Extraction of boundaries for built-up urban areas
Urban space is a representation of the development level of a city, and the dimension, structure and morphology of an urban space can reflect the scale, function and rate of expansion of the city. The built-up urban area is an objective reflection of the construction and development of a city in terms of domain or space, and is also the premise for urban expansion study. The built-up urban area mainly refers to downtown land blocks, which are concentrated or connected with one another, and other urban construction land blocks, which are scattered in suburban areas but are closely related to the downtown area of the city. The built-up urban area represents the status of construction land use for city development in different periods, and symbolizes the scale and dimension of city development. As a result, water bodies, farmlands, hilly lands, waste lands and forest lands can be integrated into one class of ground objects. In other words, binary processing can be carried out for classified images, then two classes of ground objects, i.e., land for construction and land not for construction, can be derived (Fig. 3). Also, man-machine interactive method was adopted in combination with false-color remote sensing images for manual visual interpretation, then the interpreted images were digitized to delineate directly the boundaries for the built-up urban area, and to extract topic images with boundary outline for built-up urban area of Guangzhou City in 1979, 1988, 1995 and 2002. Extraction of boundaries for built-up urban area follows the principle of spatial continuity of a city, and land blocks of small areas and not for urban construction, together with water bodies within the scope of a city, are defined as being part of the built-up urban area, while land blocks of big area and not for urban construction shall be excluded (Chen & Xu, 2005). Moreover, land blocks for agricultural construction in the marginal urban areas shall not be included in the built-up urban area, but large central towns in a city will be counted as built-up urban areas.