In Simultaneous Localisation and Mapping (SLAM), it is well known that probabilistic filtering approaches which aim to estimate the robot and map state sequentially suffer from poor computational scaling to large map sizes. Various authors have demonstrated that this problem can be mitigated by approximations which treat estimates of features in different parts of a map as conditionally independent, allowing them to be processed separately. When it comes to the choice of how to divide a large map into such ‘submaps’, straightforward heuristics may be sufficient in maps built using sensors such as laser range-finders with limited range, where a regular grid of submap boundaries performs well. With visual sensing, however, the ideal division of submaps is less clear, since a camera has potentially unlimited range and will often observe spatially distant parts of a scene simultaneously. In this paper we present an efficient and generic method for automatically determining a suitable submap division for SLAM maps, and apply this to visual maps built with a single agile camera. We use the mutual information between predicted measurements of features as an absolute measure of correlation, and cluster highly correlated features into groups. Via tree factorisation, we are able to determine not just a single level submap division but a powerful fully hierarchical correlation and clustering structure. Our analysis and experiments reveal particularly interesting structure in visual maps and give pointers to more efficient approximate visual SLAM algorithms.
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