Abstract: Over the past two decades, metropolises in China witnessed unprecedented growth rates in urban areas. The rapid growth of urban space could lead to a series of problems. The fact that China has the largest population manifests the negative consequences of urban expansion. A rigorous and quantitative comparison of urban growth among metropolitan areas will lend basic support to policy making on regional urban development and will help city planners, economists, environmentalists, etc. This study analyzed the spatiotemporal patterns of urban expansion at different distance from city center in 27 metropolises in China using multi-temporal remotely sensed data. We use three satellite images obtained from the Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) sensor system circa 1990, 2000 and 2010 for each city to develop maps of urban areas. A concentric partitioning method was employed to define urban core area and urban fringe on the basis of built-up density calculated in a series of 1km concentric rings. Urbanization Concentricity Index was proposed to measure the degree to which residential or non-residential development is close to urban core area. Then we calculated and analyzed Urbanization Intensive Index across a series of 1km buffer zones for each city to identify the location-based spatiotemporal patterns of urbanization. UII offers a measure of speed and intensity of urban expansion. The results showed that the spatial structure of these cities was closely related to national policies for regional development, demonstrating obvious regional characteristics. In general, the UCI value of the cities in western and northeastern China is higher in central and eastern China. Most cities exhibited lower UCI value in 2010 compared with 1990, showing a tendency toward dispersion. The highest UII for each city usually occurs around the urban core area and move outward over time. The UII value for second- and third-tier cities became quite small in rings more than 20 km away from city center. The area close to city center witnessed higher UII value in the first decade as compared with the second one. On the contrary, the UII value for the buffer zones away from the city center was much larger in the second decade. Urban form also correlated to the level of economic development to some extent. In the first decade, first-tier cities expanded more intensely and broadly than other cities. Some Second-and third-tier cities experienced significant urban expansion in the second decade, demonstrating a tendency towards decentralization in their development pattern. In 1990, almost all metropolises in China exhibited a trend of comprising one single large central area for the entire landscape. With the expansion of urban land and rapid new nuclei development, some developed cities exhibited a transition of urban space from being mono-centric to multi-nucleated in form, with the emergence of some sub-centers. However, some relatively less developed cities still retained their mono-centric urban space. It can be inferred that urban structure tends to transform from being monocentric to polycentric with the continuous development of economy.

Key words: urban expansion, urbanization concentricity index, urbanization intensive index, spatial structure