Spatial-temporal Evolution Of Urban Expansion In Five Provinces Of Northwest China Based On DMSP/OLS And NPP/VIIRS Data

As an underdeveloped area in China,Northwest China is relatively backward compared with the central and eastern regions in terms of urbanization development level and ecological environment.However,the development potential of the northwest region is great,especially with the development level of urbanization as the main influencing factor,and the northwest region is far more influential than the eastern and central regions.The implementation of the western development strategy can not only speed up the development of the central and western regions,but also contribute to China's modernization.As the main body of the ancient Silk Road in China,the five provinces in Northwest China are important bridges for economic and trade cooperation between China and Central Asian countries.Urban development has a great impact on the coupling and coordinated development of ecological environment and urban land.Moreover,the research on the spatial change of urban expansion also provides a theoretical basis for the planning of future urban development.For a long time series of urban areas,the application of night light data can provide a strong basis for human activities.DMSP/OLS and NPP/VIIRS are easy to operate and widely used in the field of urban expansion.Taking Shaanxi Province,Gansu Province,Ningxia Hui Autonomous Region,Qinghai Province,and Xinjiang Uygur Autonomous Region in Northwest China as the research objects,using DMSP/OLS annual stable light image and NPP/VIIRS annual synthetic night light image as the main data sources,the NPP/VIIRS night light image is simulated as DMSP/OLS night light image,The nighttime light integration data set of five northwest provinces from 1992 to 2018 was formed.This paper calculates a variety of related indicators of urban expansion,studies and analyzes the characteristics of urban expansion in the five northwest provinces,and analyzes the driving forces of urban expansion in the five northwest provinces from the aspects of physical geography,population,social economy,and other factors.The main conclusions are as follows:(1)Based on the data of two kinds of night light images in 2012 and 2013,the NPP/VIIRS night light images in 2015 and 2018 are simulated as night light images with similar DMSP/OLS night light image attributes.The correlation coefficient(R~2)of the fitting curve of two kinds of night light images in 2012 and 2013 is greater than 0.9,which can meet the research needs.The urban spatial information extracted by two kinds of nighttime lights is compared with the urban built-up areas extracted by Landsat images and land use data.(2)With its advantages,the scale of the city is expanding,the human activities of the city are more active,and the brightness of night lighting is also increasing.The index of urban expansion intensity of Shaanxi Province is the highest during the study period.Although the administrative area of Qinghai Province is large,the intensity index of its urban expansion is the lowest among the five northwest provinces.The annual growth rate of urban expansion in Shaanxi Province and Xinjiang Uygur Autonomous Region was the most stable and kept a gradual growth trend during 1992-2018.The urban expansion speed of the five provinces in Northwest China has maintained the trend of growth in general,and the acceleration of urban expansion indicates that people's living standards are constantly improving.(3)As time goes on,the compactness of the towns in the five provinces of Northwest China has been kept in a state of relatively diffuse distribution,low compactness,and irregular shape in space.The spatial distribution gradually tends to irregular shape from regular circle.Qinghai Province is suitable for gathering life due to the terrain and environmental factors,so the compactness of urban expansion is relatively high.The fractal dimension value of Xinjiang Uygur Autonomous Region remains?1.5,and the fractal dimension value of Qinghai Province is less than 1.5 before 2015,and other provinces show instability.(4)During the study period,the center of gravity of standard deviation ellipse of urban expansion in Shaanxi Province shifted from south to north,from Weicheng District of Xianyang city to Huangling County of Yan'an City.The area of standard deviation ellipse increased from 55192.03 km~2 in 1992 to 214939 km~2.The ellipse shape of the standard deviation of urban expansion in Ningxia Hui Autonomous Region gradually increased during the study period,and the change of the center of gravity transfer distance was irregular.The center of gravity of standard deviation ellipse of urban expansion in Qinghai province moves from northeast to southwest.The standard deviation ellipse shape of urban expansion in Xinjiang Uygur Autonomous Region kept increasing during the study period,and the center of gravity was located in Jinghe County of Bortala Mongolian Autonomous Prefecture.(5)In the analysis of the driving force of urban expansion in the five provinces of Northwest China,the comprehensive geographical factors such as the terrain and geomorphic characteristics of the location restrict the urban spatial expansion,urban spatial structure,and the external form of the city.Combining the characteristics of urban population change of five provinces in Northwest China with the urban spatial information extracted by night light data,it is found that the growth of urban population is closely related to the expansion of urban scope,and urban population plays an important role in the process of urban development.Urban social economy has a direct impact on the speed of urban expansion and the level of urbanization.The economic development level of the five provinces in Northwest China is constantly improving,which accelerates the expansion of urban space.To sum up,in the context of the coordinated development of the five provinces in Northwest China,the urban spatial change characteristics and driving factors of the five provinces in Northwest China from 1992 to 2018 are analyzed by using the integrated two kinds of night lights.The results show that the urban expansion changes of the five provinces in Northwest China continue to grow.The city's economic development one belt,one road,and five East and East mutually open patterns of the northwest provinces and cities have created good development opportunities and brought about great changes to the urbanization of five provinces and cities in Northwest China.Integrating the two kinds of night light data with the different temporal and spatial resolutions has a certain reference value to expand the spatial evolution of urban areas where human activities gather.