. This knowledge will facilitate morerealistic derivations of the space-time prism that willin turn provide users with more appropriate actionpossibilities. Understanding these decision-makingprocesses may also assist in developing methods forderiving personal preferences. In order for LBS torealize its true potential, it must account for userpreferences and past experiences. These advancesmay then allow scheduling or activity planning tooccur without an explicit user request. Work in thisarea may benefit from frameworks that store histo-ries of previous user requests using multidimension-al database designs (Smyth, 2001; Jensen et al.,2002) which are analyzed to determine trends intime and across space. An understanding of thesetrends may then be used in our spatio-temporal the-ory to weight action possibilities representedthrough a user’s mental affordance. It should be not-ed that methods involving the analysis of stored be-havior raise serious questions with regard to person-al privacy and surveillance. The usefulness of thesetechniques must also be measured in relation to theirpotential invasive practices (Smyth, 2001). Methodsfor doing so are open research questions.

Decision support for mobile users should alsoconsider the effects of geographic and temporalscales on an individual’s information requirements.The current framework incorporates the extendedtheory of affordances with an assumed uniformscale. However, it seems likely that the informationrequired when traversing street networks in an au-tomobile is very different from that when walkingin a shopping mall, as perception is largely a func-tion of situational context. These considerations areequally valid at different temporal scales. Usersmay require different assistance when schedulingactivities within a day as opposed to throughout theweek. These considerations affect how users willquery a LBS, what information should be provided,

and how it should be presented. Accommodationsfor this may be directly incorporated into our the-ory by applying body-scaled ratios (section 3) atdifferent scales to physical and mental affordances.The results from this work will no doubt have directimpacts on user interface designs and human–sys-tem interaction.

Attention must also be given to the implementa-tion specifics of measuring affordances in real timefor mobile users. The framework presented herefails to consider the difficulties inherent in measur-ing user locations, as well as measuring the space-time locations in the environment that afford actionpossibilities. Concrete implementations will needto consider the uncertainties occurring among both.With regard to user locations this will require anunderstanding of how user tracking affects the real-time calculation of space-time prisms. Work in thisarea is currently ongoing, and may benefit from ex-isting research on moving objects databases – seeSistla et al., 1998; Moreira et al., 1999; Pfoser andJensen, 1999; Leonhardi and Rothermel, 2002 –and measurement theories for time geography(Miller, forthcoming, b).

Work must also be conducted in the area of for-mal representations and implementations. Much ofthis work will require the representation of spatio-temporal sets and perhaps formulating a multi-agent system that would allow for using the frame-work’s asynchronous communication possibilities.These works could be augmented by a direct inter-face with the OpenLS specification to make theservices more dynamic and central to the userneeds. Representations of moving space-time sta-tions are also required to provide the communica-tion possibilities of users with mobile phones orwireless communications. The underpinning ofthis work will then be the synthesis of all these ar-eas of research into a formal theory of LBS.

At the present time, researchers have yet to con-sider LBS as anything more than a location-de-pendent wireless extension of the current GIS ap-plication framework. This is particularly apparentin the lack of attention given to the temporal prop-erties of activities and the unique situational con-texts of individual users. Current applications alsofail to consider activities with unique spatial de-pendencies, such as those provided through tele-communications and asynchronous interaction.The integration of time geography and affordancetheory addresses some of these issues, but futurework is required before a functional theory may re-alistically be implemented.



AcknowledgementsXxxxxx xxxxxx xxxxxx xxx xxxxxxxxxx xxxxxxxx xxxx xxxxxx xxxxxx xxxxx xxx xxxxxxxx

Martin Raubal,Institute for Geoinformatics,University of Münster,Robert-Koch-Str. 26–28,48149 Münster,Germany, http://ifgi.uni-muenster.de/E-mail: [email protected]

Harvey J. Miller,Department of Geography,University of Utah,260 S. Central Campus Dr. Rm. 270,Salt Lake City,UT 84112-9155,USA, http://www.geog.utah.edu/E-mail: [email protected]

Scott Bridwell,Department of Geography,University of Utah,260 S. Central Campus Dr. Rm. 270,Salt Lake City, UT 84112-9155,USA, http://www.geog.utah.edu/E-mail: [email protected]

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