| The law of urban spatial expansion has an indication of measuring the level of regional urbanization and population concentration.The rapid expansion of urban space will also lead to intensified contradictions between population,resources and the environment.Therefore,studying on the process and pattern of urban spatial expansion is of great significance for comprehending regional development status quo and man-land relationship.Urban agglomeration is the main form of new-type urbanization in China,and it is also the most dynamic and potential core area in the future economic development pattern.The Beijing-Tianjin-Hebei,the Yangtze River Delta and the Pearl River Delta,which are typical representatives of urban agglomerations in China.Therefore,studying on the urban spatial expansion characteristics of the three first-class urban agglomerations is valuable for understanding the development of Chinese urban agglomerations.The urban built-up areas derived from daytime remote sensing images can only reflect urban spatial expansion characteristics in terms of LUCC,which is more suitable to characterize the physical form of the urban built-up areas.By contrast,the newly big data of social sensing is applied to reflect the actual development level and city vitality.Nighttime light(NTL)data,which records lamplight from towns and villages at night,is concrete representation of human socioeconomic activities and highly correlated with GDP,population distribution,power consumption,et al.Therefore,to carry out a long-term sequence of urban agglomeration function and vitality analysis,NTL data is a better choice.DMSP-OLS NTL data and NPP-VIIRS NTL data are of good quality,convenient operation,opening and sharing so that have been generally used in the research of nighttime light remote sensing.This study took the Beijing-Tianjin-Hebei,the Yangtze River Delta and the Pearl River Delta as study areas.At first,in view of the discontinuity in the archival time between DMSP-OLS NTL data and NPP-VIIRS NTL data,power function model was used to transform NPP-VIIRS NTL data(2012—2017)to simulate DMSP-OLS NTL data so that we generated a temporally consistent NTL dataset of study areas from 1992 to 2017(consisting of 1992—2013 DMSP-OLS NTL data and 2014—2017 NPP-VIIRS NTL data).Then,referring to the statistical data of urban built-up area,the optimal threshold of nighttime light was determined to extract urban scope.Based on this method,urban scope with insufficient economic vitality contained in the statistical data was effectively stripped off and those low-grade,high-vitality urban areas that did not belong to the statistical scope were identified,which innovated the perspective of data application.Finally,analysing the urban spatial expansion process and pattern of the three first-class urban agglomerations in terms of scale,hierarchical structure,spatial direction and spatial pattern.Through the above analysis,we can have a better comprehension of the development characteristics of the three first-class urban agglomerations in China from a new perspective and provide a reference for regional development policy formulation.The main conclusions of the study are as follows:(1)In terms of NTL data integration,based on the correlation analysis of the two images in overlapping years,the operation of NPP-VIIRS image simulation of DMSP-OLS image can be realized.After the simulation operation,compared with the original NPP-VIIRS image,the correlation between the original DMSP-OLS image and the simulated image is significantly improved,and the standard error is significantly reduced.In 2012 and 2013,the correlation coefficient r values reached 0.88 and 0.85,respectively.RMSE)decreased from 18.8 and 18.1 to 5.3 and 6.1.The overall error of extracting the built-up area of the city based on the integrated image is only 4.28%,indicating that the integration method is feasible and the integration data can be used in the study of urban spatial expansion of the three major urban agglomerations.(2)Using the spatial comparison method of statistical data to determine the optimal threshold of NTL data to extract the city range.The result of the extraction is the numerical result of coupling the statistical data to the greatest extent,which reflects the regional economic vitality more,although it has higher consistency with the urban built-up area.There is no guarantee that the spatial distribution of the extraction results and the statistical area of the statistical data achieve the best fit.On the other hand,according to different levels of statistical data to obtain the optimal light data threshold,the city connotation of the extraction results are also different.The city extraction results of night lights can reflect the development vigor and spatial heterogeneity of urban agglomerations on a larger spatial scale;at the same time,it can effectively strip the urban space with insufficient economic vitality included in the statistical data and identify those that are not within the statistical scope Low-level,highly dynamic urban areas accurately identify the spatial pattern of balance within urban agglomerations.