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Spatio-temporal GIS design for exploring interactions of human activities.

Article Excerpt
Introduction

Human lives involve various activities in a space-time context. As social beings, people cannot avoid interacting with others in activities. These interactions enable flows of information and material among people and keep the society functioning. Because activities are distributed at different locations, travel is usually used to help people participate in activities and complete interactions. Considering travel as a derived demand, transportation researchers have recognized that the spatial and temporal distribution of activities can determine where and when people travel (Damn and Lerman 1981; Kitamura et al. 1990; Hanson 1995). Therefore, a better grasp of spatial and temporal characteristics of human activities and interactions can help researchers gain a better understanding of the transportation system, and the urban landform in the long run.

Hagerstrand (1970) proposed a framework to examine the relationships between various con straints and human activities in a space-time context. The framework was advanced with further efforts made by Hagerstrand and his research collaborators and is now known as Time Geography (Wachowicz 1999). In an integrated space-time system, time geography uses the concept of space-time path to describe an individual's trajectory in physical space over time and the concept of space-time prism to depict the space-time extent that can be accessed by an individual under certain constraints.

Human activities and interactions are predominately performed in physical space, through physical presence or physical contact of the participants. With a space-time explicit representation of an individual's trajectory, the space-time path concept of time geography provides an effective support to the study of spatial and temporal characteristics of human activities and interactions. However, recent developments in information and communication technologies (ICT)--such as cellular phones and the Internet--are changing how people carry out their activities and interactions. The information and communication technologies enable a different space, which can connect people electronically and transmit information among people more efficiently than physical space. This space has been named virtual space or cyberspace in the literature (Janelle and Hodge 2000).

Virtual space allows people to participate in activities remotely instead of by physical presence in physical space. This new presence mode through virtual space is known as tele-presence. Therefore, people now can choose to conduct activities through either physical presence in physical space or tele-presence in virtual space. The availability of tele-presence is changing the conventional role of physical space to contain human activities. The use of ICT frees certain activities and interactions from various spatial and temporal constraints. For example, people can enjoy more freedom in space by getting in touch with others using their cellular phones because they do not have to attach to fixed locations. People are no longer restricted to the open hours of airline agencies to book air tickets, and they can do it on the Internet at any time in a day. The freedom and flexibility gained from the use of ICT have important implications to the spatial and temporal aspects of human activities. With representation for physical space only, the space-time path concept of time geography falls short of portraying human activities and interactions in virtual space. Therefore, the concept needs to be extended to handle activities in both physical and virtual spaces, and support the exploration of human interactions in today's society.

Geographic information systems (GIS) are specifically designed to handle spatial data. Due to the spatial nature of human travels and activities, GIS have been considered as a powerful approach to study human activities (Pipkin 1995). Several attempts have been made to represent and explore activity/travel data in GIS (e.g., Shaw and Wang 2000; Wang and Cheng 2001; Frihida et al. 2002). Although these studies demonstrate the potential of GIS in supporting the organization of activity/ travel data and the exploration of human activities at the individual level, they focus on human activities in physical space only.

Kwan (2000a) presented an attempt to visualize an individual's activities in both the physical and the virtual space, using the space-time path concept and a multi-scale GIS environment to portray the three different spatial ranges of an individual's activities through physical presence and tele-presence. However, most existing studies concentrate on the representation and exploration of activities of a single person, and limited efforts have been made to study the interactions among people in a GIS environment. Therefore, a GIS design, which can effectively organize human activity data and help explore interactions of human activities in physical and virtual spaces, remains a challenging research topic.

This paper presents a spatio-temporal GIS design to support the exploration of spatio-temporal characteristics of human activities and interactions in physical and virtual spaces. An extended space-time path concept of Hagerstrand's (1970) time-geographic framework was used to help organize and visualize human activities and interactions in a GIS environment. Spatio-temporal analysis functions were developed within the framework to help explore the spatial and temporal patterns of human interactions.

This paper is organized into five sections. The next section discusses theoretical issues involved in the study. The following section presents a spatio-temporal GIS design that supports representation and spatio-temporal analysis of human activities and interactions. Based on the proposed GIS design, I then implement a prototype system with a hypothetical activity dataset in ArcGIS. Customized analysis functions and user interfaces in the prototype system are also demonstrated in this section. A discussion of potential applications of the system and future improvements concludes the paper.

Human Activity and Human Interaction

Every human activity takes place in a particular spatial and temporal context (Golledge and Stimson 1997). As activities are often distributed at different locations in space, an individual has to trade time for space so that she or he can be physically present at activity locations and participate in the activities (Hanson 1995). Travel is a conventional means for people to trade time for space, which means that traveling across the space takes time (Miller and Shaw 2001). Because time is a scarce resource, the more time is used for travel, the less is left for activities. Therefore, both space and time can constrain a person from performing certain activities. Using an integrated space-time system, time geography provides an effective framework to examine the relationships between constraints and human activities in a space-time context (Hagerstrand 1970). The framework has been frequently used by researchers to portray and study spatial and temporal characteristics of human activities in physical space (e.g., Carlstein et al. 1978; Parkes and Thrift 1980; Ellegard 1999).

Time geography considers the time dimension to be equivalent to that of space in the study of human activities. The framework adopts a three-dimensional orthogonal coordinate system to portray spatio-temporal aspects of human activities, with time as the third dimension added to a two-dimensional space. The two-dimensional space is used to measure the location changes of objects, and the time dimension is used to order the sequence of events and synchronize human activities.

Two fundamental concepts--space-time path and space-time prism--were developed in the time-geographic framework to depict an individual's activities in space and time. A space-time path is the trajectory of an individual's movements in physical space over time (Figure 1). A space-time path provides a space-time explicit representation for an individual's activities, including the starting/ending time and location of an activity, and the sequence of activities. Therefore, the spatial and temporal characteristics of an individual's movements are integrated under the concept of space-time path (Miller 2005).

[FIGURE 1 OMITTED]

A space-time path is composed of a series of tilted and vertical segments. The tilted segments indicate an individual's movements in space...

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