Learning from HIC-AL School of grassroots urbanism
Keywords:urban vitality, spatial relationship
With the acceleration of urbanisation in China, most urban spatial structures have changed from "single center" to "multi center"; urban centers are also gradually showing the characteristics of functional compounding. Scholars' research on urban centers has experienced a transformation from geographical centers to functional places providing trade, finance, administration and other services (Christaller, 1933). Since then, the connotation of urban center is no longer limited to the central location of urban geographical structure, but more from the perspective of the functions undertaken by the center itself. In recent years, with the continuous enrichment of the perspective of urban space research, scholars' research on urban space is no longer limited to the dominant elements such as space and function, and begin to emphasise the non-material space in urban research. Sociological content is added to the connotation of urban center, and it is considered that urban center is not only the functional core of the city, but also a material spatial form that condenses the sense of identity of citizens (Shi, Yang,2013,p.86).Taking the temporal and spatial characteristics of residents' behavior as an aspect of describing the urban center, it is believed that the urban center should have a certain use intensity (Zhang, Zhang&Zhou,2017,p.183).For the research on the spatial relationship of urban centers, many scholars took several large cities as examples to analyze the current situation of their urban center system and the influencing factors behind it, and gave the connotation and research framework of the urban center system. However, most of these studies focus on the hierarchical structure or spatial distribution, and use the individual attribute of the center as the research basis for the urban center system (Shi, Yang，2011，p.29; Zhang, 2012,; Wei, Xiu&Wang,2014, p.83). In fact, with the continuous development of information technology, the relationship between the centers is often generated through traffic flow, information flow, capital flow, technology flow, there are also obvious element links between urban centers.
The application and development of big data technology provides a better platform for studying the development status and spatial relationship of urban central areas. Many scholars try to visualize different types of big data, and use the address data representing different functions of the city to describe the spatial distribution of urban functions (Wang, Zhen,2014,p.58;Zhao,Liang&Guo, 2018,p.72). Analyze the flow status of urban residents in urban space according to the track data left by urban residents on the data platform (Liu,Biderman&Ratti,2012,p.72;Ding, Niu,2015,p.16,Niu, Ding,2015,p100). However, the city is a complex life body. Using a certain kind of data from one aspect alone cannot accurately describe the urban spatial form.
Therefore, based on the existing research of urban center and the dual attributes of urban space material and behavior, this paper combines urban construction space with human space-time behavior, and puts forward the concept of urban vitality center, elect the POI data of AutoNavi which can represent the urban function and the thermal map data of Baidu map which can represent the spatial agglomeration characteristics of urban population as the basic data to identify the spatial distribution of urban vitality centers, and analyze the spatial relationship of urban vitality centers from two aspects , grade and spatial connection.