A model, based on detailed descriptions of spatial configuration and a probabilistic approach to the user's choice, is proposed to measure the relationship between demand and supply locations in urban local systems. According to this model, the articulation of the public space grid, associated to the uneven distribution of facilities, generates a powered supply network to which demand locations are related. Choice, as well as demand satisfaction, will then be a function of the relative position (centrality) and attractiveness of supply locations. The model gives a simultaneous account of the spatial opportunity of demand and the spatial convergence of supply. Concurrently it can offer a picture of the stability of space in terms of possible land-use changes.