1. A computer-implemented process for providing a geometric model database for use in a ubiquitous computing environment to respond to queries about the environment's geometric state, comprising using a computer to perform the following process actions: accepting information about the geometric state of the environment; building a geometric model database of the environment based on an initial input of said information, comprising, establishing a set of entities that are of interest in the environment, each entity of which is represented by at least a coordinate frame unique to that entity, and characterizing the location of each entity in the environment in terms of the coordinate frame of at least one other entity, rather than in terms of a coordinate frame common to all entities; maintaining the geometric model database by modifying it based on the input of updated information about the geometric state of the environment; and responding to queries concerning the geometric relationships between entities in the environment using the geometric model database.