Analysis On Dynamic Changes Of Urban Expansion Based On Topography And Landform

Urbanization is the product of the development of human society to a certain stage,and it is the natural result and representation of human being's ability to adapt and transform the natural world.The urban expansion is the most direct form of urbanization.With the continuous development of productivity,non-agricultural industries continue to gather in cities,and rural populations continue to flow into cities,resulting in increased urban land demand and continuous expansion of urban spatial scope.The urban expansion on the two-dimensional plane alone cannot meet the research needs.Therefore,it is particularly important to study the changes of urban spatial expansion from the perspective of two-dimensional plane and three-dimensional plane,so as to enrich the theory of urban expansion.As the foundation of urban development,topography and landform have a direct or indirect impact on the planning management and layout of the city.Unreasonable construction activities have caused a series of urban problems.Therefore,it is of great practical significance to view the process of urban expansion from the perspective of topography.The research results can provide reference opinions for urban planning and layout,and it can provide effective and effective decision support for urban sustainable development.This paper takes Shaanxi,Gansu and Ningxia as a research area,based on multisource data,from two scales of provincial and prefecture-level cities,combined with expansion indicators,center of gravity model and gravity model to analyze the spatial and temporal changes of urban expansion from 1995 to 2015.The index factors such as elevation,undulation,slope,water system and genetic landform were selected to explore the topographic and geomorphic conditions of the study area and to analyze the distribution of built-up areas in different topographic and landform units in different periods.With the help of standard deviation ellipse model and buffer analysis method,the distribution patterns of built-up areas in different geomorphic units and the temporal expansion characteristics and direction of patches in the largest built-up area were analyzed.Using the building height data in 2019 as the benchmark and the historical high-precision image of the study area downloaded by Google Earth as a supplement,the height of the building that changed in the main urban area of Lanzhou from 2008 to 2019 was manually interpreted.Statistic and analysis of the mutual conversion of building types during this period,and compare the changes in the skyline of the changing areas in 2008 and 2019.The main conclusions are:(1)Overall,the total area of urban built-up areas in Shaanxi,Gansu and Ningxia area increased by about double from 1995 to 2015.The expansion dynamics,expansion speed and expansion intensity of various urban built-up areas in Shaan-Gan-Ning area showed an overall "increasing-decreasing-increasing" development trend.The expansion method changes from internal filling to external expansion.In 2015,the strength of cities' external economic ties and the expansion of cities in Shaanxi,Gansu and Ningxia were significantly positively correlated.On the provincial scale,the scale of urban expansion in Shaanxi Province is relatively the largest,and the distribution of urban built-up areas in Shaanxi Province presents a development trend of "high in the middle and low in the north and south".Ningxia's urban built-up areas have the fastest growth scale,and urban expansion mainly follows the Yellow River Economic Belt.The distribution of built-up areas shows a development trend of "high in the north and low in the south".The overall development speed of Gansu cities is slow.Due to the limitation of the typical valley-shaped landforms of the "two mountains and one river" in the provincial capital Lanzhou,the city has insufficient space for expansion.The distribution of urban built-up areas in the province shows a development trend of "west high and east low".At the prefecture-level city scale,there are development groups represented by central cities such as Xi'an,Lanzhou,Yinchuan,etc.The prefecturelevel cities with significant urban expansion and change characteristics are Xi'an,Xianyang,Weinan,Baoji,Yinchuan,Wuzhong,Shizuishan and Lanzhou.The provincial capital Xi'an has a greater ability to radiate and drive the city than Lanzhou and Yinchuan.(2)Urban construction activities prefer to develop and grow in areas with low terrain,low undulation and low slope with good terrain conditions.Based on the characteristics of genetic landforms,urban built-up areas are mainly distributed in flowing water landforms,some built-up areas are distributed in loess landforms and dry landforms,and the remaining built-up areas are less distributed in other landform types.Based on the distribution characteristics of the river,it is found that the closer to the river,the more the built-up area and the faster the built-up area grows.When the distance between the built-up area and the river is from near to far,the cumulative area of the built-up area increases from rapid to gradually flat.From 1995 to 2015,the builtup area in the main urban area of Xi'an City can be spread out in a pancake-like manner,and the direction of expansion is not significant,and the expansion is almost not restricted by terrain.The built-up area of Yinchuan City gradually expands from the southeast-northwest direction to the north-south direction,and then the direction of expansion obviously changes to the southwest-northeast sloping expansion.The expansion of the built-up area is reflected in the fan-shaped expansion method,and the expansion is less restricted by the terrain.Affected by the topography and landform conditions,the entire built-up area of the main urban area of Lanzhou has been expanding from southeast to northwest.The direction of its expansion is more significant than that of Xi'an and Yinchuan,and its expansion is restricted by the terrain.(3)In 2008,the city's early low-rise buildings and multi-storey buildings will be the first to be transformed and eliminated in the planning and design process.The skyline of the urban building change area is low and undulating.With the improvement of urban development capacity,many non-construction land in the urban fringe was converted into construction land in 2019,and the newly added low-rise building area accounted for 34% of the total new area.From 2008 to 2019,more than 4.98 square kilometers of non-construction land were converted Into building land of different heights.There are 5.28 square kilometers of low-rise buildings such as shantytowns or urban villages,and a small number of dilapidated multi-storey buildings have been demolished or have been converted into non-building areas.4.89 square kilometers of non-construction land is converted into construction land.Other buildings have less type conversion area.There are more high-rise and super high-rise buildings in the eastern and central regions,and the building height of Xigu District,which is located on the edge of the urban area,has also increased significantly from 2008,but there are fewer significant super high-rise buildings.There have been new changes in the city 's east-west skyline.