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<Paper uid="W03-0102">
  <Title>Pointing to places in a deductive geospatial theory</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
2 Outline of GeoLogica
</SectionTitle>
    <Paragraph position="0"> Questions are posed to GeoLogica in a subset of English and translated into logic by a natural language parser, the system Gemini (Dowding et al., 1993). The logical form of the question is rephrased as a theorem and presented to the theorem prover SNARK (Stickel et al., 2000). (A knowledgeable user of GeoLogica may prefer to bypass the parser and phrase the query directly in logical form.) The geospatial theory that SNARK uses for this application consists of a set of axioms, logical sentences that provide definitions and describe properties of important spatial constants, functions, and relations, including those in the logical form of the query. When SNARK proves a theorem, it shows that the theorem follows logically from the axioms in the theory. SNARK also has an answer-extraction mechanism; in addition to proving the validity of the theorem, it can extract from the proof an answer to the query encoded in the theorem, using mechanisms originally developed for automated program synthesis as well as question answering.(Manna and Waldinger, 1980) Using the appropriate axioms, SNARK transforms and decomposes the query to simpler and simpler subqueries.</Paragraph>
    <Paragraph position="1"> If the right subqueries of the query are answered, SNARK can extract the answer to the main query from the proof.</Paragraph>
    <Paragraph position="2"> Answers may be logical terms or, when demanded, visualizations, such as maps or satellite images. There may be many proofs of a theorem, and each proof may yield a different answer; it is possible to induce SNARK to find more and more proofs of the same theorem, and hence more and more answers to the query.</Paragraph>
    <Paragraph position="3"> SNARK has some geospatial knowledge that is built into its axioms, but it has access to a far larger body of knowledge through its procedural attachment mechanism. Procedural attachment allows subqueries to be answered by external knowledge sources, which may be programs or data bases and may reside on any machine accessible through the Web. We shall use the generic term &amp;quot;agent&amp;quot; for an external program invoked by the procedural attachment mechanism.</Paragraph>
    <Paragraph position="4"> The capabilities of agents are advertised by axioms in the theory, and the agents themselves are linked to symbols of the theory, so they may be invoked when they are appropriate. The procedural-attachment mechanism allows SNARK to behave as if the information possessed by the external agents were represented as axioms in the geospatial theory.</Paragraph>
  </Section>
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