北京城市形态的分形集聚特征及其实践意义

The fractal urban form of Beijing and its practical significance

基于1984、1999年Landsat TM遥感图像,根据分形理论,用半径法研究了北京城市形态,发现北京城市具有分形性质。北京市建设用地从中心向外扩散的集聚分形结构存在明显的标度区转折现象,其中第一标度区是能够揭示北京城市形态演化特点的有效标度区。1984、1999年半径维数的数值变化反映了建设用地密度集聚扩散的不同特点。本文根据集聚分形的标度区建立了一种新的城市范围定义,这种定义可以减少通常定义中的主观因素和不可比因素,标度区大小具有一定的理论意义。

This paper presents a study on the fractal urban form of Beijing. We use the city built-up area to define the urban form. It includes all the built-up area of houses, squares and roads, but the roads connecting central cities are not included, nor the water and vegetation. The data is obtained from the Landsat TM images of Beijing. Two TM scenes from 1984 and 1999 are used to cover the city and its adjacent areas. Based on the unsupervised classification using the IMAGINE ERDAS 8. 6, the authors conducted an aborative correction on the map so that the classification is accurate enough. ESRI ARC GIS 9.0 is used to get the area data. The authors use the area-radius relationship method in this paper. In this view, the fractal dimension is a way to clarify the convergence and decentralization of urbanized area. The authors found that the city of Beijing bears clear fractal characteristics. The computation shows that there exist double scaling range. The first scaling range is good to reveal the fractal evolution of Beijing. The scaling range is 8km in 1984 while it is 14km in 1999. The extension of the scaling range shows that the fractal characteristic in 1999 is better than that in 1984. The dimension values also indicate the building density of Beijing. In 1984, the density remains almost unchanged in a region of 10 km distance from the center （Tian＇an Men）. In 1999, the building density increases from the center to a region with a radius of 16.5 km. Both of the building densities decrease outside the corresponding ranges of 1984 and 1999. The fractal model from the area-radius relationship method in this paper has important theoretical significance, which needs further studies. The fractal computation in this paper has important practical significance. The scaling range of a city is a special variable that carries individual characteristics. It is also a comparable variable among cities. Based on this, the authors identified a new city definition using the scaling range. The new definition can avoid the subjective and incomparable factors of the ordinary definition method. The scaling range also has important theoretical significance.