| Urban agglomeration is a new regional unit for a country to participate in global competition and international division of labor.It is the main platform for China to participate in international economic cooperation and competition.It is also a key gateway for China to go to the world,China city group is one belt,one road city,Eighth Central City Groups in China.The area is responsible for the strategic construction of the new Silk Road Economic Belt.It plays an important role in the development of the western region and the construction of the "one belt and one road".Nighttime light such as DMSP/OLS and NPP/VIIRS are new data sources for the study of urban agglomeration,which is convenient for the study of long time series of urban agglomeration.As there are many problems in the original DMSP/OLS Data,such as inter annual discontinuity,Digital Number saturation,etc.,it is necessary to correct the original image before using it.Among the correction methods,"invariant target area method" is widely used,but there are still some problems to be explored,such as the selection of invariant target area outside the study area,not considering the weak change of invariant target area in long time series,and the selection of reference image for continuous correction.Taking Guanzhong Plain Urban Agglomeration as the research area,this paper will explore a correction method suitable for Guanzhong Plain urban agglomeration.8 periods of radiometric calibration images are analyzed,4 periods of radiometric calibration images are selected as reference images to participate in saturation correction;combined with literature analysis,a reasonable reference image is selected for continuous correction.As the correction accuracy of DMSP/OLS will affect the follow-up research,this paper tests the accuracy of the corresponding DN values from three levels of urban agglomeration DN value,municipal Gross Domestic Product and county-level GDP,and the results show that the saturation problem has been greatly improved,and the correction accuracy is better.Based on the corrected data,DMSP/OLS and NPP/VIIRS luminous data are integrated,and then the dynamic threshold method is used to extract the urban built-up area.Then it analyzes the spatial-temporal evolution characteristics of the built-up area of Guanzhong Plain urban agglomeration,analyzes the luminous scale of the urban agglomeration by using the statistical methods such as the total amount of night light,the mean value,and the standard deviation ellipse;analyzes the boundary morphological characteristics of the built-up area of the urban agglomeration by using the fractal dimension and aggregation index;and analyzes the structure of the built-up area of the urban agglomeration by using the primacy degree,Gini coefficient,and rank scale rule This paper tries to analyze the spatial-temporal evolution characteristics of the urban expansion process of Guanzhong Plain Urban Agglomeration in a long time series.The main conclusions are as follows:(1)In this paper,the pre-processing method of night light image,saturation correction and mutual correction method are used.It is proved that the method can successfully correct the radiance of night light data in Guanzhong Plain urban agglomeration area,and the correction effect is good.(2)The total NTL of Guanzhong Plain Urban Agglomeration in 1992 was 31925,an increase of 245853 from 1992 to 2019,and the growth rate of TNL reached 770%in 28 years.Guanzhong Plain Urban Agglomeration in TNL Under the condition of small scale,the increment surpasses,which shows the rapid development process of Guanzhong Plain Urban Agglomeration;the nighttime lighting growth of Guanzhong Plain Urban Group mainly contributes from Xi'an city;the analysis of three provinces included in Guanzhong Plain Urban Agglomeration shows that the increase of lighting in Shaanxi Province is the largest;the urban agglomeration of Guanzhong Plain is growing positively.(3)In recent 28 years,the temporal and spatial dynamic characteristics of night light in the built-up area of Guanzhong Plain urban agglomeration are as follows:?In recent 28 years,the main driving force of night light evolution in the built-up area of Guanzhong Plain urban agglomeration is the east-west direction,in which Xianyang and Xi'an have a strong pulling effect on the spatial scale of the built-up area of Guanzhong Plain urban agglomeration.?In recent 28 years,the urban agglomeration in Guanzhong Plain presents the trend of spatial expansion contraction expansion.?In the past 28 years,the built-up area pattern of urban agglomeration is mainly external expansion,and the pattern of built-up area pattern has become more complex.?The agglomeration degree of luminous scale in the built-up area of Guanzhong Plain urban agglomeration is not stable,showing a trend of agglomeration dispersion agglomeration.?The primacy degree(S2)is greater than 2,which indicates that Xi'an has a strong agglomeration effect on other cities from 1992 to 2019.From the long time series,the S2 value of Guanzhong Plain Urban Agglomeration shows a trend of decrease increase decrease in 28 years,which indicates that the radiation driving effect of the first city on other cities first decreases,then increases,and then decreases.?Zipf index(|q|)is greater than 1,and fluctuates around 2,indicating that the distribution of urban luminous scale is relatively concentrated from 1992 to 2019,indicating that Xi'an and other large cities are well developed,while the rest of the small and medium-sized cities are underdeveloped.From the long time series,the |q| value of the built-up area of Guanzhong Plain Urban Agglomeration shows a trend of first decreasing,then increasing,and then decreasing in 28 years,which indicates that the distribution of luminous scale in the built-up area of Guanzhong Plain urban agglomeration is first dispersed,then concentrated,and then tends to disperse. |
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