Abstract: Urban form is defined as a figure presentation of spatial pattern of city's external shape, and, together with urban pattern and urban morphology, constitutes the three subjects at different levels in the research field of urban morphology. Current studies on this subject ignored the identification of the growth process from sprouting, growth, maturity, to ageing, and its impacting factors such as historical, political and social structure. Urban form is the lowest level in the urban morphology field. However, it has a very close relationship with urban growth process and the influence factors, it is an important knowledge basis for further understanding urban pattern and urban morphology, and it is also beneficial to grasping the expansion process of the boundary of urban built-up areas and making rational measures to control the trend of urban growth. Furthermore, coastal cities that sprouted in the estuarine always have unique growth characteristics different from others cites, because of their dependence on waterway transportation in early times. For example, early urban expansion nearby a port may be related to estuaries, rivers i.e., and the correlation would weaken in the process of social and economic development. How to quantitatively describe this evolution process and influence factors? This is a question that still needs to be further discussed by using new methods and defining new spatially related indices. Therefore, based on multi-type historical maps and multi-temporal remote sensing images, this paper first employs geographic information system (GIS) software ArcGIS Desktop 10.01 and remote sensing (RS) images processing platform ERDAS Imagine 2011 to extract the information of built up area boundary of Shanghai from 1842 to present, a 170-year time period, and secondly, calculates the fractal dimension (FD) and related circumscribing circle (RCC) of built-up area boundary by Fragstats 4.1 software to measure the complexity of urban boundary expansion, use ArcGIS Desktop 10.01for statistical analysis of the fan-shaped areas of built-up area expansion in different directions such as East, West, South, North, to identify the directional characteristics, and define an index urban hydrophilic property (UHP) to express the attraction of Huangpu River and Wusong River to built-up area boundary extension. All of these metrics finally are used for analyzing the historical evolution process and characteristics of the built-up area boundary of Shanghai City. In addition, with the reference to the historical literature, this paper discusses the impact of social transformation on the urban form evolution of Shanghai. The results show that: (1) FD and RCC of urban boundary showed a rise pattern of "M"; (2) urban boundary expansion showed a "single-direction" or "full-direction" mode during different historical periods; (3) the attractive effect of Huangpu River and Wusong River on built-up area boundary expansion has weakened due to the development of public transport system on the land; (4) complexity and directions of urban built-up area boundary showed significant stage characteristics along with the historical evolution process, which is mainly driven by different levels of social and economic development in the historical periods. The conclusion of the study is of reference value to Shanghai government in planning a more sustainable urban form in the future, and also makes a significance contribution to the extension of the theories and methods of urban morphology study.

Key words: GIS, historical process, remote sensing, Shanghai City, urban expansion, urban form