Analysis On Urban Built-up Land Expansion Of Nanjing Metropolitan Area Based On Multi-temporal Remote Sensing Data

In today's world,urbanization has become an irresistible trend.Since the Reform and Opening,China's urbanization process has gradually accelerated.The expansion of urban space is the concrete form of urbanization process.Urban space is expanding,evolving and gathering.The integration degree of regional economy and spatial gradually strengthened,urban agglomeration came into being.The urban agglomeration,as an advanced form of urban spatial organization,its spatial form evolution,expansion process and driving force,had attracted the attention of many scholars in recent years.According to the "Yangtze River Delta Urban Agglomeration Development Plan",the Yangtze River Delta urban agglomeration is subdivided into "one core,five laps,four belts ",Shanghai is positioned as the core city of Yangtze River Delta,Nanjing is the only megacity.As one of the five metropolitan circles,Nanjing metropolitan area,including three cities of Nanjing,Zhenjiang and Yangzhou,will become a regional central metropolitan circle which promotes the development of surrounding cities.In this paper,Landsat remote sensing images were used as the main data sources in 1985,1993,2001,2009 and 2016 to carry out the monitoring of the spatial expansion of the Nanjing metropolitan cities(Nanjing,Zhenjiang and Yangzhou).Based on the remote sensing and GIS spatial analysis method,the urban impervious surface information was extracted and the boundary of the urban built-up area was delineated.The temporal and spatial characteristics,the morphological characteristics,the expansion model and the rationality of the spatial expansion of Nanjing metropolitan area in the past 32 years were discussed.At the same time,the driving factors of the expansion of the built area were analyzed,and the following conclusions are drawn:(1)Through the comparison of NDBI,IBI,BCI,CBI four kinds of urban built-up index to extract the impervious surface information,the advantages and disadvantages of the four methods were compared from the qualitative and quantitative aspects.It was found that the BCI index has a high degree of discrimination on different types of objects when extracting impervious surface information,which is an effective method to extract impervious surface information.On the basis of the results of preliminary extraction,by calculating the distance weighted imperious surface cohesion density,it can better reflect the accumulation of impervious surface in Nanjing metropolitan area.(2)During the period from 1985 to 2016,the urban built-up area of Nanjing metropolitan area was increasing year by year.The expansion intensity of each city increases first and then decreases.The centroid of Zhenjiang continues to move southeast,and the urban built-up area extends along the northwest-southeast.The centroid shift and the city growth orientation in Nanjing and Yangzhou has its own emphases in different periods.From 1985 to 2016,Yangzhou's compact ratio was larger than the Nanjing and Zhenjiang,the city space was more compact.Nanjing's compactness index shows a decreasing trend,which indicates that the urban space is becoming discrete.From the aspect of Boyce-Clark radial shape index,it changes the largest in Zhenjiang,and shows an upward trend year by year,which indicates that the shape and contours of built-up areas in Zhenjiang are becoming more and more irregular.(3)Through the development conditions and space morphology of three cities over the years,the model of urban expansion was determined.The conclusions are as follows:Yangzhou belongs to the central concentric circle expansion,Zhenjiang belongs to the Zonal axial extension,and Nanjing belongs to the Multi Center network expansion mode.The rationality of urban expansion is evaluated further by using the urban expansion elasticity coefficient,which is reasonable when the value of the urban expansion elasticity coefficient is less than 3.The elasticity coefficient of urban expansion in Nanjing is close to 3,which is a reasonable urban expansion.While the urban expansion elasticity coefficient in Zhenjiang and Yangzhou are on the high side,because the population growth rate is too low,indicating that there is an imbalance between urban spatial expansion and population development.The driving factors influencing the expansion of urban built-up area in Nanjing metropolitan area include administrative division,natural geographical environment,economic development,policy guidance,etc.(4)This paper analyzes the impervious surface growth of the urban administrative boundary of Nanjing metropolitan area,and finds that the impervious surface growth of Baohua town and Huangmei town are obvious.Select Baohua town,Huangmei town for the Nanjing metropolitan Integration experimental area for specific analysis.From 2000 to 2016,Baohua town,Huangmei town have changed from rural towns that have no obvious signs of urban construction into a certain scale of satellite towns.The built-up area of Baohua town and Huangmei town in 2016 is 10.39,5.32 square kilometers respectively.