Spatial information processing requires a robust capability to describe geometric properties such as position (location), direction and distance. Information may be spatially referenced to local structures (e.g., building interiors) and regions (e.g., maps), or spatially referenced to the Earth as a whole (e.g., global weather). Information may be spatially referenced to other celestial bodies (e.g., astronomical, orbital, and geomagnetic observations). Information may also be spatially referenced to objects defined within artificial contexts such as virtual realities. In each of these cases, a spatial reference frame is defined, with respect to which the values of geometric properties may be determined.

It is often necessary to represent position in several different spatial reference frames, simultaneously, according to the context in which the position is to be used. Each spatial reference frame corresponds to a particular way of expressing position. Spatial reference frames may be relative to moving objects (e.g., planets), and therefore have values that are a function of time. It is necessary to specify the time to which the spatial position refers, and the time for which the spatial reference frame is defined.

The Spatial Reference Model (SRM) provides a conceptual model that allows a set of spatial reference frames to be defined in such a way that they describe geometric properties uniquely. The SRM supports unambiguous specification of the positions, directions, distances, and times associated with spatial information. It also defines algorithms for precise transformation of positions, directions and distances (for a given time) among different spatial reference frames.

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