Polycentric Urban Spatial Structure Identification And Performance Analysis

With the rapid development of China’s economy and the advancement of urbanization,population and industries continue to gather in urban areas,and the spatial distribution of factors have led to significant changes in the urban spatial structure.Due to the excessive concentration of socio-economic activities in urban areas,there is a certain degree of disharmony between urban development and environmental,economic,and social sustainability.Thus,the polycentric urban spatial structure is considered as an important spatial strategic approach for sustainable urban development in the future,and is expected to promote effectively economic and more equitable spatial development.However,whether the polycentric urban spatial structure conforms to the laws of urban development and the realization of urban sustainable development still lacks strong empirical support.Therefore,clarifying and grasping the effects of the polycentric urban spatial structure on environmental,economic,and social performance on can provide scientific reference for China to better implement the concept of“sustainable development”,develop sustainable urban development patterns,and optimize urban spatial structures.Currently,most studies have focused on the performance impact analysis of polycentric urban spatial structure based on a single spatial scale or a single dimension,ignoring the impact of polycentric urban spatial structure on performance in different dimensions(environmental economic social)with different scales.Thus,this study takes the “identification-pattern-effect” of the polycentric urban spatial structure as the main line of research.On the basis of fusing multi-source nighttime light data to develop a long-term nighttime light dataset,we quantified morphological and functional polycentric urban spatial structures in an intra-city scale,respectively.Then,we constructed a multidimensional performance impact analysis framework covering the three aspects of environment,economy,and society.Next,on the basis of using nighttime light data to identify urban entities,we analyzed the spatial structure characteristics of polycentric urban spatial structure within a intra-city scale,and further assessed the effect of the polycentric urban spatial structure on environmental,economic,and social performance.The main research contents and results of this study are as follows.(1)We proposed a framework for correcting and analyzing long time series heterogeneous nighttime light data.The dataset is developed to provide necessary data support for quantifying and analyzing the polycentric urban spatial structure.We calibrated DMSP-OLS data(1992～2013)by using a quadratic model based on a“pseudo-invariant pixel” method.Thereafter,an exponential smoothing model was used to predict and patch missing data in the monthly SNPP-VIIRS data(2013～2019).Outliers and noise were also removed from the annual data.In addition,a sigmoid model was employed to generate improved simulated DMSP-OLS(SDMSP-OLS)data(2013～2019),which were appended with the calibrated DMSP-OLS data(1992～2013)to develop improved DMSP-OLS-like data(1992～2019)in China.Finally,we qualitatively and quantitatively compared these data with published NTL data to examine data availability.Results showed that choosing invariant pixels to calibrate DMSP-OLS data can minimize discontinuity.The correlation between the SNPP-VIIRS data synthesized by the patched monthly SNPP-VIIRS data and the official annual SNPP-VIIRS data was higher than those of the two existing correction methods.Spatial patterns of pixels in the improved SDMSP-OLS data in 2013 were more similar with the DMSP-OLS data than those in the published data.Strong correlations likewise existed between the total(average)pixel values of the improved SDMSP-OLS data(2013～2019)and the DMSP-OLS data in 2012.We also found that the improved DMSP-OLS-like data held strong linear correlations with different statistics.(2)We realized the quantification of morphological and functional polycentric urban spatial structures based on long time series nighttime light data,revealing the evolution trend of polycentric urban spatial structure in different dimensions.And then we further clarified the differences and similarities of performances covering the environment,economy,and society between morphological and functional polycentric urban spatial structure.Firstly,the total nighttime light intensities in provinces were used to represent the urban size,and then quantified the degree of morphological polycentric urban spatial structure.Then,the Radiation Model was utilized to develop a directional measurement approach via nighttime light data to calculate the urban interactive light index.This index effectively demonstrated the interaction intensities among cities to quantify the functional polycentric urban spatial structure after validating its rationality.Finally,on this basis,taking provinces as the research units of the inter-city scale,we accessed the effect of polycentric urban spatial structure on environment-economic-social efficiency and differentiated from the morphological and functional dimensions.The results showed that urban interactive light index was similar to Baidu’s population migration index and the economic connection network constructed by the traditional gravity model.Thus,it proved the nighttime light data can effectively quantify the urban interaction intensities within the provinces.Overall,the morphological polycentric urban spatial structure in China demonstrated multi-centralization and a trend of progressive development from east to west.Similarly,the functional urban spatial structure also exhibited polycentric characteristics.Although the index of functional polycentric urban spatial structure fluctuated in some provinces,it showed an upward trend overall.In comparison to morphological polycentric urban spatial structure,functional polycentric urban spatial structure hold greater positive significance for sustainable urban development,as they tend towards the development of urban networks.It would promote the occurrence of spillover effect and borrowing size,enabling small and medium-sized cities around the central city to access the benefits of agglomeration economies and minimize the negative impact of resource mismatch and inefficiency.(3)On the basis of extracting urban entities,we quantified the polycentric urban spatial structure.And the environmental sustainability of the polycentric urban spatial structure was confirmed,no evidence was found to support the contribution to the sustainable development of economy and society.Firstly,redefining the urban entity,we used K-means algorithm to identify the urban entity.Secondly,combining POI data and SNPP-VIIRS data,the local spatial autocorrelation was used to automatically identify urban center cluster pixels with the mask of urban entities.And then we quantified the degree of polycentric urban spatial structure and analyze its spatio-temporal characteristics in China.Taking the prefectures as the research unit of intra-city,we analyzed the performance of polycentric urban spatial structures covering environmental,economic,and social aspects.The results showed that the result of urban center identification was consistent with the urban planning scheme and the coverage of POI data.And urban spatial development pattern tended to be polycentric in China.Polycentric urban spatial structure can effectively reduce carbon emissions,while it damaged economic efficiency and failed to narrow the development gap between different centers.Consequently,in the context of governments’ inclination towards optimizing the urban spatial structure towards polycentricity at the intra-city scale,while this approach has a significant positive impact on the environment,it may lead to an overall decline in regional economic and social performance.Generally,the polycentric urban spatial structure can effectively improve the sustainability of the environment,economy,and society at the inter-city scale(provinces),but can only reduce carbon emissions at the intra-city scale(prefectures).Therefore,the effect of polycentric urban spatial structure on the environmental,economic,and social performances needs to be analyzed according to the specific scale,targeted planning strategies should be adopted,and appropriate urban spatial structure should be selected according to local conditions to achieve sustainable urban development.