For study of urban expansion change, multi-temporal remote sensing images will be superimposed with one another for a detailed analysis. Here superimposed analysis serves as an important means for study of land use change, as it enables both qualitative and quantitative analysis of urban expansion and its rules in urban expansion study. Therefore, by means of superimposed analysis, topic images showing the outline of the built-up urban area of Guangzhou City as acquired from four different temporal phases will be superimposed with one another, to derive a topic image for urban expansion of Guangzhou City (Fig. 4).