Looking through Hägerstrand’s dual vistas: towards a unifying framework for time geography

Journal of Transport Geography 23 (2012) 5–16

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Looking through Hägerstrand’s dual vistas: towards a unifying framework for time geography Daniel Sui Department of Geography, Center for Urban & Regional Analysis (CURA), 1036 Derby Hall, 154 North Oval Mall, The Ohio State University, Columbus, OH 43210-1361, USA

a r t i c l e

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Keywords: Time geography Hägerstrand Representations of space and time

a b s t r a c t The publication of Hägerstrand’s seminal paper on the conceptual framework of classic time geography in 1970 has inspired a steady stream of research in multiple fields related to transportation, planning, geography, ecology, environmental science, and public health. Just like most other scholarly work, Hägerstrand’s time geography has also received its fair share of criticism from scholars of different philosophical persuasions. In response to these criticisms, Torsten Hägerstrand himself made significant efforts to extend his original framework on time geography to move beyond paths, constraints, prisms, and projects during the last 30 years of his life. Yet these diverse later efforts by Hägerstrand are still scattered in the literature and have not exerted as much influence on time geography research as Hägerstrand’s (1970) original paper. The goal of this paper is to take stock of the major extensions Hägerstrand made for time geography between 1970 and 2004. By linking Hägerstrand’s shifting views on time geography to the ancient Greek conceptualization of space and time, a unifying framework based upon the Greek manifolds of space and time is proposed for time geography. The new framework not only restores the multiple dimensions of time geography as Hägerstrand envisioned later in his life, but also links the diverse ongoing time geography research efforts to themes derived from the framework. Future research challenges in time geography are also discussed. Ó 2012 Elsevier Ltd. All rights reserved.

‘‘Let me say only that publications [in time geography] so far, reflect only limited aspects of what I have in mind’’. Hägerstrand (1983a, p. 254)

1. Introduction Hägerstrand’s time geography has attracted followers in multiple disciplines in the social sciences, natural sciences, and engineering across North America, Europe, and Asia throughout the past 40 years (Chai et al., 2002; Thevenin, 2011; Persson and Ellegård, 2012). Interesting advances and applications along multiple fronts have been reported in the literature since Hägerstrand’s (1970) seminal paper was first published (Kellegrew and Kroksmark, 2000; Takahashi et al., 2001; Raubal et al., 2004; Yu and Shaw, 2005; Sunnqvist et al., 2007; Liversage, 2009; Winter and Yin, 2010, 2011). Recent interests in time geography reached another peak in the past 5 years due to the increasing availability of spatial and temporal data through user generated content, various location-aware technologies, and the growing convergence of GIS with social media (Svee et al., 2009; Licoppe and Inada, E-mail address: [email protected] 0966-6923/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jtrangeo.2012.03.020

2010; Sui and Goodchild, 2011). Yet it is worth noting that despite Hägerstrand’s many publications aiming to extend his classic version of time geography beyond paths, constraints, prisms, and projects during the last 30 years of his life, both in English and Swedish, his followers as well as his critics seem to be disproportionally inspired by Hägerstrand’s (1970) paper on ‘‘what about people in regional science?’’ This is partially caused by the fact that Hägerstrand himself never systematically synthesized and presented an updated version of time geography before his untimely death in 2004, although he offered various fragments on what he conceived of as an extended version of time geography. This paper aims to undertake the challenge of piecing together fragments of Hägerstrand’s ideas for a more complete version of time geography. In his last paper officially published posthumously, Hägerstrand (2004) observed that ‘‘we humans have two vistas towards which we can turn our eyes. One consists of the heavens and the other the neighboring landscape all the way to the horizon. In the sky above, the sun, the moon and the planets regularly pass by against a backcloth of seemingly fixed stars. The view towards the heavens exhibits stability and regularity. By contrast, the most characteristic trait of the view of the earth is the immense wealth of forms and complexities through which matter spontaneously manifests itself. The paraphernalia of human culture displays similar variation. The

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surrounding landscape meets us as a remarkable mixture of natural and manmade things, unpredictable life forms, conflicts and compromises. That part of the world enclosed within the skin of the observer also belongs to the landscape. It can partly be reached through a perceptive eye, but it remains more concealed than conspicuous. Difficulties comprehending that nearby us seem to stand in contrast to the clarity of what is distant (p. 315)’’. Hägerstrand’s (2004) dual vistas on human condition – the celestial (towards the heavens in abstract space and time) and the terrestrial (towards the earth in concrete places/landscape and embedded time) – can serve as a starting point to better understand his holistic vision of time geography and to underscore that time geography extends far beyond the commonly used concepts of accessibility, paths, and space–time prisms. The primary goal of this paper is to look through Hägerstrand’s dual vistas to develop a unifying framework for time geography to better capture and understand the multiple dimensions of Hägerstrand’s vision for time geography. To achieve this goal, I will first take stock of the major conceptual extensions Hägerstrand himself made to time geography, which built upon his seminal 1970 paper wherein he formally outlined the initial framework for time geography. Remarkable similarities can be found between Hägerstrand’s changing views on time geography and the ancient Greeks’ view of space and time. Secondly, the manifolds of space and time as conceived by ancient Greeks are introduced, followed by a description of a unifying conceptual framework for understanding the multiple dimensions of time geography. Thirdly, this paper tries to link the diverse threads of time geography research reported in the literature so far using this framework. In addition to restoring the multiple dimensions of time geography as Hägerstrand envisioned later in his life, I will also attempt to show that the framework is capable of connecting time geography research to current theoretical and methodological frontiers in multiple disciplines and illuminating possible new research challenges in the years ahead. This paper is organized into five sections. After a brief introduction, major conceptual shifts Hägerstrand made in the later part of his life are summarized in Section 2. Section 3 introduces ancient Greeks’ dual conceptualizations of space and time and discusses how they are related to Hägerstrand’s views on space and time. Section 4 discusses how time geography can be reconceptualized and what areas need further exploration. The last section contains a summary and conclusion.

