Study On The Expansion Of Urban Built-up Area And The Internal Element Characteristic Of Chengdu-Chongqing Urban Agglomerations

Urbanization,the process of transforming human production and lifestyle from traditional rural types to urban types,is an inevitable trend of the modernization development in China.The most obvious feature of urbanization is the expanding scale of urban construction;the rural land has changed into urban construction area.As an important indicator in the study of urban expansion,the built-up area will help to rapidly and accurately grasp the scale and development trend of urbanization.Development strategies can be formulated according to the certain situation of each city.The research on nighttime light data has been deepening in recent years.The data have been applied to the research of built-up areas due to their availability and the ability to accurately measure the level of economic development.The expansion of the urban built-up can only analyze their own development considerations,It is better to combining with the economic relationship to appraise whole structure form of the urban agglomeration.Therefore,this paper extracts the long serial of the built-up area based on nighttime light data,and then analyzes the spatio-temporal characteristics of the urban built-up area and the changes of driving factors in different periods,and then combinging the socio-economic statistics in order to analysis diversification of the urban agglomerations' spatial structure of elementes.The research area of this paper is Chengdu-Chongqing urban agglomerations.The data sources are nighttime light data of DMSP-OLS(Defense Meteorological Satellite Program-Operational Linescan System)and NPP-VIIRS(Suomi National Polar-orbiting Partnership-Visible Infrared Imaging Radiometer Suite).The research duration is from 2002 to 2017,according to the national “four trillion” economic stimulus plan and the time points of the approval of the Regional Planning for the Chengdu-Chongqing Economic Zone,the study duration was divided into three phases in 2007 and 2011.The method of combining the empirical threshold method with the socio-economic statistics is used to extract the urban built-up area,then using the econometric models of urban centrality,economic contact intensity and spatial fracture point model to analysis the feature of spatial factors as“point,line,plane”of Chengdu-Chongqing urban agglomeration in 2002-2017,in order to research th integral situation of urban agglomeration.The specific research conclusions include the following aspects.(1)The method based on the combination of the empirical data threshold method and the statistical data helps to accurately extract the urban built-up area.However,for choosing the thresholds,it is necessary to take the differences in the internal development of the urban agglomeration into consideration.Different light threshold shows in diferent regions and different time periods;a single threshold setting will make the partial built-up area scale too large or too small.(2)The overall development level of Chengdu-Chongqing urban agglomeration is lower than that of the developed areas in the central and eastern regions.The intensity of land development and utilization and the level of urbanization are lower than the level of developed regions.From the perspective of spatial expansion of the built-up area,the overall expansion is slow.In terms of expansion rationality,due to the slow expansion rate,the overall human-land relationship in the urban agglomeration is in a relatively harmonious state.(3)The internal development of urban agglomerations is uneven.The development level of the two core cities of Chengdu-Chongqing urban agglomerations is much higher than that of other cities,and the planning and reality of urban scale structure in the Regional Planning for the Chengdu-Chongqing Economic Zone.Besides the two core cities,the key regional cities and the small towns supported by them are less affective;they are not able to share the urban functions of the core cities yet.(4)The spatial pattern of “one axis,two belts,two cores and three zones” designed in the Chengdu-Chongqing urban agglomeration plan was initially formed.The nuclear status of Chengdu and Chongqing is stable,and its economic subordination covers the entire urban agglomeration.In terms of Economic connection strength,the economic links along the Yangtze River and Chengdu-Deyang-Mianyang-Leshan City Belts are at the highest level in the corresponding period.