This thesis is a critical inquiry about the spatial relationships between occupancy and the physical structure of squares and parks in city centres. It focuses on usability and the spatial capacity of places, from two different angles. Firstly, it discusses the actual uses mapped in places, using repeated observation on different days, times and weather conditions. This results in empirical knowledge about dimensions and spatial requirements, especially for some long-stay active uses, such as ball games in parks and skateboarding in squares, and how long-stay passive uses, such as sitting, might relate to them, as well as how transitory activities relate to both long-stay engagements. In addition, it illustrates how some activities can be contiguous, while some others require 'buffer' zones between them for effective use. Secondly, this thesis addresses uses imagined in parks and squares by urban landscape designers, using two approaches: mapping out likely uses in detailed maps of selected places, and revealing a physical structure of a particular place by knowing its behavioural patterns. On this basis, this thesis examines designers' tacit knowledge about the usage-spatial relationship and, highlights potential applicability, the role and value of empirically gained knowledge in the design of parks and squares. It shows that designers' beliefs and awareness about uses in places, in some aspects, differ from actual use. From this point of view, it reveals a need for effective design-research integration and stresses the importance of empirical knowledge and its incorporation in design. The thesis promotes GIS as a successful practical tool to build, develop and maintain a body of empirical knowledge using interactive GIS maps as its scripts. Concerning the implementation of such knowledge in urban public open space design, operationally, a visualisation of research findings and its related concerns to decision-making, evaluation and management, is of key importance.