2. Trajectories of Hägerstrand’s time geography, 1970–2004 Part of the conceptual motivation for time geography was Hägerstrand’s growing dissatisfaction of studying geography as areal differentiation with time conceptualized as ‘slices’ (Gren, 2009). Hägerstrand insisted that human geography should be contextualized with time. Stemming from his earlier work on diffusion and his observations concerning the highly contextualized nature of innovation and the diffusion process, Hägerstrand’s framework on time geography was heavily influenced by concepts in human ecology. Hägerstrand (1970) initially presented the classic version of time geography to regional scientists as an approach to move beyond aggregated data and focus on individuals’ movements in space and time. According to Hägerstrand (1970), how individuals, groups, and institutions navigate through time–space and everyday life is defined by the availability of these two interrelated resources. Time and space serve as a shared social arena/stage on which people have the opportunity to interact with other individuals, agencies, and institutions (Hägerstrand, 1978, 1984a). In particular, Hägerstrand (1984a, 1984b) emphasized that time–space interactions are inherently limited and constrained and highly

dependent on the daily geographies of people. By the late 1970s, time geography became an established conceptual framework for understanding the interconnectedness of everyday life and the many (often taken-for-granted) events that shape and contextualize geographies on the ground. Time geography is widely considered a major breakthrough in the history of geographic thought because it establishes human geography as a vibrant field for investigating localized structures and individual agency. Path, project, pocket, prism bundle, constraints, and station are core concepts in time geography that have been used to weave a web of interconnected and shared social experiences. Hägerstrand and his followers believe that it is the mapping of this web and the networks of individuals’ social relations that can be used to elucidate the key sociospatial bundles and bound interactions in space–time, which produce the material and symbolic landscapes (Wong and Shaw, 2011). Like all other conceptual breakthroughs in geography, Hägerstrand’s time geography had been critically scrutinized by scholars of different philosophical persuasions (Gregory, 1994; Gren, 2001). As Hägerstrand (1989) himself admitted, time geography has received its fair share of criticism, which can be grouped into three categories. First, humanists argue that time geography is inherently reductionist and carries the baggage of physicalism. Hägerstrand himself struggled to balance comprehensive models with the need to simplify. In time geography, humans are reduced to actors following certain traveling paths to and from stations (or locations) determined primarily by capability, coupling, and authority constraints. As such, critics have alleged that time geography has been reduced to little more than a graphic exercise. Second, as a research method, time geography is data intensive and requires massive resources to effectively establish the time geography of an individual, place, or local culture. Because of the intensity of the field work needed to map individual paths, the scale of the analysis tends to be highly localized. As a result, time geography has been regarded as overly ambitious and problematic beyond the local scale. Third, time geography became embedded within a larger structure-agency debate that consumed the discipline from the late 1970s through the mid-1980s (Gregory, 2000; Gatrell, 2006). Time geography has been criticized by humanists and structuralists alike as either too dependent on structure or too dependent on agency. Feminist scholars have criticized time geography for being overly masculinist in its representation of social reality (Dyck, 1990; Rose, 1993). Hägerstrand was one of the few intellectual giants capable of finding wisdom and inspiration in the least likely place: the minds of his vocal critics. Hägerstrand also happened to be one of few geographers who not only worked with his disciples, but also collaborated closely with those who did not share his views about the world and geography. After studying the critiques about his work in time geography, Hägerstrand (1983a) admitted that he was ‘‘trying to walk on only one leg (p. 254)’’. During the last 30 years of his life, he worked hard to develop multiple legs to expand time geography. Hägerstrand’s extension efforts for time geography can be summarized along three directions – seeking alternative and more inclusive conceptualizations of agency, space, and time. 2.1. Agency: from people to society–technology–environment Since Hägerstrand (1970) unveiled his vision, time geography has been gradually accepted as a common approach in human geography that treats time and space as resources to study the constitution of social life from an integrated spatial–temporal perspective. Time geography has been largely cited in the literature as a framework to study human activities in space and time. This is not surprising since Hägerstrand’s earlier work focused primarily on the movement and migration of people, as well as the spatial

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diffusion of technological innovations. Although matter and environment were mentioned in his 1970 paper, the dominant agent in the classic version of time geography is people, with particular emphasis on space–time conditions for human activities and human interactions (Miller, 2004, 2005b). In his later writings, starting in the mid-1970s, Hägerstrand made consistent efforts to emphasize the fact that people are embedded as part of, not apart from, nature (Hägerstrand, 1973, 1976). Hägerstrand’s first systematic discussion on time geography in Swedish made it quite clear that time geography should focus on the triad of society-technology-environment (Gren, 2009). Hägerstrand (1995) argued that ‘‘. . . it is an illusion to believe that human society can isolate itself from nature. The best we can hope for is a reasonably safe co-evolution (p. 35)’’. He also recognized how our meaningful actions and intentions are fraught with unintended consequences, often leading to meaningless changes in the world. Hägerstrand (1993) warned of the devastating consequences of our actions if we do not put our acts together to change the world in a positive way. Hägerstrand’s shifting view from people to nature for time geography was succinctly summarized in his book chapter ‘‘How about nature in regional science’’ (Hägerstrand, 1993) – a sequel to his earlier paper on ‘‘how about people in regional science’’ (Hägerstrand, 1970). In this paper, Hägerstrand clearly articulated the importance of situating human activities in the broader context of nature and environment; he even proposed using life cycle analysis with a focus on industrial metabolism as the primary methodology for time geography to study the movement of matter in space and time. Obviously, the extended version of time geography is more matter-realistic than people-based. Hägerstrand’s phenomenological domain of interest is in matter and corporeal entities (things beyond things) – the world of materiality found at the meso-level of the everyday (Hägerstrand, 1985, 1995). In this sense, time geography, more than anything else, is a deeply ontological framework that links nature, society and technology. Time geography is quite reminiscent of an ‘‘old geography’ interested in the transformation of habitats over time that included not only its people and artifacts but also its natural base (Turner, 2002). That is why Hägerstrand’s time geography is often described as a ‘‘situational ecology’’ (Gregory, 2000). Time-geography must be understood not only as an attempt to advance human geography and the other social sciences in the direction of contextual theory, but also a development of a longstanding view of geography as human ecology – an important perspective that is often missed or under appreciated by most practicing time-geographers. Among Hägerstrand’s followers in geography, only Carlstein’s (1982) work has an explicit ecological dimension, but a handful of scholars in other disciplines have more recently applied Hägerstrand’s framework in non-human settings, such as tracking the movement of grizzly bears (Baer and Butler, 2000) and dolphins (Ware, 2008). Garton (2011) reported work on modeling animal populations and habitats and their viability in space and time. Additionally, Jones and Cloke (2008) examined the growth of trees in space and time using Hägerstrand’s framework. In summary, Hägerstrand’s return to the theme of environment and nature is not surprising as he grew up in an environment that put him in intimate contact with nature (Hägerstrand, 1983a, 1983b). Hägerstrand’s time geography emphasizes the continuity and connectedness of sequences of events that take place in situations bounded in time and space and whose outcomes are thereby mutually modified by their common localization (Hägerstrand, 1975, 1978). Although he presented time geography as a socioenvironmental web model, his emphasis on corporeality, contextuality, and collateral processes has often been interpreted in a much narrower sense. Viewing his writings on time geography as a whole, it is apparent that Hägerstrand’s time-geography must be understood firmly within the human ecological tradition of human

