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July2004 |
Present realities and the future of Internet gis
Recent History and Current State
To establish comprehensive web-based remote sensing tools and internet mapping services, many new web technologies were adopted currently in the GIS community, including new programming languages, new software architectures, and new communication protocols. This paper will only focus on two representative web technologies: Internet Map Servers and Java programming language. The establishment of Internet Map Servers provides integrated web-based map browsing, spatial query, and map overlaying capabilities (Plewe, 1997). Java language is commonly used for the development of on-line GIS tools and spatial analytical functions. The history of Internet Map Servers can be traced back to the development of the Xerox Map Viewer in 1994. The Xerox Map Viewer was using the Web Browser via HyperText Markup Language (HTML) format and Common Gateway Interface (CGI) programs to provide interactive mapping functions via the Internet. The technical framework of the Map Viewer is followed by many early on-line GIServices applications. Another important research project is the Alexandria Digital Library Project (http://www.alexandria.ucsb.edu/), which provides comprehensive services for on-line spatial queries, map browsing.
In general, most Internet map servers adopt a three-tier architecture for the system implementation (Figure 2). The first tier is called "the client tier" which includes the user-side web browser and user-resident Java applets/HTML documents. The client tier is used by the user to make requests and to view maps and remote sensing data. The second tier is the middleware tier that includes the Web Server and the Server Connectors (such as Servlet connectors or ASP connectors) to bridge the communication between clients and the map servers. The third tier is the data storage tier that includes the map server and the database server. The three-tier software architecture of web-based GIS provides customizable functions for different mapping applications and scalable implementation for different hardware. Another contributing web technology is the Java language for developing on-line remote sensing analytical and change detection functions. The Java language is a pure object-oriented language, designed to enable the development of secure, high performance, and highly robust applications on multiple platforms in heterogeneous, distributed networks. From the computer programming perspective, Java is similar to C and C++ programming languages which are capable of advanced functions, such as preprocessor, unions, pointers, and multiple inheritance.
Current Java System Development Toolkits (JDK) provides a series of well-defined Application Programming Interfaces (API) for image processing and display, such as Java 2D API and Java Advanced Imaging (JAI). The Java 2D API is a set of object classes for advanced 2D graphics and imaging, encompassing line art, text, and images in a single comprehensive model. The Java Advanced Imaging APIs are used for manipulating and displaying images and range in complexity from simple operations such as contrast enhancement, cropping, and scaling to more complex operations such as advanced geometric warping and frequency domain processing. These APIs are used in a variety of applications including geospatial data processing, medical imaging, and photography.
On the other hand, there are also several limitations in Java applets. First, the Application Programming Interfaces (API) of Java applets cannot provide a data streaming functions from servers to web browsers. The I/O functions of Java applets cannot handle external events outside the web browsers or applets. Also, there are not effective data compression tools or formats in the Java API, which may cause a very long download time for large-size remotely sensed imagery. An alternative solution is to create Java applications rather than Java applets.
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