These concepts have been developed by Whitehand into an approach that seeks to identify the decision-making behaviour underlying land-use change. This is based on the premise that the town plan at any one time is the outcome of the perceptions, principles and policies of individuals or agencies which exercise the necessary power. The westward extension of the city of Glasgow in the eighteenth century illustrates both the economic power of landowners and the influence of the burgage-plot pattern of land-holding on urban from. More recent evidence of the influence of landowners, developers and planners on urban structure is provided by Whitehand's study of residential infilling in Amersham in Berkshire, in which he explores the decision-marking processes underlying urban change, focusing on negotiations between developers and the local planning authority. In similar vein, Moudon has studied the evolving residential morphology of the North American city. These attempts to explore the backgrounds, motivations and actions of the major agents in the creation of townscapes at the local level represent a major advance on the earlier descriptive classifications of town plans. However, the difficulty of undertaking such detailed investigations increases as one looks further into the urban past.