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geography and its overarching goal is to create a method of achieving ‘‘a contextual synthesis in the man-environment area’’ (Hägerstrand, 1973, p. 87). 2.2. Space: from abstract space to concrete place/landscape One central representational problem Hägerstrand (1970, 1983a) identified was that there did not yet exist a ‘‘neutral frame of reference’’ for describing and measuring the impacts of processes in society in such a way that their geographic implications and consequences for its people as individuals became visible. The conceptualization of space in the classic version of time geography, as reflected in concepts such as path, pocket, prism, station, bundle, and project, is largely abstract space defined by an area or a region with Cartesian coordinates that make the paths and trajectories of human movement measurable and mappable cartographically. Advances in geospatial data collection technologies such as Global Positioning Systems (GPSs) and data analysis and mapping tools such as Geographic Information Systems (GISs) have enhanced the abstract view of space as a container in time geography (Yu, 2006), but Hägerstrand himself was never fully satisfied with the abstract conceptualization. He admitted that he loved maps, but the great adventure of his scholarly life was to try to transcend the map (Hägerstrand, 1983a). In retrospect, throughout his career, Hägerstrand’s conception of space changed from absolute (a container, Newtonian view) to relative/relational (an outcome of an ontology of matter, Leibnizian view) to concrete place/landscape (a dwelling, Aristotelian view).1 For Hägerstrand, space is less an a priori category, and much more a consequence rooted in the ontology of matter. He has always maintained a keen sense of concrete place and landscape. Hägerstrand’s vision for time geography is to represent ‘‘togetherness’’ and ‘‘thereness’’ holistically (Hägerstrand, 1984a). From very early on, he realized that an absolute conceptualization of space, though measurable and mappable, was not the ideal way to represent space because it often led to information loss. Hägerstrand (1982) observed that ‘‘the best approximation we have in geography to a concept capable of grasping the momentary thereness and relative location of all continuants is landscape’’ (p. 323). To Hägerstrand, landscape is a holistic concept that is capable of capturing everything existing on a delimited territory on earth’s surface, both materially and immaterially. To best represent landscape, Hägerstrand (1982) borrowed the word ‘‘diorama’’ – literally meaning ‘‘look through’’ in Greek – a term used for arrangements in museums which depict animals and people suspended in their natural environmental setting. Using a present analogy, Hägerstrand’s diorama can be regarded as an analogue, low-tech, static version of Google StreetView. By using diorama to represent place and landscape, Hägerstrand emphasized that the essential characteristics of space do not lie in the visual property, but in the ‘‘thereness’’ aspect. According to Hägerstrand, the beauty of diorama as a concept for representation is that all sort of entities are in touch with each other in a mixture produced by history, whether visible or not. Diorama – indicating the presence of human beings and their minds and tools in a natural setting – can serve as a better representation of space to understand the drama of life. In this sense, his view of the world is remarkably similar to Shakespeare’s view that ‘‘all the world’s a stage’’ (from William Shakespeare’s As You Like It; for more details visit http://en.wikipedia.org/wiki/All_the_world’s_a_stage). By analogy, to understand the drama of life, we need to understand the actors and their roles and activities. For Hägerstrand, diorama 1 For distinctions among Aristotelian, Newtonian, and Leibnizian spaces, see Curry (1996) and Thrift (2003); for distinctions between space and place, see Agnew (2005, 2011).

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is the best conceptual representation of the stage for these activities. By shifting the conceptualization of space to concrete place and landscape, Hägerstrand had in a sense also achieved his goal of transcending the map. This entailed Hägerstrand’s subtle shift from an orthogonal view of the earth’s surface (according to the map) to a more oblique view of the world (according to the landscape). In fact, oblique views of the world dominated for quite some time in history before more formal mapping techniques were developed. As illustrated in some of the earlier maps, paintings and drawings (Casey, 2005), visualizations of earth’s surface from an oblique view are more intimate and personal, but they can only cover rather limited geographical areas. An orthogonal view, while more abstract, enables cartographers to cover more extensive geographic areas and perform more complex operations such as overlays and spatial analysis. Most conventional maps represent a predominantly orthogonal view of earth’s surface. It is a more objective view and makes calculations easier as reality is often depicted as abstract points, lines, and polygons. The orthogonal view, which assumes a perspective vertically above the ground, is not our natural way of seeing the world, as Hägerstrand (2004) so persuasively argued. Although humans have acquired the cognitive capability of the orthogonal view for millennia, as demonstrated by the marvelous maps made throughout history, seeing the world through an orthogonal view requires some training and imagination. In contrast, by representing space as diorama through the concept of landscape, Hägerstrand shifted his perspective of reality from an orthogonal to an oblique view, which is more personal, aesthetic, and subjective. Instead of assuming a viewing position directly above the ground, an oblique view of earth’s surface – one of Hägerstrand’s two vistas – is more realistic and natural for most people. His visual notation system in time geography represents an oblique view of the landscape rather the vertical view as he used earlier in his diffusion studies. Hägerstrand used the representation of abstract space, such as path and prism, only as means to better understand concrete places as defined by spatial–temporal patterns of actions and how they modify landscapes. Understanding Hägerstrand’s shifting view of space from one defined by abstract coordinates, to place, as defined by landscapes, is crucial for fully appreciating Hägerstrand’s multiple dimensions of time geography. 2.3. Time: from symbolic to embedded time Hägerstrand (1982) sought to make geography ‘‘rise up from the flat map with its static patterns and think in terms of a world on the move, a world of incessant permutations (p. 323). He conceived of projects as ‘‘clusters of acts, individuals and times necessary for the completion of any goal-oriented behavior as taking the sense of colonizing the future and turning it into a mappable phenomena (p. 325)’’. In many ways parallel to Hägerstrand’s changing view of space, his conception of time has also evolved from time as a chain of moments moving forward in an undifferentiated manner to time as ‘‘embedded in the situational permutations of matter in space’’ (Hägerstrand, 1988, p. 34). Although Hägerstrand called his approach to geography ‘‘time geography’’, his view of ‘‘time’’, unlike his view of space, was rarely discussed explicitly. In fact, he published only one paper in English during his entire career dealing explicitly with time, which was not until 4 years after his retirement (Hägerstrand, 1988). This paper is a good summary of Hägerstrand’s shifting views of time, moving from symbolic to embedded time. According to Hägerstrand, symbolic time is an abstract and ‘freestanding’ entity, invented to summarize a large number of experiences and observations. Symbolic time can be cyclical or linear. Embedded time refers to time folded in the visible and tangible reality. It is also referred to as ‘‘storied

time’’ (Alverson, 2001), as defined by events or stories in some primitive societies. For symbolic time (such as clock or calendar time), the present is interpreted as a sharp now-line that moves forward into a not yet existent future. Embedded time implies no clock-sharp present common to everything. Instead the full lifetime or the whole time of existence for each corpuscle stands out as its present. For this reason, Crang (2011) linked Hägerstrand’s conceptualization of time to the shadowless eternal now of Nietszche’s midday hour of the world. Hägerstrand (1989) noticed that ‘‘. . . seen as a whole, the shape of a tree is a diagram of its successive spatial stages through what we call time (p. 5)’’. By moving away from symbolic to embedded time, Hägerstrand (1988) argued that four aspects of embedded time seem to be of critical importance and in need of further exploration: (1) the accelerated speed by which distance is covered; (2) the colonization of the future; (3) the relationship between the natural population of organisms (including humans) and the fabricated population of material artifacts; and (4) problems of piecemeal engineering of innovations. Hägerstrand (1988) further warned the danger of running our lives strictly according to symbolic/clock time, and argued that ‘‘symbolic time disconnected from an understanding of embedded time may permit ideas to develop which do more harm than good when they later become forced into practice (p. 42)’’.

