Distributed GIS

Distributed GIS is an integrated framework to combine multiple graphic information systems (GIS) resources and GIS workstations and servers located in different physical places for high-level interoperability and federation of GIS operations and user tasks. Distributed GIS can provide various geographic information, spatial analytical functions, and GIS Web services by linking multiple GIS and geographic information services together via wired or wireless networks.

The designation distributed reflects that the hardware and software components of distributed GIS are physically distributed in different computers, which are connected via the Internet or other types of networks. Interoperability is the key issue for the establishment of distributed GIS, because distributed [Page 116]hardware machines, programming languages, operating systems, and other online resources may vary drastically.

Distributed GIS is a paradigm shift in the development of GIS. To provide online geographic information services effectively, most distributed GIS applications utilize open and interoperable computing environments and protocols (such as World Wide Web, File Transfer Protocol, and Z39.50 protocol) and distributed programming languages (Java, JavaScript, Python, or C#) to connect multiple machines and servers together.

The architecture of distributed GIS is platform independent and application independent. It could provide flexible and distributed geographic information services on the Internet without the constraints of computer hardware and operating systems. Figure 1 shows three different types of GIS architectures.

Traditional GIS are closed, centralized systems, incorporating interfaces, programs, and data. Each system is platform dependent and application dependent. Client-server GIS are based on generic client-server architecture. The client-side components are separated from server-side components (databases and programs). Client-server architecture allows distributed clients to access a server remotely by using distributed computing techniques, such as Remote Procedure Calls (RPC), or by using database connectivity techniques, such as Open Database Connectivity (ODBC). The client-side components are usually platform independent, requiring only an Internet browser to run. However, each client component can access only one specified server at one time. The software components on client machines and server machines are different and not interchangeable. Different geographic information servers come with different client-server connection frameworks, which cannot be shared.

Distributed GIS is built upon distributed system architecture. Tanenbaum and Steen defined a distributed system as a collection of independent computers that appears to its users as a single coherent system. The most significant difference between traditional GIS and distributed GIS is the adoption of distributed component technology and distributed computing languages, which can be used to access and interact with multiple and heterogeneous systems and platforms. Under a distributed GIS architecture, there is no difference between a client and a server. Every GIS node embeds GIS programs and geospatial data and can become a client or a server based on the task at hand. A client is defined as the requester of a service in a network. A server provides a service. A distributed GIS architecture permits dynamic combinations and linkages of geospatial data objects and GIS programs via networking.