Topological and metrical methods for representing spatial knowledge have complementary strengths. We present a hybrid extension to the spatial semantic hierarchy that combines their strengths and avoids their weaknesses. Metrical SLAM methods are used to build local maps of small-scale space within the sensory horizon of the agent, while topological methods are used to represent the structure of large-scale space. We describe how a local perceptual map is analyzed to identify a local topology description and is abstracted to a topological place. The map building method creates a set of topological map hypotheses that are consistent with travel experience. The set of maps is guaranteed under reasonable assumptions to include the correct map. We demonstrate the method on a real environment with multiple nested large-scale loops.

Loop-closing has long been recognized as a critical issue when building maps of large-scale environments from local observations. Topological mapping methods abstract the problem of determining the topological structure of the environment (i.e., how loops are closed) from the problem of determining the metrical layout of places in the map and dealing with noisy sensors. A recently developed incremental topological mapping algorithm [E. Remolina et al. (2004), B. Kuipers et al. (2004)] generates all possible topological maps consistent with the experienced sequence of actions and observations and the topological axioms. These are then ordered by a preference criterion such as minimality or probability, to determine the single best map for continued planning and exploration. This paper presents the planarity constraint and analyzes its impact on the search-tree of all topological maps consistent with (non-metrical) exploration experience. Experimental studies demonstrate excellent results even in artificial environments where loop-closing is particularly difficult due to large amounts of perceptual aliasing and structural symmetry.