3. Hägerstrand’s changing view of space and time: an ancient Greek connection Evidently, Hägerstrand made steady efforts during the last 30 years of his life to extend time geography, as reflected in his shifting views of the key conceptualizations related to agency, space, and time – three integral components of time geography. However, unlike his original version of time geography, Hägerstrand did not put together a coherent, comprehensive, extended version of time geography, although he worked hard to develop its elements. One of the goals of this paper is to develop a framework that aims to connect the diverse strands of Hägerstrand’s time geography as reflected in his efforts starting in the early 1970s until his death in 2004. To achieve this goal, two Greek notions of time, chronos and kairos, and their spatial counterparts, choros and topos, are discussed, which will be linked to Hägerstrand’s reconceptualizations of space and time.2 The unifying framework for time geography is built upon the dual Greek conceptualizations of space and time and is presented in Section 4. There are voluminous literatures on alternative conceptualizations of time in general (Hall, 1983; Schivelbusch, 1986; Gould, 1987; Aveni, 1989; Grosz, 1995; Miller, 2002; Galison, 2003) and the meaning of time in geographical and GIS research in particular (Thrift, 1977a, 1977b; Tuan, 1978; Parkes and Thrift, 1979, 1980; Kellerman, 1989; Hornbeck et al., 1995; Dodgshon, 1998; Frank, 1998; Jones, 2004; Shaw, 2006; Crang, 2005; Glennie and Thrift, 2011). A comprehensive discussion of human conceptualizations and experiences of time in a more general sense is far beyond the scope of this paper; a brief review of the dual conceptualizations of time by the ancient Greeks will suffice for the present purposes. According to Rämö (1999), ancient Greeks had a dual conceptualization of time – chronos and kairos, which should not be treated 2 I am aware of the growing literature in both physical and social sciences against presenting space and time as a dichotomy (Massey, 1999, 2001). May and Thrift (2001) has an edited volume on TimeSpace. Wallerstein (1997, 1998) also argued that the future of social sciences hinges on the concept of TimeSpace. In the context of time geography, space and time are presented as a dichotomy for pedagogical purposes, but in practice, different versions of time geography operate, to a different degree, according to the more holistic concept of TimeSpace.

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as dichotomous oppositions, but rather as complementary aspects of human understandings of time. Chronos concerns the exact quantification of passing time expressed in successive readings using rationalized and decontextualized devices and tools such as a clock or a calendar. From an Aristotelian point of view, the notion of kairos still remains closely connected with human right/timely moments to act judiciously, and not when someone (or a thing) just happens to be in a particular place at any given moment (Miller, 1992; Couclelis, 1998). In a strict Aristotelian sense, kairos is always an idea closely connected to phronesis – the wisdom to act at the right time and right place. According to Smith (1969), three distinct but related concepts are involved in the notion of kairos: (1) timing (the right time); (2) a time of tension that calls for a decision; and (3) an opportunity to accomplish some purpose. Smith’s tripartite characterization of kairos parallels Agnew’s (2005) tripartite definition of place – location, locale, and sense of place.3 Parallel to the dual conceptualizations of time, the Greeks also had a dual conceptualization of space – choros and topos, which can be roughly regarded as space and place. According to Curry (2005), geography was understood for quite some time primarily from the perspectives of choros and topos. ‘‘Geo’’ entered into the scholarly and popular lexicon at a much later age in human history. For Curry (2005), the difference among topos, choros, and geo is primarily a matter of scale. In many ways similar to chronos and kairos, these two Greek notions of space possess a different degree of familiarity. Still, this Greek notion of choros might serve as a useful complement to topos in its distinction between abstract space (choros) and concrete place (topos) (Walter, 1988). Casey (1997) further observed that Western thinking about the world had continuously neglected the perspective of place and been increasingly in favor of space. Scholars now seem to have consensus that a partial focus on either space or place might serve as an impediment to a more holistic understanding of the world. The difference between choros (space) and topos (place) is that, when the former (choros) is a geometric or cartographic extension, the latter (topos) is a contextual localization without sharp demarcations. The utility of such a distinction is demonstrated empirically in health research (Malpas, 2003; Cummins et al., 2007; Rainham et al., 2010), navigation (Kowalski et al., 2007), and landscape studies (Olwig, 2011). Methodologically speaking, Hägerstrand struggled through his career to develop a common language to describe human activities in space and time. He argued that ‘‘. . . such a needed common language can best be developed if we start far back, with a very general kind of space–time’’ (Hägerstrand, 1975, p. 12). For quite some time, I was puzzled by what he really meant by starting ‘‘far back’’ until I read his posthumously published paper on two vistas, in which he quoted Aristotle extensively: ‘‘Of the things constituted by nature some are ungenerated, imperishable, eternal; others subject to generation and decay. The former are excellent beyond compare and divine, but less accessible to knowledge. The evidence that might throw light on them and on the problems, which we long to solve respecting them is furnished but scantily by our senses. On the other hand, we know much of the perishable plants and animals among which we dwell. We may collect information concerning all their various kinds, if we take the pains’’. [quoted in Hägerstrand (2004, p. 315), from Aristotle, De partibus animalium, trans. William Ogle, in Richard McKeon, ed., The Basic Works of Aristotle, (New York: Random House, 1941)]

3 Otherwise known as homology between time and space, see Kellerman (1989) for more details on its geographical implications.

