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Procedures for live GIS access over web
Navita Relan, Pushpalata B. Shah, I.C. Matieda Space Applications Center, ISRO Ahmedabad 380015
Introduction
The Natural resources Information System (NRIS) is a countrywide network of spatial information systems, inter linked with each other. Under this NRIS umbrella, it is planned to use the Internet technology over an Intranet for spatial information dissemination. This will empower all the technocrats, decision-makers, managers and administrators, who do not have the means or need to have their own GIS departments, software and/or staff. As the emphasis is on natural resources, a major component of the information will be thematic map data that has to be served dynamically. Again, the spatial representation of the demographic data in the context of infrastructure, will be a better tool for planning purposes. Currently most GIS vendors provide this technology through their priced proprietary products called Internet Map Servers or Web Servers. However, other techniques such as CGI, Java-Servlets and CORBA can also be used for dynamic map serving as well as application customization. Using these techniques applications have been developed and tested. This paper deals with the comparative study and analysis of all these techniques. Architectures for dynamic map serving over a Web Any web based GIS application has a 3 - tier architecture for the complete chain of events, right from querying to getting back the results in a browser. The three tiers are described below. First tier - HTML/Applets User friendly HTML pages / Applets provide the Graphical User Interface (GUI) which presents form/s to the user for specifying his queries. Second tier - Server Side Programs The items of interest selected in the HTML forms are collected and passed by the browser to WEB server which in turn invokes the server side program specified in the action tag of the HTML form. The server side program parses the query string/s and then performs the required processing. Third tier - GIS / DATA Server The server program passes the user inputs to the GIS server, which performs the required processing. After execution of the query, the result is generated on the fly and sent back to the browser in the form of HTML pages. The result mainly is a map (vector data). For presenting this map/s to the client in the browser, it has to be converted as raster data. In case of non-spatial queries the outputs are raster images as well as tables. Any application developed using a proprietary "WEB SERVER" or "INTERNET MAP SERVER" has the limitation of being tied up with the specific GIS package in the background. However the use of CGI or Java-Servlets or CORBA does not restrict the choice of the GIS package. That is, these three approaches are GIS independent. They are OS independent also. The figure below gives a pictorial representation of the three-tier architecture for dynamic web map serving. The Three Tier Architecture for Dynamic Web Map Serving 
The In-house development The web implementation of the SDSS shell was attempted using all the three techniques viz. CGI, JAVA-Servlets and CORBA. The GIS package used is ARC/INFO version 7.2.1 on IRIX 6.3. The same has also been developed and tested on WINDOWS/NT with ARC/INFO version 7.1.1. Each of these is described in the subsequent sections. CGI approach CGI-c is an ANSI C-language library for the creation of CGI based World Wide Web applications. Here, whenever client requests a document, the server fetches it and sends it to client. But if requested URL is a CGI program then server acts as a middleman between client and CGI program. Server in this case just passes the client input parameters to CGI program. On the basis of inputs received from the HTML form, the CGI program, performs the GIS access task, and the environment for executing ARC amls is ensured through 'rlogin' command. The map results are saved as bitmap (.bmp) files. This raster information along with tabular information is posted to the client browser as an HTML page, which is generated on the fly. Currently Arc macrocode scripts are used for extracting GIS data. The effective response time will improve, if GIS objects replace these scripts. CGI is easy to program; can be written in programming as well as scripting language. Also, it has an open standard i.e. CGI is not bound to any particular web server.
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