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Developing an Internet-GIS Application using GML
 Ehsan Mohammadi M.Sc. Student, Dept. of GIS Eng. Email: mohammadiehsan@yahoo.com Ali Aien M.Sc. Student, Dept. of GIS Eng. Email: ali_aien@yahoo.com Ali A. Alesheikh Assistant Professor, Dept. of GIS Eng. Email: ali_alesheikh@hotmail.com K.N. Toosi University of Technology Vali_asr St, Tehran, Iran, P.C. 19697 Tel: +98 21 8789357 Fax: +98 21 877 9476 1. Introduction The ability to extend the audience of GIS to a large circle of users via Internet is growing as well as growth of Internet technology itself. Progress of the Internet technology is the result of many attempts and through these attempts, a great collection of data formats, programming languages, hardware and software appeared. A large part of Internet progress depends on the suitable approaches of dealing with data and information, and markup languages (MLs) play the main role in this progress. A markup language is a way of describing a document by placing tags in the document. Markup languages differ from many other programming languages, in containing loops, conditional logics, subroutines and some other programming structures. There are many markup languages with different applications these days. The most important MLs are: SGML, HTML, and XML. SGML stands for Standard Generalized Markup Language and is a powerful markup language in handling large quantities of structured data, but it is complex to use. HTML is the language of the web. It defines the way that images, multimedia and text are displayed in web browsers. It is simple to learn and use, but is not good at describing what data means. It is just a picture of information. XML stands for eXtensible Markup Language and was designed to describe data. It is created to structure, store and to send information. It also allows everyone to create his own information, send anything to anywhere for anybody. It has the capability of SGML and simplicity of HTML. It is also free and clear for any user. XML also is used to create other markup languages for particular applications. For instance, GML is a very useful and simple markup language for GIS applications that is extended by XML. In order to provide an Internet-GIS application, GML must include spatial and non-spatial information together. Some technologies help GML to show the graphic of spatial information, like SVG. SVG itself is a world of graphical tools, which can be handled with using just text. Integration of these technologies, that follow the principals of markup languages, helps designers to implement simple and versatile applications via Internet. 2. XML XML is a computer language for defining MLs and is used to create structured documents. Defining the contents of a document is equivalent to creating structured information in the document. XML opens a great view for creating other markup languages, and allows other applications to have a specific ML only for themselves. The focus of XML is on data and what data is. It means that the duty of defining tags and data is not predefined for XML and designers can do it according to their aims. Any XMLWriter allows the designer to describe parts of XML document, which composes of two main components: The structure of XML document and DTD or schema. The first component includes of a prolog and elements. The prolog for an XML document states some information to the parsers. This information expresses that this document is marked up in XML and can contain XML processor instruction. The prolog also includes text encoding, declaration of special pieces of text, and the DTD (Document Type Definition) or schema being used. The elements come after the prolog and let XML tags be written here. The second component is DTD, the grammar of the XML page. It actually is a tool to create and describe XML tags. Once a DTD is created and a document has been written based on that DTD, the document will be compared to the DTD that will cause the validation of the document. If the XML document follows the rules listed in the DTD, then the document is said to be valid, otherwise it is called invalid. Schema has the capability of DTD but differs in some characteristics, like, it is predefined for a specific application, then, there is no need to define how XML application will work. To use the XML document, some programs are needed to distinguish between text and information contents in the text. Parsers are programs that facilitate the task for users. Parsers can read XML documents and parse the XML information into data and markup. Internet Explorer 4 was the first web browser that included XML parser. Some of the renowned XML parsers are IE5, Expat, and Lark. The specific advantages of XML are:
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