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Hägerstrand’s worldview in general, and his views on space and time in particular, were inspired by ancient Greek thought. But for reasons that are still mysterious to me, Hägerstrand did not reveal explicitly in his publications that he conceived his time geography according to ancient Greek thoughts of space and time, even though he mentioned once that his time geography would be better characterized as a ‘‘chorochronology’’ approach (Hägerstrand, 1995, p. 95). In retrospect, Hägerstrand’s conceptualizations of space and time are remarkably similar to those of the ancient Greeks. For his classic version of time geography, Hägerstrand admitted that he had to simplify reality in order to make it manageable; using his own words, he treated reality more like a ‘‘bubble-chamber of the physicist’’ (Hägerstrand, 1973, p. 77). To capture and record people’s movements in space and time, his initial conceptualization of space and time needs to be abstracted – Cartesian space (with X and Y coordinates) and clock/calendar time (depending on the temporal scales involved). In other words, Hägerstrand’s classic time geography relied heavily on abstract space and time, which corresponded to the concepts of choros and chronos derived from ancient Greek thought. Although Hägerstrand’s time geography resonated well with regional scientists and quantitative geographers, time geography was criticized as ‘‘a dance macabre’’ by humanists (Buttimer, 1976) because it omitted crucial dimensions of human temporality that is essential to human life – the internal realms of experience and meaning. Hägerstrand conceded that intellectually he was trying to walk on only one leg in his earlier version of time geography due to his conceptualization of space as merely ‘‘choros’’ and time as ‘‘chronos’’. In contrast, his emphasis on ‘‘landscape’’ and its representation as ‘‘diorama’’ later in his career, which has a Greek origin, shifted his conceptualization of space to the Greek concept of topos. Although there are some subtle differences, Hägerstrand’s concept of ‘‘embedded time’’ moved him closer to the Greek conceptualization of time as kairos. There are more than twenty different metaphors invoked to understand embedded time (http:// grammar.about.com/od/rhetoricstyle/a/x.htm). Kario is a single concise term that can capture the multitude of non-chronological time. Now that we have discussed how Hägerstrand’s views on time and space have changed and how these changes are related to ancient Greek conceptualizations of space and time, we can reconceptualize time geography according to space–time manifolds – a task hinted by Hägerstrand in his various speeches and publications between 1970 and 2004, but one that he never quite fleshed out explicitly. 4. Time geography reconceptualized: towards a unifying framework By linking Hägerstrand’s changing views on space, time, and agency for time geography (Fig. 1) with ancient Greek views of space and time, we can better integrate Hägerstrand’s scattered publications on extending time geography during the last 30 years of his life, thus gaining a clearer picture of what Hägerstrand had in mind for time geography. 4.1. Vistas into Hägerstrand’s mind? From the perspective of ancient Greeks’ dual conceptualization of space (choros vs. topos) and time (chronos vs. kairos), time geography can have at least the following nine versions, which are represented as four lines (Cr–Cs, Cr–T, K–Cs, K–T), four triangles (Cr– Cs–T, K–Cs–T, Cr–K–Cs, Cr–K–T), and one trapezoid (Cr–K–Cs–T) on Fig. 1. I believe these nine different approaches to time geography are what Hägerstrand had in mind, as reflected in his writings

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culminated in his development of mobility analytics (Miller, 2010b), but also initiated a paradigm shift in GIScience from place-based to people-based GIS (Miller, 2005c). Incorporating a time-geographic perspective into GIScience research has stimulated research on individual movement (Westervelt, 1999) and a more general chorochronos approach for spatial–temporal database development (Frank, 1999). Yu (2006) and Shaw et al. (2008) also did ground-breaking work to integrate time geography with GIS, and their ArcGIS time geography extension tool is publicly available (Shaw and Yu, 2009b).4 In Europe, Kraak and Koussoulakou (2005) also developed a space–time visualization tool based upon Hägerstrand’s time geography framework.

Fig. 1. Alternative conceptualizations of space, time, and agency for time geography.

between 1970 and 2004. All these different approaches for time geography can be applied to study agency in the context of humans/societies, technologies, and environments. Although he touched upon elements of each individual version of time geography, Hägerstrand never systematically presented his full vision of time geography. In this section, a preliminary effort is made to present a unifying framework for time geography, with the goal of tying together ongoing time-geographic research and stimulating discussion on the prospects of further research. 4.1.1. Time Geography I – the chronos–choros (Cr–Cs) approach This approach to time geography conceptualizes space as choros (an area or region defined by Cartesian coordinates) and time as chronos (clock/calendar time). This is the classic version of time geography. Hägerstrand (1995) called it chorochronology. The chronos–choros approach to time geography tends to be quantitative and mappable. Not surprisingly, most time geography research conducted by transportation geographers and GIScientists tends to take this approach. Shaw (2010) and Neutens et al. (2011) provided comprehensive reviews of chronos–choros time geography in the context of transportation research, with particular emphases on the operationalization of space–time prisms within transport networks, and the application of these network-based space–time prisms in empirical studies on accessibility. Timmermans et al. (2002) provided an overview of single-facet models (trip-chaining, departure time decisions and time allocation) vs. multiple-facet models (constraints-based models, utility-maximizing models, computational process models, and microsimulation models) in time geography. Raubal et al. (2004) developed a user-centered time geography for location-based services. Raubal et al. (2007) further applied the time geography framework for ad hoc sharedride trip planning using mobile geosensor networks. Neutens and his colleagues examined space–time opportunities for multiple agents and they also developed new procedures to deal with timing and synchronization for joint activity participation (Neutens et al., 2007). Miller (1991) first introduced the space–time prism concept to model accessibility in a GIS framework. Miller followed up his initial work during the past 20 years by developing what he called measurement theory (Miller, 2005a), linking time geography to activity theory (Miller, 2005b), and proposing a field-based approach for time geography (Miller and Bridwell, 2009). Miller’s chronos–choros (Cr–Cs) approach to time geography not only

4.1.2. Time Geography II – the chronos–topos (Cr–T) approach In the literature, Bakhtin (1981) used ‘‘chronotope’’ as a literary genre, meaning time–space, but he really meant concrete place, not abstract space. This approach to time geography conceptualizes space as topos (concrete place or landscape) and time as chronos (clock and calendar time). Hägerstrand (1982) used the diorama as a metaphor/model to represent place or landscape. Competitive sports can be understood better from the chronos–topos perspective in the sense that the outcome of the competition is measured by clock time and games usually take place in clearly defined fields or settings. Moore et al. (2001) created the Spatio-Chronological rugby Union Model (SCRUM) model, in which time geography was applied to study Rugby. The visual tools of time geography were shown in the context of a video information system (Moore et al., 2003a, 2003b). In the business context, the chronos–topos (Cr–T) approach was also used to study just-in-time (JIT) management delivery (Nandhakumar and Jones, 2001) and time management in software development (Nandhakumar, 2002). 4.1.3. Time Geography III – the kairos–choros (K–Cs) approach This approach to time geography conceptualizes space as choros (abstract space) and time as kairos (embedded/event time). Virtual communities such as online social networks exist in a special type of abstract space – choros; embedded, event time is a more appropriate conceptualization of time for virtual communities. Although alternative conceptualizations of time were discussed early on in time geography research, alternative conceptualizations of time in empirical studies have been scant in the time geography literature. Schwanen’s (2006) work is one of the few that comes close to the deployment of embedded/kairos time in time geography. Schwanen (2006) argues that time should not be expressed solely with reference to clock time because boundaries on what is acceptable or appropriate depend not only on clock time but also on the times of the body and especially the time inherent to the dynamics in the juxtapositions and presence/absence of human beings and inanimate objects within a bounded physical space. His interview excerpts clearly reveal how clock time interacts with embedded time in the context of how parents in two-worker families perceive and cope with coupling constraints during workdays (Schwanen, 2006). Schwanen (2008) developed his framework further to handle uncertainty and foreground the practical, material, and situational aspects of space -time behavior. 4.1.4. Time Geography IV – the kairos–topos (K–T) approach This approach to time geography conceptualizes space as topos (concrete place/landscape) and time as kairos (embedded/event time). Dance has been invoked as a metaphor for life (Hall, 1983; Zukav, 1980) and for understanding time and complex social relationships in social science research (Lichty, 2003; Blumenfeld-Jones, 4 To download the tool, visit http://web.utk.edu/~sshaw/NSF-Project-Website/ download.htm.

