A Comparative Analysis of the Characteristics of High-Speed Railway Network in China’s Metropolitan Areas

The role of high-speed rail network in the metropolitan areas is becoming more and more important, which has become one of the main ways of the city personnel contacts. This paper mainly uses the social network analysis method to revamp the six national metropolitan areas: metropolitan area of the middle reaches of the Yangtze River, metropolitan area of Hachang, metropolitan area of Chengyu, metropolitan area of Yangtze River Delta, metropolitan area of Zhongyuan, metropolitan area of Beibu Gulf make a comparative analysis. The macroscopic and microscopic angles are used to calculate and compare the overall network characteristics, centrality, core-periphery structure and cohesion subgroups. It is mainly due to the fact that the high-speed rail network structure of the six major metropolitan areas needs to be improved. Metropolitan area of the middle reaches of the Yangtze River and metropolitan area of Zhongyuan are relatively good the integrity and the connectivity. Metropolitan area of Yangtze River Delta is the highest utilization rate of resources. Metropolitan area of Hachang is the highest network density. Metropolitan area of Chengyu is outstanding between core and periphery. Metropolitan area of Beibu Gulf network of organizations is broken. Therefore, when layout of high-speed rail infrastructure, we should consider the high-speed rail network resource utilization, balance high-speed rail traffic within the metropolitan areas, thus enhancing the city’s high-speed rail accessibility and convenience.

strategic focus of local and national conditions. The competition between regions is no longer limited to the competition between cities and cities, instead, the formation of the central city as the representative and other cities the common division of labor and cooperation between the metropolitan areas of mutual attraction and exclusion. In order to promote the development of metropolitan area, and better play high-speed rail fast, convenient, large transport characteristics, it is necessary to study the structural characteristics of high-speed rail network of metropolitan areas. Friedmann and Miller (1965) first began to study the metropolitan areas of network and the main research direction is the relationship between urban and urban hierarchy networks. At present, the research on the spatial structure of metropolitan area mainly focuses on four aspects: First, Zhu (2006) and Bai (2014) study the mechanism and the causes of the metropolitan area; second, explored the structural characteristics of metropolitan areas from a city's properties, such as Nie et al. (2017)  Fourth, Wang and Wu (2013) explored the relationship between metropolitan areas from spatial structure measure. In the aspect of high-speed rail, Vickerman (1995), Levinson (2012) and Kim (2000) have studied the influence of the reachability of high-speed rail in Europe and South Korea on regional spatial structure. Jiang et al. (2010) studied the impact and change of the reachability of metropolitan areas from the specific line; FANG Da-chun and SUN Ming-yue (2015) Using the social network analysis method from the perspective of network density, centrality and cohesion subgroup, analyzed the spatial structure characteristics of metropolitan areas in the Yangtze River Delta from the high-iron age. It can be seen that the metropolitan area research is still at the theoretical stage both theoretical and empirical, mainly focused on the specific impact of spatial structure, rarely from the network point of view to explore, so the network of metropolitan areas have further in-depth to study. And from the www.scholink.org/ojs/index.php/wjssr perspective of high-speed rail to analyze the metropolitan area is very few, and most of the high-iron research focused on the relationship within the city group or from a single specific line to explore, and the comparison between metropolitan area is picky.
Therefore, the social network analysis has a wide range of prospects in the study between metropolitan areas. However, the current urban planning mainly research from the direction of spatial, and less from the perspective of social networks. Urban space design can cause human activities to make more meaningful human environment and the natural environment in order to improve the quality of human space environment, thus changing the quality of human life, but to reveal the interconnection between the space is empty. The social network analysis, it is based on things and things between the integrity, differences, the degree of analysis, so it is a favourable complement to the spatial analysis. This paper analyses the spatial characteristics of the metropolitan area in the mainland by the spatial structure of high-speed rail, and can compare the differences among metropolitan areas, and try to describe the characteristics of the existing urban high-speed rail network in mainland China from different aspects, and provide valuable reference for the future construction of high-speed rail and the operation of metropolitan area.

