We have shown how the nine tiles in the projection-based model for cardinal directions can be partitioned into sets based on horizontal and vertical constraints (called Horizontal and Vertical Constraints Model) in our previous papers (Kor and Bennett, 2003 and 2010). In order to come up with an expressive hybrid model for direction relations between two-dimensional single-piece regions (without holes), we integrate the well-known RCC-8 model with the above-mentioned model. From this expressive hybrid model, we derive 8 basic binary relations and 13 feasible as well as jointly exhaustive relations for the x- and y-directions, respectively. Based on these basic binary relations, we derive two separate composition tables for both the expressive and weak direction relations. We introduce a formula that can be used for the computation of the composition of expressive and weak direction relations between “whole or part” regions. Lastly, we also show how the expressive hybrid model can be used to make several existential inferences that are not possible for existing models. 1. Introduction Relative positions of regions in large-scale spaces, and particularly in the geographic domain, are often described by relations referring to cardinal directions. These relations specify the direction from one region to another in terms of the familiar compass bearings: north, south, east, and west. The intermediate directions northwest, northeast, southwest, and southeast are also often used. Some models for reasoning with cardinal directions are the cone-shaped [1, 2], projection-based models (ibid), and direction matrix [3–5]. Papadias and Theodoridis [6] describe topological and direction relations between regions using their minimum bounding rectangles (MBRs). However, the language used is not expressive enough to describe direction relations. Additionally, the MBR technique yields erroneous outcome when involving regions that are not rectangular in shape [4] Some work has been done on hybrid direction models. Escrig and Toledo [7] and Clementini et al. [8] integrated qualitative orientation and distance to obtain positional information. Isli [9] combined Frank’s [1, 2] cardinal direction relations model and Freksa’s [10] orientation model to facilitate a more expressive reasoning mechanism. Sharma and Flewelling [11] infer spatial relations from integrated topological and cardinal direction relations. Liu and colleagues [12] have developed reasoning algorithms which combine RCC-8 [13] for topological relations (discussed in Section 4) and the cardinal direction
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