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2008). Not surprisingly, choreography and dance have been used to describe Hägerstrand’s time geography, from Pred’s (1977) choreography of existence to Seamon’s (1979) place ballet to Lenntorp’s (2011) drama of life. Similar to competitive sports, dance also usually takes place in the clearly defined space of a stage. Unlike competitive sports, the quality of dance is not judged in chronos time. Different choreographers may interpret dance movements differently according to his/her unique understandings of how the dance portrays life, or aspects of it. Thus, kairos time, rather than chronos time, is a better framework for understanding dance from a temporal perspective. Ahlqvist et al. (2010) recently developed a methodology to visualize and map dance movements. Using actor-network theory (ANT), which conceptualizes time and space as essentially kairos–topos, Schwanen (2007) presented one of the few studies on the movement of matter from a perspective of time geography IV. By incorporating ‘‘livelihood’’ into the concept of stations (as defined in classic time geography) in western Ostergatland farms in 19th century Sweden, Hoppe and Langton’s (1986, 1994) time geography approach for studying economic development also implicitly took a kairos–topos approach. Although not always presentable in visual or cartographic form, a kairos–topos (K–T) approach to time geography brought us closer to being able to understand and appreciate tempos and movement of life at both individual and societal levels, as demonstrated by Allan Pred’s (1981, 1986) early work along the kairos–topos tradition. 4.1.5. Time Geography V – the chronos–choros–topos (Cr–Cs–T) approach This approach to time geography conceptualizes time as chronos (clock/calendar time) and space as both choros (an area defined by Cartesian coordinates) and topos (concrete place and landscape). Couclelis (2009) developed new frameworks to extend classical time-geographic concepts to represent and analyze activities and interactions in a hybrid physical–virtual space. According to Couclelis’ (2009) novel conceptual framework for human activities in an ICT-rich world, people’s space–time behavior is modeled not just in a three-dimensional physical space, but in a multidimensional space that involves both physical and virtual space. A chronos–choros–topos approach improves the classic time geography by representing and analyzing the increasing variety of activities carried out with the help of ICT. Couclelis’ work is complemented by Yu and Shaw (2008). Shaw and Yu (2009a) further developed and implemented a space–time geographic information system (GIS) to organize activity and interaction data as spatio-temporal processes in an integrated space–time environment. Their space–time GIS design offers a useful analytical environment for implementing the chronos–choros–topos approach for time geography to study the dynamic human activity and travel patterns facilitated by rapid advances in information and communication technologies (ICT). 4.1.6. Time Geography VI – the kairos–choros–topos (K–Cs–T) This approach to time geography conceptualizes time as kairos (embedded/event time) and space as both choros (abstract space defined by Cartesian coordinates) and topos (concrete place and landscape). The emerging location-based social networking websites such as Foursquare, Gowalla, MyTown, and BriteKite usually involve both abstract space (virtual space) and concrete places (specific locations for concrete actions, such as a meeting place or restaurant). Although clock time is used/implied in locationbased social networking, kairos time involving the actual events tends to be the norm. Foss and Coulclelis’s (2009) recent study of fishermen’s travel implicitly adopted the K–Cs–T approach for time geography via a combination of event time, agent-based models, and constraints from the local environment. Seamon’s (1979) earlier study on the geography of the lifeworld also implicitly

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assumed the movement, rest, and encounter in both abstract space and concrete places through kairos (event) time. 4.1.7. Time Geography VII Chronos–kairos–choros (Cr–K–Cs) This approach to time geography conceptualizes space as choros (an area defined by Cartesian coordinates) and time as both chronos (clock/calendar time) and kairos (embedded/event time). As pioneered by Schwanen’s (2006, 2008) work, time geography approaches in transportation studies increasingly conceptualize time as both chronos and kairos. There is also an emerging literature in GIScience that focuses on an event approach (Worboys, 2005), which in a broader sense also takes a chronos–kairos–choros approach. Based upon her earlier work integrating GIS with time geography (Kwan, 2004), Kwan and her students recently recorded oral histories and geonarratives in their GIS-based timegeographic research (Kwan, 2008; Kwan and Ding, 2008). Oral histories and geonarratives often entail, although implicitly, a shift of conceptualization of time from chronos to kairos. Obviously, there is a growing trend in transportation studies and GIScience to take a chronos–kairos–choros approach to time geography, which has been demonstrated to represent people’s daily lives more realistically. 4.1.8. Time Geography VIII Chronos–kairos–topos (Cr–K–T) This approach to time geography conceptualizes space as topos (concrete place or landscape) and time as both chronos (clock/calendar time) and kairos (embedded/event time). Allan Pred’s (1977, 1983, 1984) qualitative work linking structuration theory to time geography represents the chronos–kairos–topos approach to time geography. Emerging from Buttimer’s (1976) humanistic critique of time geography, rhythm analysis implicitly assumes a Cr–K–T approach to time geography. Although the literature on rhythm analysis is predominantly qualitative thus far (Lefebvre, 2004; Mels, 2004; Edensor, 2006, 2010), the sequence alignment method, developed by molecular biologists for DNA analysis, seems to offer a promising quantitative methodology to conduct rhythm analysis for time-geographic research (Shoval and Isaacson, 2007). Perhaps more significantly in a conceptual sense, rhythm analysis has subtly shifted time geography from an ocular focus on the visual to a aural focus on the auditory, and has resulted in a fuller appreciation of multi-sensuality (Lefebvre, 2004). Hägerstrand always dreaded the inadequacies of the map and the visual for representing what he had in mind for time geography, as he admitted in his autobiography, ‘‘I see, almost literally, the opulence of the world as a moiré of processes in conversation (p. 239)’’ (Hägerstrand, 1983a). 4.1.9. Time Geography IX Chronos–kairos–choros–topos (Cr–K–Cs–T) This approach to time geography conceptualizes space as both choros (abstract space defined by Cartesian coordinates) and topos (concrete place and landscape) and time as both chronos (clock/ calendar time) and kairos (embedded/event time). In retrospect, I believe the Cr–K–Cs–T approach represents Hägerstrand’s holistic vision of a grand synthesis for time geography. Although Hägerstrand himself did not present his extended version of time geography in such as comprehensive manner, he obviously touched on individual elements of his extended version of time geography. Although Schwanen and De Jong (2008) came close to presenting a case study in Time Geography IV as they tried to link the dual conceptualization of space (choros and topos) and time (chronos and kairos), the empirical literature based on a holistic approach to time geography is still lacking. Recent advances in data collection, analytical methods, and new theoretical development seem to move us closer to conducting empirical studies according to the full version of Hägerstrand’s time geography – a chronos–kairos–choros–topos approach. Before we can practice Hägerstrand’s

