Multi-server Internet Gis: Standardization and Practical Experiences

An approach to an open infrastructure for geographic information on the Internet is presented in this chapter. This infrastructure enables data providers to publish their data independently, while enabling end-users to access data from several providers simultaneously, and integrate the data locally in a geographic browser. Our goal is that an end-user finds accessing geographic information in this environment as easy as if he would be working with a state-of-the-art GIS package with all data that he is interested in on his own computer. The key elements that are required are: a common format for publishing meta-data on each server, a common SQL derived query protocol, standard transfer file formats, and standard certificate based authentication procedures, for access control and (optionally) billing. An experiment with this approach has been carried out, with three data providers in The Netherlands: The Dutch Cadastre, the municipality of Almere, and the cable-TV company Casema. This chapter presents the major design decisions, the choices for the prototype environment, and the relationship to ongoing specification and standardization processes for geographic data. In particular the relationships with the proposed European CEN standards, and the recently accepted specifications from the OpenGIS consortium are described. The current wave of GIS software for Internet is based either on the file downloading paradigm, or on the picture paradigm (presenting a map as a JPEG picture), or on the client-server paradigm (creating a closed interaction between a client and a single server). Neither of these approaches can capitalize on the main potential of the Internet: integrated and easy access to a vast amount of geographic information on various servers. In addition to that the interaction protocol between client and server is typically proprietary, which means that someone who browses geographic information needs software from the same vendor as is used by the publisher. To make GIS popular on the Internet one needs to create for geographic information the same level of uniformity as the World Wide Web has done for text. The brilliance of the World Wide Web lies in the combination of the hypertext model with the Internet, together with a formatting standard for text (HTML). The hypertext model, however, does not work for geographic data: it is not particularly useful to jump from one map to the next. So another basic metaphor has to be used. The standard model for geographic data on the computer is the layer model. The layer model of geographic information systems relates to a paper map like the hypertext model relates to text on paper. To make geographic data on the Internet attractive one has to set a standard for the layer model, so that we can obtain a topographic layer from one source, a pollution layer from another, and a property layer from a third source, and dynamically merge them in a geo-web browser. To achieve this the following standard protocols have to be defined (in addition to support for authentication and billing): 1. a method to inquire which layers are available, and what data they contain; 2. a method to ask a particular layer from a particular source; and 3. a standard format for returning this information. Many aspects of these protocols are subject to ongoing standardization efforts. The format for returning geographic information is essentially a description of a transfer file format. The method for querying the meta-data has a clear relationship with the meta-data standards (CEN-META 1996), and Clearinghouse related activities (Absil van de Kieft and Kok 1997). The protocol for querying geographic information is new. The CEN has acknowledged the need for something like this; see (CENQUERY 1997), which specifies names and semantics of required spatial operators. The closest relevant specification for the query protocol is the OGC specification for SQL with simple geometric features. Despite the fact that it is relatively easy to identify protocols that can be used to address part of the problem, no comprehensive proposal exists so far that can be used to achieve open access to geographic data publishers on the Internet.