(3)In terms of urban spatial expansion characteristics of the three first-class urban agglomerations:(1)effectively distinguish the contribution of county-level towns and cities above the municipal level in the development of urban agglomerations.In the context of seeking consistency with the statistical unit,the extraction results of the three urban agglomerations are generally lower than the entire extraction range,and the total deviation is about 2.76%.The county-level towns in the Beijing-Tianjin-Hebei urban agglomeration,the Yangtze River Delta,and the Pearl River Delta urban agglomerations contributed 4.07%,3.43%,and 0.31% to the city limits respectively.Among them,the Beijing-Tianjin-Hebei urban agglomeration has a vast hinterland,and county-level cities and towns are an important part of the regional economic vitality.The overall contribution rate to the city is the largest,and it has maintained an increasing trend except for 1992-1995.A large number of them,some county-level cities and towns in the early 1990 s have strong economic vitality,but a large number of provinces and cities in the region have withdrawn counties and set up cities around 2000,and the number of county-level cities and towns has decreased,gradually lower than Beijing-Tianjin-Hebei urban agglomeration after 2005;cities in the Pearl River Delta The hinterland of the group is small,and the city scope is mostly distributed in the Pearl River estuaries such as Guangzhou,Dongguan,Foshan,and Shenzhen.The spatial aggregation effect is obvious.The contribution of county-level towns to the city scope is always the smallest among the three city clusters.(2)The built-up area of the three major urban agglomerations continues to expand,and the intensity of expansion has experienced three stages of low-speed growth,high-speed expansion,and steady expansion under the "new normal",showing obvious timing characteristics;at the same time,the night lights of the three major urban agglomerations The total intensity and the average intensity both showed an upward trend.The Yangtze River Delta urban agglomeration had the highest total light intensity at night,followed by the Pearl River Delta urban agglomeration,and Beijing-Tianjin-Hebei the smallest.The average light intensity in the built-up areas of the three major urban agglomerations was gradually converging.(3)In terms of the characteristics of the hierarchical structure of the size of the urban agglomeration,the Beijing-Tianjin-Hebei urban agglomeration takes Beijing as the first city,and Beijing and Tianjin as the growth poles.A certain structural system has been formed between the cities of all levels,but the Gini coefficient is relatively large and the city The difference in development is obvious;the Yangtze River Delta urban agglomeration takes Shanghai as the first city,forming a well-developed urban system of the secondary urban agglomerations such as Suxichang,Nanjing Metropolitan Circle and Hangzhou Bay.The cities of all levels are well developed and the Gini coefficient is small.The development is relatively average;the Pearl River Delta urban agglomeration takes Guangzhou as the first city,and also has many large cities such as Shenzhen,Dongguan,and Foshan.The urban built-up area has broken through the barriers of administrative boundaries,the urban agglomeration has a high level of development,and the urban system is relatively complete.(4)The unbalanced urban expansion of the three urban agglomerations presents different patterns.The Beijing-Tianjin-Hebei urban agglomeration has formed two major growth pole cities,Beijing and Tianjin,and the city has expanded into a “dual-core” model under the leadership of Beijing and Tianjin.The spatial imbalance is significant,and a perfect city scale system has not yet been formed;cities in the Yangtze River Delta and Pearl River Delta The cluster shape has obvious clustering characteristics,and the spatial balance is stronger.The path,direction and distance of the center of gravity migration also reflect the different expansion strengths and modes of action of the three urban agglomerations.The overall distance of center of gravity migration in the Yangtze River Delta spans the largest,followed by the Pearl River Delta,and the Beijing-Tianjin-Hebei migration.(5)The overall urban spatial expansion pattern presents the characteristics of continuously expanding hot spots and shrinking cold spots.The spatial pattern of urban agglomerations in the Yangtze River Delta and Pearl River Delta has changed a lot,and the core cities in the region have a strong leading role,while the hotspots and cold spots in the Beijing-Tianjin-Hebei urban agglomeration are relatively stable.The average value of Z score in the hotspots of the Beijing-Tianjin-Hebei urban agglomeration is the largest,indicating that the hotspots have a stronger centripetal agglomeration,followed by the Yangtze River Delta and the Pearl River Delta urban agglomerationTo sum up,in the new policy-context that national development strategy of the Beijing-Tianjin-Hebei coordinated development,integrated regional development of the Yangtze River Delta and construction of the Guangdong-Hong Kong-Macao Greater Bay Area,this study obtained high-precision urban spatial expansion atlas and evolutive characteristics in three first-class urban agglomerations based on integrated NTL data since 1990 s.Also,we integrated two kinds of NTL data with different archived time and attributes so that extended time range of NTL remote sensing research,and was valuable to NTL data integration research in the future. |
PDF Full Text Request