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time geography holistically, there are a few areas that require further study. 4.2. Pockets for further exploration Calls have been made in the past about new paths and new beginnings for time geography (Thrift and Pred, 1981; Hallin, 1991; Lenntorp, 1999). Neutens et al. (2011) tried to develop a new research agenda for time-geographic research with a focus on transportation. Due to the recent advances in data collection technologies, space–time analysis methods, newly available data sources, and new theories, it is again time for interdisciplinary researchers interested in time geography research to broaden our research agenda in time geography. Inspired by the unifying framework discussed above, I would like to highlight the following three areas for time-geographic research to stimulate further discussions. 4.2.1. Data avalanche & representational realism The explosive growth of geo-tagged user-generated content, coupled with advances in other location-aware technologies such as radio-frequency identification (RFID), quick response (QR) codes, WiFi, and smart phones, are moving us on a fast track to knowing where everybody and everything are located on the surface of the earth, at all times. We will continue to witness what Miller (2010a) called the data avalanche or computer scientists called big data (Caverlee, 2010) or the exaflood (Swanson, 2007). We now have technologies that can not only monitor human mobility in intimate spatial and temporal detail, but also track a particular product for its entire life cycle from inception to disposal, or from cradle to grave. As Manovich (2011) observed, some disciplines in the social and behavioral sciences (such as sociology, economics, political science, and geography) have been confined in the past to what is known as ‘‘surface data’’ about the many, while other fields in the social sciences (such as psychology, psychoanalysis, anthropology, ethnography) have been confined to ‘‘deep data’’ about the few. For the first time in human history, we, as social scientists, no longer have to choose between data volume and data depth. We may now have deep data about many, which potentially means an unprecedented level of empirical analysis of human activities from a time geography perspective. There is, however, one important caveat. Although we have made important progress in recent years in harvesting spatial and temporal data from multiple online sources, the quality and credibility of those data for scholarly research still warrant further investigation. We need to explore new ways in which the fusion of GIS with social media can be deployed to promote the human-as-sensor paradigm (Goodchild, 2007) in spatial data generation and continue to develop procedures that are capable of handling both quantitative and qualitative data (Backstrom et al., 2010; Yardi and boyd, 2010) and mobile phone data (Pfaff, 2010) at both individual and aggregate levels (Ratti et al., 2006; Sevtsuk and Ratti, 2010). The availability of abundant spatial–temporal data provides time-geographic researchers unprecedented opportunities to study people’s mobilities across multiple spatial and temporal scales. As demonstrated by the website ‘‘We Feel Fine’’ (http://www.wefeelfine.org), we now can not only know how to track people about their daily routine activities (Phithakkitnukoon et al., 2010), but also have a glimpse on people’s subjective/emotional feelings at a particular place and time during the day. 4.2.2. Methods of space–time mobility analytics Our growing capabilities of time-critical mapping and peoplebased GIS offers us a new opportunity to have a better understanding of the spatial dynamics of human behavior and societal

transformation, but attaining this goal demands better tools to study spatial dynamics. New breakthroughs have been made in time–space transformations of geographic space for time-geographic analysis (Ahmed and Miller, 2007). Yuan (2011) has recently called for the development of more robust data analysis and synthesis methods for studying spatial dynamics. This need is more urgent in the context of time-geographic research. As of today, we still do not have generic tools to automatically discover relevant information for a particular application over the Web by general users, although a range of specialized tools and websites (such as Radian6 and Alterian) have been developed. To process the massive amount of data from various socialmedia sites with different levels of uncertainty, one crucial need is to rapidly synthesize information, ideally in real time. Provenance and uncertainty of different sources should be maintained in synthesis, which is still a challenging issue in the transportation and GIScience communities. How to conflate geospatial data with varying levels of accuracy, different levels of detail, and different generalizations is still an open question. The general trends seem to point to the direction of artfully combining both quantitative and qualitative analysis (Goetz et al., 2009; Cope and Elwood, 2009). More research is needed along this direction to link GIS with the emerging mobility research methodologies (Fincham et al., 2010; Lee and Kwan, 2011). Lefebvre’s (2004) approach for rhythmanalysis could also be used as a new operational approach for multiple versions of time geography; his ‘alignments’ of urban rhythms – arrhythmia, polyrhythmia, eurhythmia, and isorhythmia – were all borrowed from the medical literature to describe rhythms in the human body according to kairos (embedded time) rather than chronos (clock time). Young (1988) questioned the tendency to replace a rhythmic, cyclical, sun-centered society with an artificial, clock-regulated, metronomic one. Time geography can potentially be used to improve people’s quality of life if we can find innovative methods of linking our previous understandings of the diverse hidden rhythms existing in both nature and society (Zerubavel, 1981) to the new daily routines of contemporary life (Ahas et al., 2010). 4.2.3. New levels of theoretical development Calls to study time geography from a complexity perspective were made more than a decade ago (Ellegård, 1999), but only recently have we seen growing attention to human mobility dynamics by a diverse group of interdisciplinary scholars (Barabási, 2005; Brockmann et al., 2006; Candia et al., 2008; González et al., 2008; Song et al., 2010; Sun et al., 2011). By extending time geography from its original conceptualization of a chronos–choros approach to the holistic version of the chronos–kairos–choros–topos approach that involves multiple conceptualizations of space and time, we also open doors for a much broader theoretical development and engagement for time-geographic research that draws on cutting-edge theories from both physical and social sciences as well as the humanities. In the 1970s and 1980s, time geography attracted the attention of social theorists of different persuasions, ranging from humanists and structurationists, to feminists and Marxists. Activity theory, utility maximization, probability theory, and complexity theory have also been invoked to inform time-geographic research since the 1990s. Now, with the growing interdisciplinary interest in time geography, we are poised for a new level of theoretical engagement in time-geographic research – actornetwork theory (Latour, 2005), non-representational theory (Thrift, 2008; Anderson and Harrison, 2010), social practices theory (Blake et al., 2009), and performitivity theory (Olwig, 2008; Macpherson, 2010). The multiple versions of time geography as derived from the unifying framework in this paper also demand a pluralistic, eclectic theoretical approach to guide time-geographic research in the years ahead.

