Geography Markup Language
Geography Markup Language (GML) is an XML application that provides a standard way to represent geographic information. GML is developed and maintained by the Open Geospatial Consortium (OGC), which is an international consortium consisting of more than 250 members from industry, government, and university departments .
GML has been used as a standard for transporting geographic data but it can also be a useful format for storing it. Since GML is an application of the XML standard to geographic data, the XML database systems can also be used for the management of GML. However, the nature of data that GML represents and the query techniques it requires is quite different from other XML based languages. So far, only specifications for GML query languages have been proposed, but they are yet to be implemented. In this study, we employed Oracle9i, an XML-enabled database, to store XML and GML data. However, the conventional query languages (e.g. SQL and its extended versions) are not suitable for direct querying and updating of GML documents. Even the effective approaches working well with XML could not guarantee good results when applied to GML documents. XQuery is a powerful standard query language for XML. This study will investigate a query language specification to support spatial queries over GML documents by extending XQuery. The data model, algebra, and formal semantics as well as various spatial functions and operations of this proposed query language are presented here
Today, the Internet is the main platform for data sharing. The development of Internet has demanded that our technologies be extensible and comprehensible. Thus, to share and integrate geographic data in the Internet environment requires a standard data format, which is interoperable, extensible and suitable for Internet Technology. OGC, whose mission is to address the lack of interoperability between systems that process geo-spatial data, has established the XML-based standard, known as GML: "The Geographic Markup Language (GML) is an XML encoding for the transport and storage of geographic information, including both the spatial and non-spatial properties of geographic features" GML provides an XML-based encoding of geo-spatial data. It can be viewed as a basic application framework for handling geographic information in an open and non-proprietary way. Like any XML encoding, GML represents (geographic) information in the form of text, thus it can be readily intermixed with a wide variety of data types including text, graphics, audio and more .GML documents, like XML, are both human-readable and machine-parsable.So, they are easier to understand and maintain than proprietary binary formats. Like XML, GML also separates content of geographic data from its presentation. GML mainly describes the structure of geographic data without regard to how the data can be presented to a human reader
Since GML is based on the XML standard, it can readily be styled into variety of presentation formats 19 2.5. Application of XML standard to geographic data including vector and raster graphics, text, and sound Graphical output such as a map is one of the most common presentations of GML