Various Views on Spatial Prepositions

which the primary object is located the reference object. In order to use projective prepositions, the speaker has to establish a reference frame (that is, an orientation) that determines the direction in which the primary object is located in relation to the reference object. The reference frame can be established in different ways. One way is to use the intrinsic orientation of the reference object. In this case, the regions that are above, below, in front of, behind, to the left of, and to the right of the reference objects are the regions which are adjacent to the top, bottom, front, back, left side, and right side, respectively. If the intrinsic orientation of the reference object is used to establish the reference frame, I am referring to the intrinsic use of the corresponding prepositions. Thus, in intrinsic use, two arguments are needed for a locative description: the primary object and the reference object (Herrmann et al. 1986). If the reference object doesn't have an intrinsic orientation, or its intrinsic orientation isn't used for establishing the frame of reference, factors of the situational context determine the reference frame (Wunderlich 1986) and, thus, impose an extrinsic orientation on the reference object. In this case, I am referring to the extrinsic use of prepositions. Relevant contextual factors are, for example, the accessibility of the reference object, its motion, other objects in its vicinity, or the gravitation of the earth (see How is the Reference Frame Determined in Extrinsic Use?). Thus, in extrinsic use, three arguments are needed for a locative description: the primary object, the reference object, and a contextual factor (Hays 1987; Herrmann et al. 1986; and Wunderhen describing spatial relations in natural language, we often use spatial prepositions such as in, behind, or above. Spatial prepositions such as these, which are used in order to describe the location of one object in relation to another, are called relational prepositions (Clark 1973). Some of these prepositions, such as in, at, and near, only refer to topological relations between the objects. Others, such as in front of, behind, left of, right of, beside, above, and below, also convey information about the direction in which one object is located with respect to the other. Following Herskovits (1986), I call these projective prepositions. They are also called directional prepositions (Richtungspräpositionen) (Wunderlich and Herweg 1988). Projective prepositions can be used in different ways. For example, if we say, “The ball is in front of the car,” it can mean that we want to locate the ball in relation to the car from the point of view of the speaker, with respect to the orientation of the car itself, or with respect to the actual direction of the motion of the car. In the first case, I speak of deictic use; in the second case, of intrinsic use; and in the third case, of extrinsic use (Wunderlich 1985). This article presents a survey of various approaches to the issues concerning the deictic, intrinsic, and extrinsic use of projective prepositions. These approaches belong to different disciplines, such as linguistics, psychology, and AI. In addition, the system CITYTOUR is introduced and compared to some other AI systems that deal with spatial prepositions. In the following sections, I call the object that is to be located the primary object and the object in relation to In this article, principles involving the intrinsic, deictic, and extrinsic use of spatial prepositions are examined from linguistic, psychological, and AI approaches. First, I define some important terms. Second, those prepositions which permit intrinsic, deictic, and extrinsic use are specified. Third, I examine how the frame of reference is determined for all three cases. Fourth, I look at ambiguities in the use of prepositions and how they can be resolved. Finally, I introduce the natural language dialog system CITYTOUR, which can cope with the intrinsic, deictic, and extrinsic use of spatial prepositions, and compare it with the approaches dealt with in the previous sections as well as to some other AI systems.