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<Paper uid="P06-1131">
  <Title>Sydney, July 2006. c(c)2006 Association for Computational Linguistics Incremental generation of spatial referring expressions in situated dialog [?]</Title>
  <Section position="4" start_page="1041" end_page="1042" type="intro">
    <SectionTitle>
2 Data
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    <Paragraph position="0"> If we consider that English has more than eighty spatial prepositions (omitting compounds such as right next to) (Landau, 1996), the combinatorial aspect of relational scene model construction becomes apparent. It should be noted that for our purposes, the situation is somewhat easier because a distinction can be made between static and dynamic prepositions: static prepositions primarily  denote the location of an object, dynamic prepositions primarily denote the path of an object (Jackendoff, 1983; Herskovits, 1986), see (2). However, even focusing just on the set of static prepositions does not remove the combinatorial issues effecting the construction of a scene model.</Paragraph>
    <Paragraph position="1"> (2) a. the tree is behind [static] the house b. the man walked across [dyn.] the road In general, static prepositions can be divided into two sets: topological and projective. Topological prepositions are the category of prepositions referring to a region that is proximal to the landmark; e.g., at, near, etc. Often, the distinctions between the semantics of the different topological prepositions is based on pragmatic contraints, e.g. the use of at licences the target to be  Static prepositions can be used in dynamic contexts, e.g. the man ran behind the house, and dynamic prepositions can be used in static ones, e.g. the tree lay across the road. in contact with the landmark, whereas the use of near does not. Projective prepositions describe a region projected from the landmark in a particular direction; e.g., to the right of, to the left of. The specification of the direction is dependent on the frame of reference being used (Herskovits, 1986).</Paragraph>
    <Paragraph position="2"> Static prepositions have both qualitative and quantitative semantic properties. The qualitative aspect is evident when they are used to denote an object by contrasting its location with that of the distractor objects. Using Figure 1 as visual context, the locative expression the circle on the left of the square illustrates the contrastive semantics of a projective preposition, as only one of the circles in the scene is located in that region. Taking Figure 2, the locative expression the circle near the black square shows the contrastive semantics of a topological preposition. Again, of the two circles in the scene only one of them may be appropriately described as being near the black square, the other circle is more appropriately described as being near the white square. The quantitative aspect is evident when a static preposition denotes an object using a relative scale. In Figure 3 the locative the circle to the right of the square shows the relative semantics of a projective preposition.</Paragraph>
    <Paragraph position="3"> Although both the circles are located to the right of the square we can distinguish them based on their location in the region. Figure 3 also illustrates the relative semantics of a topological preposition Figure 3. We can apply a description like the circle near the square to either circle if none other were present. However, if both are present we can interpret the reference based on relative proximity to the landmark the square.</Paragraph>
    <Paragraph position="4">  mantics of topological and projective prepositions</Paragraph>
  </Section>
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