Research Objects
Metropolitan area, as a gathering place of population, capital and technology, has become a universal form of the spatial form of human social and economic development. In this paper, the State Council approved six national metropolitan areas as the object of this study (metropolitan area of the middle reaches of the Yangtze River, metropolitan area of Hachang, metropolitan area of Chengyu, metropolitan area of Yangtze River Delta, metropolitan area of Zhongyuan, metropolitan area of Beibu Gulf), and to high-speed rail data as their analysis of the bridge. The population of the six metropolitan areas accounts for 53% of the total population and the proportion of GDP accounts for 49%. Therefore, the six major metropolitan areas are highly representative. The development and distribution of the six major national metropolitan areas is shown in Figure 1 and   Source: Encyclopedia of Baidu.

Research Methods
Social network analysis is a method to study the structural relationship between social networks, and to study the actors and actors, actors and the Internet and the network and the interaction between the network and the overall structure by the use of graph theory and mathematical model. The theory of the network regards the "relationship" as an analytical unit, and regards the structure as a model of the relationship between the actors, where the structure can be either a structure of action, a political structure, a social structure, and an economic structure (Stanley Wasserman, 1994). The social network method provides a visual analysis tool for studying the network structure of the metropolitan area, and scientifically quantitatively analyzes the network structure relationship of the metropolitan area. This paper mainly investigates the differences between the central and the equivalence of different nodes in the metropolitan area of Chinese mainland based on high-iron space, the consistency of the whole network and the regional pattern of regional cities through the Overall Density Analysis, Centrality, Core/Periphery Structures and Subgroup Cohesion.
(1) Overall density analysis. The network density reflects an indicator of the degree of closeness between individuals in the network. The metropolitan area of cities is defined as the ratio of the number of connection relationships between cities that are actually owned and the maximum number of theoretical relationships that may be owned. The greater the network density, the closer the relationship between the network nodes.
(2) Centrality analysis. Network centrality measures the degree to which nodes are centralized throughout the network. Network Centrality can be divided into central degree and central potential. (3) Subgroup cohesion analysis. Subgroup cohesion refer to the fact that some nodes in the network are so closely related that they are combined into a subgroup. Subgroup cohesions can be obtained to analysis of small groups in the network phenomenon of seriousness from the reciprocity of relationships, the proximity or accessibility among subgroups, the frequency of relationships among subgroups, the relationship between the density of internal members of subgroups contrast to and the relationship between internal and external members Density and so on, and then look at the overall development of the network as a whole. There may be a whole very loose, but the core of small groups are very high cohesion.
(4) Core-peripheral structure analysis. Core-Peripheral structure refers to the whole network in which the city is at the core position, which cities are marginalized. With a quantitative way to scientifically determine which cities are in a more important position, which nodes are in a relatively minor position.

Node Selection
The choice of nodes is based on the prefecture-level administrative units in the metropolitan area. The nodes do not indicate the size of the administrative units, but only the stations where the trains arrive.
The size of the nodes indicates the relative importance of the nodes in the metropolitan areas. The larger the node, the more cities that connect the city to the metropolitan area.

Edge and Edge Rights
This article based on the 12306 railway website query results. If the two nodes in the train, we say that the existence of the two nodes edge. The determination of the edge rights to the basis of the number of trains per day as a basis. The more rough, the more the number of trips. While considering the train is round-trip, so the train came out with the train is not equal.

Urban Agglomeration Network Structure
This paper uses UCINET.6 to construct the social network structure of six national metropolitan areas.

