|
Implementing Internet GIS with Java based Client/Server Environment
Java Servlets are dynamically loaded modules run inside the JVM and service requests from a server. Because the servlet is running on the server side, it does not depend on browser compatibility. Servlets can be used for any number of Web-related applications. A servlet can handle multiple requests concurrently and can synchronise requests. Also, it can forward requests to other servers and servlets. Thus servlets can be used to balance the load among several servers. Servlet can communicate a user through an applet interface embedded in any HTML page via HTTP. Figure 1 depicts the execution of Java servlets and communication with applet via HTTP. 
For developing Web pages there are ever expanding numbers of hypertext markup language (HTML) tags that can be used to format the way items look on pages. The eXtensible Markup Language (XML) has been developed to compliment HTML, taking over from simply defining what data looks like to describing what it actually is. Whilst XML uses the same tag structure as HTML, an XML element can declare its associated data to be an address or post code, a point value with associated attribute data or any other desired data element. The intrinsic flexibility of XML can provide a number of tangle benefits to users. XML can allow integration of data from disparate sources, with middleware applications pulling data from different databases and translating it into neutral XML for client-side processing. An XML document can contain descriptions of data which can be used in multiple applications, each application using the specific tags, and delivering users appropriate view for their particular task. Integration with Relational Database Management System (RDBMS) Databases are becoming an increasingly important feature of GIS and an increasingly valuable asset. Relational Database Management System (RDBMS) provides number of functions critical to the effective operation of a GIS and it is developed to manage and share data in an orderly manner and to ensure that the integrity of the database is maintained. Thus the value of user’s data can be increased greatly by the use of a Relational Database Management System (RDBMS). RDBMS enables fast storage and retrieval of large amounts of information. The interface that is used to insert, manipulate and extract data is called the Structured Query Language (SQL). The database for the Internet GIS explained in this paper consists basically of two types of data: tabular data stored in a relational database (dbf) and ESRI shape (shp) files stored on disk. In this model, the map and attribute data are removed from the shape and dbf files and placed them in MySQL relational database. Thus the map features are stored in a clearinghouse as SQL based database (MySQL) according to OpenGISâ standards (ALOV, 2002). Clearinghouse is a container which contains the list of datasets accessible via server side and their metadata. The clearinghouse can be either XML document or a set of tables in SQL database. This clearinghouse consists of tables that hold all the available attribute data and the attribute table includes fields that can be used to store binary data (BLOB). This model is more flexible and allows incremental downloading of data. The server spreads vector maps to client in the most efficient way and uses advanced caching to reduce network traffic. Other leading RDBMS like Oracle, MS-SQL Server, Interbase or MS-Access can also be used to store and manage layers and attribute data. Design and implementation of low-cost Internet GIS Designing and implementing Internet GIS with freely available software tools can be successfully done with little care. To obtain map data in standard formats we have to depend on commercial products like ArcView, MapInfo etc. Therefore, small investments are required for the generation of basic GIS data, but the cost will be almost nil by using by using products like ALOV Map (free Internet GIS software), the Java Development Kit (JDK), Tomcat Java Web Server, MySQL RDBMS etc for developing a Client/Server based Internet GIS. ArcView software is used to generate shp and dbf files and that is the only commercial product used here. ALOV Map is free portable Javaâ application for publishing vector and raster maps on Internet and interactive viewing through Web browsers. It supports the complex rendering architecture, the unlimited navigation and allows working with multiple layers, thematic maps, hyperlinked features and attribute data. Figure 2 illustrates the structure of the system comprised of ALOV Map, Tomcat Java Web Server and MySQL relational database. Map data transfer is controlled by a set of servlets in Tomcat Web Server and a map applet at the client end on request via HTTP. The servlets available in the Servlet container are: MapServlet - main servlet, ContextListener - allows to reload all objects on server, WMSServlet - implements Web Map Service and UploadServlet - upload shp and dbf files to SQL database and creates metadata and register the datasets. The following are the six simple steps to build a low-cost Internet GIS. 
Table 1. Simple steps to build ALOV Map based Internet GIS
Map window, legend, buttons like zoom out, zoom in, pan, web link, feature selection, info, text boxes for selection domains, themes, query and help are available with the applet for user interactivity. Examples for query results are given in figure 3. Maps are shown with decimal geographic co-ordinates and a zoom scale is given at the right bottom end of the map panel in the applet. Web links have been given for some layers to be connected with related web sites. The flexibility of the system is that modification or updating of map data can be done at any point with the help of UploadServlet interface. 
Conclusion The Internet is redefining the collection, management and dissemination of spatial information. In this new Internet computing environment, spatial applications are moving from the desktop to database servers where they can be professionally managed and accessed through simple Web browsers. The rapid growth of the Internet is having profound impact on GIS industry, bringing GIS out of its specialized niche into the broader information technology market. Internet offers increased dissemination and use of geographic information in all walks of life. The Java platform allows developers and business users to create exact application that run over networks. XML is a generalized way to describe arbitrary data types independent of how the data are used or presented to users. Databases are becoming an increasingly important feature of GIS since it allows many users to access large amount of data through computer networks or through the Internet. Small footprint Internet GIS applications like ALOV Map and other free server based applications enables Client/Server based computing with less investment of money and other resources. References
|