GeoPQL: A Geographical Pictorial Query Language That Resolves Ambiguities in Query Interpretation
The main problem of visual query languages for geographical data concerns the query’s ambiguity. Ambiguity derives from the fact that a query can lead to multiple interpretations for both the system and user. In fact a query can have different visual representations, and these can themselves have different interpretations. Among the reasons leading to these ambiguities, one appears to be fundamental: the user gives his own semantics to the information. However his actions may not completely represent his intentions, so the system may make an incorrect interpretation. Additionally, when a user draws two icons representing different geographical objects of a query he cannot avoid defining one or more spatial relationships between them. This is the case for any pair of icons, however the user often does not want to define spatial relationships between all pair of icons. So he cannot express his exact query and different queries must be formulated to obtain his goals.
This work proposes a Pictorial Geographical Query Language, GeoPQL, that allows the user to represent only the desired relationships and avoid undesired relationships in the query’s visual representation.
The language is based on twelve operators. The set of operators includes all the main topological operators, distance and two operators devoted to solving ambiguities in visual query representation. The paper then discusses syntactic and semantic correctness of spatial configurations and related operators in the context of the declarative geographic pictorial query language. Some possible ambiguities and their solutions are presented in order to show the language’s characteristics.
GeoPQL has been implemented as a stand alone tool which interfaces with ESRI’s ArcView®, and the main results obtained are: high expressive power, solution of the ambiguities inherent to the spatial representation of a query and exact matching between the query and the obtained results.
KeywordsGeographical Information System Query Language Geographical Data Topological Relationship Visual Language
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