Keywords: 
• distributed GIS /
• Internet/Intranet GIS /
• software architecture /
• layered model

Abstract: Developing scalable distributed computer architecture has been more important in many application domains.Most of recent researchers believe that a lack of awareness and problems of accessibility of spatial data are significant bottlenecks to increasing numbers of users and applications.Client/Server architecture GIS provides the opportunity to optimize networks resources and increase developer productivity.However, implementation at the application designs level lacks application architecture and a methodology to assist the designer and provide a degree of rigor and consistency to the design process.Traditional Client/Server architecture can not work well under the Internet/Intranet circumstance. [FK(W21。40ZQ] To develop distributed GIS applications, the application designer must first determine what components of the application are candidates for distribution and what are the criteria for distribution.The application model is provided to aid the designer in developing distributed GIS applications.It addresses the problem of functional decomposition beyond the traditional methodologies.A general layered architecture model for distributed GIS which includes four layers: user interface layer, application layer, application interface layer and data service layer, is proposed in this paper. The proposed four-layer application model is a model for segmenting the GIS application, in order to identify candidates for distribution.With respect to the open systems interconnect (OSI) reference model for network protocols, this analysis falls within the application layer.The four-layer model is characterized by: the nature of the data flowing across the layers, bi-directional data flow across the interface, limited knowledge of the internal functions and data structures of other layers, atomic self-contained functions, programming interface into and out of the layers, consisting of a set of APIs that provide location and protocol independence, and each one of the layers can be isolated and defined according to the above characterization. This paper introduces the main idea of layered architecture and two instances of the model under Internet/Intranet circumstance.With this application model, users may be easy to operate Internet/Intranet GIS, including pan, zoom, query and analysis.In Client side, users operate the GIS like that in Server side, and seamless link with main database.The tasks of Client, application Server and database Server are separated.User interface layer is implemented.These instances are implemented with Java, DCOM and ADO technologies.