2.1. Sustainable Urban Development and Spatial Structure
The focus on sustainable development issues originated in the Brundtland Commission report in 1987. This concept is defined as development that can meet the needs of the present without compromising the ability to meet those of the future generations [
11]. The connotation of sustainable development is multidimensional, and its three pillars are environmental, social and economic sustainability [
12]. From the perspective of sustainable development, cities, as consumers of energy and producers of waste, are regarded as practical places that cause unsustainable problems [
13]. Therefore, in the face of swelling urban populations, promoting the sustainable development of large urban areas is the key to achieving the global sustainable development goals [
14]. In fact, the World Commission on Environment and Development (WCED) emphasized the challenges of sustainable urban development when the concept was first proposed [
11]. In recent years, the topic of sustainable urban development has changed from whether the city can realize sustainability to how the city can achieve sustainable development [
15,
16]. For the design of sustainable cities, scholars have proposed a variety of sustainable urbanism models, including compact cities [
17], eco-cities [
18], low-carbon cities [
19], resilient cities [
20], and new urbanism [
21]. While these models describe the vision of sustainable urban development, they also emphasize the connection between urban form, that is, urban spatial structure and sustainability. The term urban spatial structure refers to discernible patterns in the distribution of human activity in cities [
22]. It reflects the organization of urban land use and is closely related to the travel patterns of residents. A sustainable urban spatial structure contributes to control the size of the city and population, reduce the traffic distance and the use of vehicles, and achieve the efficient use of land resources, thus promoting urban sustainability [
17,
23].
2.2. Identification and Characteristics of Urban Spatial Structure
Researchers believe that the characteristics of urban spatial structure include both static and dynamic aspects [
24,
25]. The static characteristics reflect the morphological features of space [
26]. Workplaces and residences are the two most important functions affecting urban development and residential living conditions. Therefore, using the spatial distribution of employment and population to describe the morphological features of urban spatial structure is a common method in existing research [
27,
28,
29,
30]. Besides, studies on US metro areas have dominated related works [
31]. Burgess abstracted the monocentric city model based on the relationship between land uses and social classes in Chicago [
32]. This model indicates that, in the early development of megacities, all or most jobs were concentrated in the urban core, while residences were arranged in concentric circles around the core area [
33,
34,
35]. With changes to the economic development mode and the evolution of transportation technology, a suburbanization process had taken place in big cities, in developed countries, by the 1960s [
36]. The city center maintained its dominance for tertiary industry, while office space, research and development institutions, university campuses, logistics parks, and residential areas gradually spread to the urban fringe [
37]. The concept of edge city [
38] and employment subcenter [
39] have proved the emergence of polycentric morphology in the process of suburbanization in the United States. Since then, empirical studies on large cities in other countries have also confirmed the existence of polycentricity [
40,
41,
42], and polycentric development has also been considered as an effective planning tool to combat unorganized urban sprawl [
43,
44]. However, another perspective emphasizes that the suburbanization process will not necessarily form a polycentric urban spatial structure, but will further promote the decentralization of jobs and people [
45]. This makes megacities form a pattern of generalized dispersion, and some recent studies in the United States and other developed countries present evidence consistent with this view [
46,
47,
48,
49].
The dynamic characteristics of urban spatial structure reflect intra-city functional linkages, which are manifested as dense functional urban regions [
50]. Existing studies have used a variety of flows to measure the functional connections, among which the traffic flow generated by human daily activities is the most frequent [
51,
52,
53,
54]. The traditional approach to obtaining travel data is that of a household travel survey, which is costly, error-prone, and not easily updated. Moreover, the sample size limitation makes it difficult to provide comprehensive evidence of human mobility [
55]. Thus, although Berry tried to reveal the spatial structure via complex flow systems in the 1960s [
56], related studies have still concentrated on the nodal regions, such as those organized by various interactions between urban core nodes and their hinterlands [
57]. The updating of research data and methods in recent years has triggered a renaissance of dynamic urban structure studies. The development of information and communication technologies (ICT) and location-aware technology has provided new data sources for detecting the dynamics of urban structure, including Global Positioning System (GPS) log data, smart card data, mobile phone data, and other trajectory data [
58]. These new data sources have provided opportunities to track human movements and obtain socio-demographic information [
59,
60]. On the other hand, researchers have discovered that the statistical characteristics of travel behavior follow a power law and have a truncated heavy-tailed distribution, meaning that people are more likely to travel repeatedly in familiar areas and/or close to their place of residence [
61,
62,
63]. These findings have resulted in the introduction of complex network theory and methods into the field of urban studies, and researchers have started to explore dynamic functional areas from large-scale trajectory data [
50,
55,
64,
65,
66,
67].
As the most populous country in the world, China’s urban development has had a significant impact on the global urbanization process and environmental issues [
4]. However, due to the limitations of data, previous studies have certain deficiencies in the understanding of the spatial structure of Chinese megacities. First, from the perspective of the static characteristics of urban spatial structure, previous datasets used in these studies mostly rely on statistical sources, which are usually renewed once every five or ten years [
68]. For example, some recent studies still rely on the population census for 2010 and economic census for 2008 [
6,
69,
70]. Besides, due to the difficulty of obtaining the spatial distribution of job statistics from public sources, Chinese scholars have had to measure urban patterns based on resident population data for a long time [
70]. However, usually employment, not population, is considered to be the key to shaping the urban form and determining economic development [
28]. Therefore, there needs to be more empirical research to explore the morphological features of Chinese megacities from the perspective of employment distribution. Second, from the perspective of the dynamic characteristics of urban spatial structure, scholars have revealed the functional urban regions formed by population flows and spatial interactions in the inner city based on different sources of trajectory data in recent years. However, it should be noted that most studies used trajectory data generated by specific types of vehicles, such as taxi trajectory data [
50,
67], rather than commuting flows. In fact, the commuting flows that connect workplaces and residences is the specific representation of dynamic spatial structure [
49]. In addition, compared with statistical data used in urban form research, the new trajectory data differs greatly in methods, scope and time in which statistics are gathered. Therefore, few studies can analyze the static and dynamic characteristics of the urban structure at the same time, because different sources of data reflect the spatial development in different periods. Third, urban studies are scale-dependent. This means that the characteristics of the urban spatial structure may be different at different research scales, and that policy making at different spatial levels will also be affected [
71]. Urban planning of Chinese megacities usually involves two spatial scales, the metropolitan area as the macro-scale, and the central area as the meso-scale. Correspondingly, planners will study the spatial structure and make policies for land use and transportation development at these two spatial scales. However, most previous studies have focused on exploring the characteristics of urban spatial structure at a single spatial scale, and there is limited research examining spatial structure at multiple scales [
70].