Urbanization Heterogeneity Of Urban Agglomeration Based On Integrating Nighttime Light Data

Urban agglomeration is a highly developed spatial form of integrated cities.As one of the most important changes in human history,urbanization profoundly affecting the development of human society and natural environment.Night light remote sensing has a wide range of applications in urbanization monitoring.Nighttime light signal obtained from the Defense Meteorological Satellite Program/Operational Linescan System(DMSP/OLS,data before 2013)and the Suomi National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite(NPP/VIIRS,data after 2012)provide reliable data sources for global urbanization monitoring.However,few studies have been concerned with the data simulation procedures for longtime analysis of spatiotemporal dynamics and heterogeneity of urbanization at the scale of urban agglomerations,due to the considerable inconsistencies.This study focuses on the Yangtze River Delta urban agglomeration,which is the most urbanized region in China.First,integrating the longtime series of nighttime light data,including Ⅰ)the NPP/VIIRS data based longtime series DMSP/OLS nighttime light data simulation method and the DMSP/OLS data based longtime series NPP/VIIRS night light data simulation method;Ⅱ)Verifying the effectiveness of the two data simulation methods by: spatial comparison before/after data simulation,pixelby-pixel based correlation performance,changes from total sum of light,and the regression performance between total sum of light and GDP,to determine the optimal nighttime light data simulation method;and Ⅲ)proposing a time changing threshold to identify the built-up areas,so that the built-up areas of the Yangtze River Delta urban agglomeration are extracted based on the nighttime light data.Secondly,the urbanization process of the Yangtze River Delta urban agglomeration since the 21 st century is divided into period 2001-2010 and period 2011-2019,and the spatiotemporal evolution and heterogeneity of the urbanization over the two periods are explored based on the longtime nighttime light data,including: Ⅰ)spatial heterogeneity analysis of builtup area tendency changes on administrative city scale;Ⅱ)time heterogeneity analysis of nighttime light intensity tendency changes on pixel scale;and Ⅲ)spatiotemporal joint analysis of urbanization in cities with different development speeds combined with GDP classification.The main conclusions are as follows:(1)The two data simulation methods: the NPP/VIIRS data based longtime series DMSP/OLS nighttime light data simulation and the DMSP/OLS data based longtime series NPP/VIIRS night light data simulation,can both provide a continuous and consistent nighttime light dataset for long term surface changes monitoring.Among them,the simulated DMSP/OLS and the original DMSP/OLS images are more similar compared with simulated NPP/VIIRS and the original NPP/VIIRS,and the pixel-bypixel correlation is higher.Considering that the DMSP/OLS data has an advantage of temporal coverage,the simulated longtime DMSP/OLS nighttime light dataset was adopted in application of long-term urbanization analysis over the Yangtze River Delta.(2)Spatial heterogeneity: Most cities in the Yangtze River Delta have generally increased in built-up areas and light intensity after 2010.Thereafter,the urban development characteristic has changed from a centralized pattern to a scattered distribution and undergoes a further urban agglomeration process.In the process,Suzhou-Wuxi-Changzhou,Hangzhou,Ningbo,Nanjing and Hefei contribute a fast development in the urban agglomeration,which construct a networked of spatial pattern distribution.(3)Temporal heterogeneity: Urban development time sequence is first dominated by the light intensity growth and then the built-up area growth.This transition of the dominant mode indicates whether the city core is fully developed.Urban development in the original city core has saturated and is spilling over to the suburbs and countryside,forcing the surrounding suburbs and countryside undergoing a fast urbanization.The transitions of a “rapid to moderate” and a “moderate to rapid” and the “hot to cold” and a “cold to hot” distribution indicate a continuous urban expansion.