The research presented in this paper introduces a relative representation of trajectories in space and time. The objective is to represent space the way it is perceived by a moving observer acting in the environment, and to provide a complementary view to the usual absolute vision of space. Trajectories are characterized from the perception of a moving observer where relative positions and relative velocities are the basic primitives. This allows for a formal identification of elementary trajectory configurations, and their relationships with the regions that compose the environment. The properties of the model are studied, including transitions and composition tables. These properties characterize trajectory transitions by the underlying processes that semantically qualify them. The approach provides a representation that might help the understanding of trajectory patterns in space and time.