*2.1. Study Area*

*2.1. Study Area* 

To verify the relationship between search query data in cyber space and population migration in geographical space, we select three urban agglomerations in China as case study: Beijing-Tianjin-Hebei metropolitan region (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD). There are 38 cities located in these regions, 10 cities from BTH, 16 from YRD, and 9 from PRD, as shown in Figure 1. These regions are chosen based on the following reasons: (1) Extensive population migration can be detected in these areas. In 2015, the migration population in these areas has reached more than 8 million in total, accounting for 30.77% in China. Exploration of migration in these regions can avoid the influence of random migration under the support of large quantities. (2) These regions with relatively higher internet penetration offer adequate search query data. By the end of 2015, internet penetrations of core cities in those three urban metropolitan areas are separately 76.5% for Beijing, 73.1% for Shanghai, 78.4% for Guangzhou, and 83.2% for Shenzhen. More widespread application of the internet can be identified in almost all the provinces cover BTH, YRD, and PRD [34]. (3) They are the most significant areas for China's urban system construction. These areas occupy approximately 5.09% land of China but account for 23.65% and 39.87% of national permanent residents and gross domestic product in 2015. Research on them can offer more information to guide the coordinated development of urban areas in China. *Int. J. Environ. Res. Public Health* **2020**, *17*, x 3 of 18 **2. Study Area and Data** 

**Figure 1.** Location of the study area. **Figure 1.** Location of the study area.

To verify the relationship between search query data in cyber space and population migration

shown in Figure 1. These regions are chosen based on the following reasons: (1) Extensive population migration can be detected in these areas. In 2015, the migration population in these areas has reached more than 8 million in total, accounting for 30.77% in China. Exploration of migration in these regions can avoid the influence of random migration under the support of large quantities. (2) These regions with relatively higher internet penetration offer adequate search query data. By the end of 2015, internet penetrations of core cities in those three urban metropolitan areas are separately 76.5% for Beijing, 73.1% for Shanghai, 78.4% for Guangzhou, and 83.2% for Shenzhen. More widespread application of the internet can be identified in almost all the provinces cover BTH, YRD, and PRD [34]. (3) They are the most significant areas for China's urban system construction. These areas occupy approximately 5.09% land of China but account for 23.65% and 39.87% of national permanent residents and gross domestic product in 2015. Research on them can offer more information to guide

The data used in this paper include the population migration data, search query data, and socioeconomic data. (1) There are three kinds of population migration data used in this study: the net inflow population, intercity population flow, and the floating population. The net inflow population

the coordinated development of urban areas in China.

*2.2. Data* 
