## 2 Time Geography and Individual Space-Time Behavior Analysis  
Individual space-time behavior refers to an individual's movements and activities through time and across space. There are several aspects for describing individual space-time behavior, such as travel type, space-time trajectory, stops, duration, speed, and sequence [16]. Two groups of quantitative methods are commonly employed for individual space-time behavior analysis. One group takes a holistic view to analyze the trajectories or activity types, such as the approach by location sequence alignment [17]. The other group focuses on a certain aspect of individual behavior, such as the travel distance from major employment centers [18]. As pointed out in literature [16], many quantitative analyses of individual space-time behavior are limited because they rely on a series of isolated events to describe continuous space-time behavior. In other words, discrete spatial variables (e.g., dividing space into several homogeneous zones) and temporal variables (e.g., using hourly average activity instead of real-time activity) are used, and these discrete variables may lead to errors and are subject to issues such as the Modifiable Areal Unit Problem (MAUP) [8]. Therefore, most studies failed to reflect the full picture of individual space-time behavior (and the environment effects of these behavior) due to the limitation of the spatial and temporal analysis scheme. Swedish geographer [PERSON] developed the conceptual framework of time geography in the late 1960s. According to [PERSON] [19], individual space-time behavior is always limited by a series of constraints of space and time, including authority constraints, capability constraints, and coupling constraints. Under these constraints, an individual travels through a 3D space-time cube, _i.e._, a two-dimensional space and a one-dimensional time. The 3D travel trajectory in this cube is called a space-time path (Figure 2). Space-time prism [19] was developed to define an individual's travel possibility across space and through time. The concept of space-time prism was introduced into Geographic Information System (GIS) by [PERSON] [20] more than two decades later to examine the spatiotemporal accessibility of individuals. According to time geography, spatiotemporal events are presented through three components--theme, location, and time; these components are measured, controlled, and fixed respectively, to form different combinations [21]. For example, in a regional air pollution map, location is fixed, time is controlled, and theme (_i.e._, pollution) is measured. However, due to the limitation in information and visualization technologies as well as that in computation power, [PERSON]'s time geography has been mostly a framework for a long time. Individual space-time behavior has been studied at relatively coarse spatial scales and discrete time slices until recently. The visualization has been mostly 2D-maps showing distribution across space at selected time points. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Staying/Sleeping/In Car**} & \multirow{2}{*}{**Walking**} & \multirow{2}{*}{**Running/Cycling**} & \multicolumn{2}{c}{**Playing/Light**} \\  & & & & & & & & **Physical Labor** \\ \cline{2-9}  & **Speed** & **Air Volume** & **Speed** & **Air Volume** & **Speed** & **Air Volume** & **Speed** & **Air Volume** \\ \hline Children & \textless{1 or }\textgreater{}24 & 5–10 & 1–5 & 12.5–17.5 & 5–24 & 30–35 & \textless{}2 & 15–20 \\ Adult females & \textless{1 or }\textgreater{}24 & 5–10 & 1–5 & 17.5–22.5 & 5–24 & 45-50 & \textless{}2 & 15–20 \\ Adult males & \textless{1 or }\textgreater{}24 & 7.5–12.5 & 1–7 & 25–35 & 7–24 & 55–60 & \textless{}2 & 20–30 \\ \hline \hline \end{tabular}  
Unit: Speed (km/h), Air volume (liter)  
\end{table} 
Table 2: Individual average air intake volume per minute (adapted from [PERSON] 1994 [15]).