The Overall Characteristics of Comparative Analysis
According to the results of Table 2 and the high-speed network model of the six major metropolitan areas constructed in Figure 2, the overall characteristics of the major metropolitan areas can be clearly demonstrated. From the high-speed network model, we can be initially seen that the metropolitan area of the middle reaches of the Yangtze River, the metropolitan area of Yangtze River Delta, metropolitan area of Zhongyuan have complex network intertwined, and have more intensive in the network visual structure. However, metropolitan area of Hachang, metropolitan area of Chengyu, metropolitan area of Beibu Gulf is relatively sparse, the network visual structure is relatively simple. This can also be further analyzed from the specific data of the number of nodes, the average degree, the average path length, the network density, and the average point weight.  connectivity and integrity to be improved (When the network density is 0, it means that each city in the city group does not intersect and the network density is 1, then the network has the highest degree of connectivity). Metropolitan area of Hachang of the network density is the most prominent, which is 0.64. While the metropolitan area of Beibu Gulf of network density is only 0.16, is still in the initial level of construction.
In general, the network structure of metropolitan area of the middle reaches of the Yangtze River and metropolitan area of Zhongyuan is the most reasonable. The metropolitan area of Yangtze River Delta is the most frequent network communication. Metropolitan area of Hachang has the highest network density, but other performance in general, that the transaction costs and expenses of the high-speed rail travel will affect the efficiency of resource use. The most bad of network is the metropolitan area of Beibu Gulf, there is still a long way to go in high-speed rail network construction.

The Network Centrality of Comparative Analysis
The comparative analysis of the overall characteristics of the metropolitan area mainly discusses the characteristics of the whole network from the macroscopic point of view, and the centrally comparative analysis of the metropolitan area is to explore the structural characteristics of the network from the microscopic point of view. As the high-speed rail is a two-way, so the centrality points of out degree and in degree separate the statistics, the purpose is to determine the high-iron network absorption and exclusion benefits. The size of the network centrality can not only reflect the concentration of resources, but also reflect the uneven distribution of resources. The greater the centrality, the more high-iron network focused on several large cities. Table 3 is a comparative analysis of the centrality structure of the six metropolitan areas.  Chengyu and metropolitan area of Beibu Gulf about 20%, the other in the 5% up and down. It can be seen that metropolitan area within the two cities can be almost between the direct travel, and the role of "intermediary" in other cities in the metropolitan area is not obvious, and the high-speed rail network is closely linked.
In general, the centrality of the six major metropolitan area is not very obvious, high-speed rail site resources distribution is more uniform, but there are also individual city center prominent phenomenon.
In the distribution of high-speed rail resources, metropolitan area of Chengyu and metropolitan area of Beibu Gulf need to control the excessive concentration of resources, to strengthen the protection of important sites. High-speed rail network is more balanced in the metropolitan area of the middle of the Yangtze River, metropolitan area of Hachang, metropolitan area of the Yangtze River Delta, metropolitan area of Zhongyu. It is necessary to improve the efficiency of resources to prevent excessive balance caused by resources.

Subgroup Cohesion and Core-Peripheral of Comparative Analysis
Using the K-plex algorithm, when the Minimum size and K values are 3 and 2, the urban aggregates based on the high-iron space are shown in Figure 3. Better of network connectivity is metropolitan area of Zhongyuan, metropolitan area of the Yangtze River and metropolitan area of the Yangtze River Delta, when the network connection strength to ≥ 10, most cities are still in this period, indicating that the network connectivity is high. Especially in the metropolitan area of Zhongyuan, 31 cities have 4 less than 10 network connection strength, the entire network of high-speed rail distribution is the most balanced, the whole network accessibility is better. And metropolitan area of Chengyu, metropolitan area of Hachang, metropolitan area of Beibu Gulf, when the network connection strength of 5, there are more cohesive subgroups. Especially in the metropolitan area of Beibu Gulf, when the network connection strength is 1, still divided into several cohesive subgroups, but the cluster subgroups are mostly single city, the whole network is more dispersed, the overall network connectivity is blocked.  From the core-peripheral structure point of view (Table 4), correlation is greater than 50%, indicating that the core cities in the high-speed rail network plays a more important role. Metropolitan area of the middle reaches of the Yangtze River, metropolitan area of Chengyu, metropolitan area of Zhongyuan of the core cities are more, the relative proportion of the core city and the edge of the city is small. The core area and the edge of the joint density is not high, which also reflects the high-speed rail network in the city group is more balanced. However, the correlation between the metropolitan area of Hachang, metropolitan area of of the Yangtze River Delta and metropolitan area of the Beibu Gulf is more than 70%. The core cities are less important, which highlights the importance of the core cities in the metropolitan area. High-speed rail site resources are relatively concentrated, then the overall network will be more like a pyramid-type structure.