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5. Exploring multiple paths of time geography: still an unfinished project? In 2005, Progress in Human Geography inauguarated a new section on ‘‘Makers of Modern Geography’’ (Johnston, 2005). Torsten Hägerstrand was chosen as the first geographer to be covered in this new section. Undoubtedly, Hägerstrand’s status as one of the key thinkers in modern geography makes him deserving of this recognition. His groundbreaking work in innovation diffusion, migration, and time geography has been written into textbooks in multiple languages. Hägerstrand’s work has inspired generations of scholars in many disciplines who have conducted research under new theoretical and technological contexts for the past 50 years. The goal of this paper has been to take stock of the major extensions Hägerstrand made for time geography between 1970 and 2004. A unifying framework based upon the manifolds of space and time is proposed to synthesize various elements Hägerstrand developed during the last 30 years of his life to extend time geography. This new framework not only restores the multiple dimensions of time geography that Hägerstrand envisioned, but also links the diverse ongoing research efforts in time geography to current wider theoretical debates within the academy. Furthermore, it opens up new possibilities and opportunities in the years ahead. Multiple new paths have been explored in time geography research in recent years, but as an intellectual pursuit, time geography is still largely an unfinished project because Hägerstrand envisioned a much grander goal for time geography – a better understanding of the human world from home to globe and from day to lifetime (Hägerstrand, 1978). This grand vision is still under-appreciated by his followers and critics alike. Towards the end of the Two Vistas paper, Hägerstrand (2004) summarized the main goal of time geography in this way: ‘‘. . .the main purpose of the approach which came to be called time geography was to open up a related perspective from the outside in which the main issue is how the myriad of objects in our lifeworld, i.e. all that existing upon the earth’s surface, get placed or place themselves through contact with one another during the lapse of time (p. 323)’’. Inspired by biochemist Edelman’s (1988) Topobiology, Hägerstrand characterized time geography as essentially topoecological in character, a way to study geography not bit by bit but all at once. In his autobiography, Hägerstrand (1983a) further explained that ‘‘. . . the whole perspective is very broad and general. For myself at least, I have managed to unite within one frame of thought all the various subject matters I have dealt with earlier: settlement, migration, social communication, diffusion, domain structure, and impact of technology. There is also the beginning of an answer to my old quest for the purpose and form of the ‘‘regional’’ approach to geography and the placing of man in nature’’ (p. 254). In fact, very early in his life, Hägerstrand indicated that he developed a general world-picture in his mind that today one perhaps would call ‘‘holographic’’. According to Hägerstrand (1983a), this holographic world view is not a special way of formulating problems, but rather a special way of ‘‘forming an image before any questions can be asked or answers sought. The arduous acquisition of this image is my single most essential experience as an adult geographer. Almost everything I have done since is somehow extrapolated from it (p. 245). ’’ Indeed, by looking through Hägerstrand’s (2004) two vistas, I hope I have helped you gain a glimpse of Hägerstrand’s rather grand vision for time geography, and not surprisingly, it is quite splendid. Like all the great minds before him, Bernhard Varenius, Alexander von Humboldt, Friedrich Ratzel, and Paul Vidal de la Blache, to name but a few, Torsten Hägerstrand is a great holistic thinker and synthesizer. Buttimer (1983) described Hägerstrand

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‘‘as a scholar who sees no intrinsic conflict between art and science, between poetry and mathematics, who can be just as thrilled by the esthetics of landscape and music as by the rigor and power of scientific models (p. 238)’’. Obviously, what Hägerstrand had in mind for time geography is not simply an analytical approach to study human movement in time and space, but also a broadly based conceptual framework for geographical synthesis. His goal in developing time geography was to counteract the study of areal differentiation in the discipline during the earlier stages of his career, and to further develop the art of putting the various materials of specialists and subfields together into a coherent and unified worldview. As revealed by the unifying framework for time geography, Hägerstrand has achieved a new level of synthesis: (1) across space, time, place, and scale; (2) across problem domains (people, technology, and environment); (3) across multiple senses (map/visual and sound/music); (4) across metaphors (forms/prisms, machines, organisms, and spontaneous events); and (5) across different geographical traditions (man-land, regional, and spatial analysis) in geography. As Taaffe and Gauthier (1994) have so ably demonstrated, transportation geography research has been intimately linked to the changing nature of geographic thought in the United States. We must somehow understand time geography at a similar level of conceptual breadth and depth in order to fully appreciate the versatility of time geography as a research paradigm that has sustained and enhanced our research interests for over four decades. Viewed at this conceptual height, there is a vast terra incognitae waiting to be explored. Indeed, time geography is still an unfinished project. As Thrift (1977b) observed, the frozen circumstances of space ‘‘only come alive when the melody of time is played (p. 448)’’. Acknowledgements The author would like to thank Shih-Lung Shaw for his invitation to contribute to this special issue. Research assistance by James Baginski and Majid Dadgar for this paper is gratefully acknowledged. Thanks are also due to Michael Goodchild, Kevin Cox, and Morton O’Kelly for their critical comments on an earlier draft. Perceptive comments by two anonymous reviewers have also significantly improved this paper. The author is solely responsible for any remaining errors. References Agnew, J., 2005. Space: place. In: Cloke, P., Johnston, R.J. (Eds.), Spaces of Geographical Thought. Sage, London, pp. 81–96. Agnew, J., 2011. Space and place. In: Agnew, J., Livingston, D.N. (Eds.), The Sage Handbook of Geographical Knowledge. Sage, Los Angeles, CA, pp. 316–330. Ahas, R., Aasa, A., Silm, S., Tiru, M., 2010. Daily rhythms of suburban commuter’s movements in the Tallinn metropolitan area: case study with mobile positioning data. Transportation Research Part C 18, 45–54. Ahlqvist, O., Ban, H., Cressie, N., Zuniga-Shaw, N., 2010. Statistical counterpoint: knowledge discovery of choreographic information using spatio-temporal analysis and visualization. Applied Geography 30, 548–560. Ahmed, N., Miller, H.J., 2007. Time–space transformations of geographic space for exploring, analyzing, and visualizing transportation systems. Journal of Transport Geography 15, 2–17. Alverson, H., 2001. From ‘‘storied time’’ to ‘‘clock time’’ in economic globalization at the new Millennium. In: Soulsby, M.P., Fraser, J.T. (Eds.), Time: Perspective at the Millennium. Bergin & Garney, Wesport, CT, pp. 177–188. Anderson, B., Harrison, P., 2010. Taking-Place: Non-Representational Theories and Geography. Ashgate, Burlington, VT. Aveni, A., 1989. Empires of Time: Calendars, Clocks, and Cultures. University Press of Colorado, Boulder. Backstrom, L., Sun, E., Marlow, C., 2010. Find me if you can: improving geographical prediction with social and spatial proximity. In: Proceedings of the 19th International Conference on, World Wide Web, pp. 61–70. Baer, L.D., Butler, D.R., 2000. Space–time modeling of grizzly bears. Geographical Review 90 (2), 206–221. Bakhtin, M.M., 1981. The Dialogic Imagination: Four Essays. Translated by C. Emerson and M. Holquist. University of Texas Press, Austin, TX.

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