A moving feature can be modelled as a combination of movements. The overall motion can be expressed as the temporal path or trajectory of some reference point on the object (the “origin”), such as its center of gravity. Once the origin’s trajectory has been established, the position along the trajectory can be described using a linear reference system (as defined in ISO 19133). The “parameterization by length” for curves (as defined in ISO 19107) can be used as a simple linear reference if no other is available. The relationship between time (t) and measure value (m) can be represented as the graph of the t→ m function in a plane with coordinates (t, m). This separation of the geometry of the path and the actual “time to position” function allows the moving feature to be tracked along existing geometry.
Figure 11 illustrates how the concepts of foliation, prism, trajectory, and leaf relate to one another. In this illustration, a 2D rectangle moves and rotates. Each representation of the rectangle at a given time is a leaf. The path traced by each corner point of the rectangle (and by each of its other points) is
a trajectory. The set of points contained in all of the leaves, and in all of the trajectories, forms a prism. The set of leaves also forms a foliation.