The dramatic changes of China's urban spatial form have brought many problems in economic, social and urban construction. These changes also put the urban environment under enormous pressure, which was so fragile that forced urban sustainable development into serious challenge. To understand the progress and mechanism of urban landscape pattern changes, building appropriate quantitative assessment model for urban spatial form is significance for not only promoting China's sustainable urban development, but also achieving the goal of rational use and protection of regional eco-environment, which were the urgent needs of urban development in China. Based on Thinh et al.'s urban compactness degree derived from the law of universal gravitation, we built a new quantitative model, the Normalized Compactness Index (NCI), to assess urban spatial form, in which the round-shape city was supposed to achieve the highest compact degree giving the same urban size. NCI was obtained by dividing Thinh et al.'s original compactness by the maximum compactness that was defined for a round-shape city, which served as a standard unit of measurement to standardized the calculation results and eliminate the impact of urban size on compactness, and thus facilitated comparative analysis between different cities or the same city among different periods.
We selected two periods (2002 and 2007) for empirical analysis in Xiamen City, using Landsat ETM + and Landsat TM remote sensing images as data sources. The whole area of city was divided into construction land and non-construction land. Construction land included building (residential, commercial, service and public facilitiy), mining, traffic and other land use types; non-construction land contained water body, wetland, woodland, farmland, bare land, etc. The results showed that: on the one hand, all of Xiamen's compact degrees of urban construction land in different spatial scales increased from 2002 to 2007; on the other hand, with the spatial scale increased, the values of NCI in two periods showed similar trend which was quick down first and slow steady followed. Therefore, we recommend that the model-scale analysis was necessary when using NCI for the comparative analysis between different cities or different periods in the same city.
The significance of building compact city was reflected in relieving resource scarcity (such as land and energy), and overall sustainable development demands in China. The lower city's compact degree, the more fragmentized the urban space layout is, the higher costs and lower efficiency of population movements, economic activities and social intercourses within the city. However, more "round-shape" for urban spatial distribution presented urban population, industry, transportation, facilities and capitals are denser, cost is lower and efficiency of traffic and resource use are higher. Meanwhile, compact city could make it more concentrated for industries as well as flows of population, goods and information. Therefore, from macro level, the theory of compact city may guide urban development.
NCI provided a new technical indicator for the quantitative assessment of urban spatial form. There is an urgent need to strengthen the empirical study of compact city, plan compact development route, and optimize regional spatial structure according to different geographical characteristics.