Spatial Structure Evolution Of Cities In China Based On Nighttime Light Remote Sensing Data

Urban area is the most concentrated places of human activity.Urbanization has facilitated socio-economic growth greatly but has also given rise to several urban problems.In-depth comprehension of urban spatial structure can provide scientific reference for urban planning and solution to urban problems.In the analysis and investigation of urban structure,the urban center is a crucial basic unit,and the identification of the urban center can help study the growth and evolution of urban spatial structure.Traditional research on urban center identification mainly relies on statistical data based on administrative units,which cannot truly reflect the dynamic changes and evolutionary characteristics of urban centers.Nighttime light remote sensing data has the advantages of large coverage,long time series,high resolution,and effective characterization of human activity.Nighttime light data has been shown to be effective for the identification and analysis of urban centers.However,existing research on urban center identification based on nighttime light remote sensing data is mostly focused on specific cities,lacking algorithms suitable for long-term urban center identification,and thus lacking analysis of the long-term evolution analysis of urban centers.Therefore,this study identified the urban center and its evolutionary characteristics of 337 cities at the prefecture-level and above in China from 2000 to2020 based on NPP-VIIRS-like nighttime light remote sensing data.By calculating the degree of polycentricity of each city,this study explored the development and evolution of urban spatial structure in China.The main contents of this study are as follows:(1)An improved localized contour tree method based on long-term nighttime light remote sensing data was proposed to identify urban centers and the evolutionary characteristics of 337 cities in China from 2000 to 2020.By comparing and analyzing urban planning documents,high-resolution remote sensing images,and existing research results,it was found that the proposed method can effectively identify urban centers and truly reflect their spatial evolution process.The analysis results of the multi-temporal urban center identification show that from 2000 to 2020,the number of urban centers in China has increased rapidly,with the average number of urban centers increasing from 3.35 to 8.08.Meanwhile,urban centers have also expanded,with the average areas of urban center increasing from 45 km~2 to 154 km~2.The changes in the number and area of urban centers are related to the level of development and geographical environment of the cities.The growth and expansion of the number and area of urban centers in eastern cities are generally more significant than in other regions.First-tier cities generally show the fusion of urban centers,while new first-tier cities show a rapid growth pattern in the number and area of urban centers.(2)Based on the long-term urban center identification results,the evolution of urban spatial structure was explored through the degree of polycentricity and the k-medoids method based on dynamic time warping.The results show that the spatial structure of the vast majority of cities in China shows a polycentric trend.The average degree of polycentricity of cities in China has increased from 0.25 to 0.37,and the development differences in urban spatial structure between regions gradually decreased as western cities emerged.Based on the evolution of degree of polycentricity of each city,the evolution of urban spatial structure can be divided into five categories:the degree of polycentricity of economic developed cities in the eastern coastal regions and important urban agglomerations continue to grow at a high level,while the northwestern China,northeastern China,and northern Anhui show slow increase in polycentricity.Most central and western cities are experiencing rapid growth in polycentricity,while cities in western and southwestern regions are developing a polycentric structure dominated by the main center.However,a few cities have a decreasing trend of polycentricity.In summary,this study utilized long-term nighttime light remote sensing data to identify the urban centers of 337 Chinese cities at or above the prefectural level from2000 to 2020.The evolution of China’s urban spatial structure was revealed,and the research findings could provide a scientific basis for policymakers to formulate effective policies aimed at optimizing urban spatial structure.