Syntactical and Semantical Correctness of Pictorial Queries for GIS

One of the main topics in geographical information systems (GIS; Chrisman, 2002; Laurini & Thompson, 1992) research concerns the definition of high level visual query languages. This arises from the need to provide the user with a visual interactive tool for data manipulation and retrieval that is independent of the data's physical organization. The use of standard query languages for spatial data handling (Rigaux, Scholl, & Voisard, 2001; Shekhar et al. 1999) has been hindered by the lack of appropriate language support. In fact, in visual query languages for GIS, a query can lead to multiple interpretations (Favetta & Aufaure-Portier, 2000). For example, suppose the user wishes to formulate the following query: "Find all the regions that are passed through by a river and overlap a forest." In this query, the user does not express interest in the relationship between the river and the forest. However, when he or she draws a shape representing a region, and another shape representing a river, he or she cannot avoid representing a spatial relationship between them, and so every representation considering a specific relationship between the two features can be considered a valid representation of the query. Different visual queries can thus represent the previous query in natural language. In Figure 1.a, the river passes through the forest; in Figure 1.b, the river touches the forest; and in Figure 1.c, the forest and the river are "disjointed." Thus, the three representations should be interpreted as three different queries and, moreover, each query has a different meaning from the original query in natural language. Owing to semantical ambiguity problems, some configuration could be semantically invalid. For example, a lake cannot include a region. However, a visual query language with clear syntax and semantics can a priori overcome many cases of the ambiguities, minimizing multiple interpretations of a query for both the system and the user. This article discusses the syntactic and semantic correctness of spatial configurations in the context of nonprocedural geographic pictorial query languages. Thus, this article considers possible ambiguities related to visual representations of a query, and it does not consider ambiguities related to interactions between system and user.