EFFECTIVE QUERY LANGUAGE AND USER INTERFACE IN GEOGRAPHICAL INFORMATION SYSTEMS (GIS)

Abstract

This dissertation focuses on the query language and user interface aspects of geographical (or spatial) information systems (GIS). While there is a lot of research going on in the area of storage management and access methods, little attention is given to query languages and user interfaces. To date, there is a paucity of query languages proposed for spatial systems. Although these languages have their unique strengths, they do not meet the needs of GIS users. A survey of these languages was carried out as part of this dissertation. The following are their major impediments: • difficult and complex syntax • lack of functionality and flexibility • unclear spatial operation semantics • insufficient spatial data types to model the real world • inadequate coverage on the graphical display aspects EGISQL, the query language/user interface proposed in this dissertation, attempts to overcome these deficiencies. The syntax of EGISQL is modelled after the de facto SQL standard. Like SOL, EGISQL hides the underlying physical implementation from the users. However, EGISQL is extended to encompass spatial concepts. Spatial data types include points, lines and polygons as the basic types. Users can define their own objects based on these basic types. EGISQL also supports metadata on spatial objects in order to facilitate enquiries about data objects. The semantics of the spatial operators and functions in EGISQL have been clearly specified. The spatial operations are based on mathematically proven binary topological relationships. These spatial operators form an orthogonal set from which more complex operators can be derived. Similarly, for spatial functions, the proposed set is adequate for basic usage but is further extensible through a programmable interface. EGISQL provides a pick function to allow the user to select objects on the screen and perform queries on it. EGISQL provides good control over the graphical representation of query results. The SOL syntax is augmented with graphical display capability for the tailoring of the display format. In addition, EGISQL allows the user to create the context for his spatial queries to provide a more meaningful background. A dynamic legend showing the meanings of the various representation is also available to users at any time to find out what the objects on the screen refer to. The proposed user interface is intended to reduce the semantic gap between the user and the system to the extent that the user can intuitively move his way around the system. The EGISQL user interface consists of three windows, one each for graphical display, non-spatial results a-d query formulation. This ensures that information is not cluttered on the same screen. EGISQL enables users to work with the results from previous queries - an important aspect of working with spatial data – and shows them to manipulate data already on the screen. The design goal of EGISQL has the GIS user in mind. The typical GIS user is not interested in convoluted syntax and physical data structures. He wants the necessary tools (operators and functions) to get his job done effectively and efficiently. EGISQL is designed to do that and to make the increased complexity introduced by the spatial dimension manageable.