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XML is an abbreviation which means Extensible Mark-up Language. It has been defined as "A Text based meta language, for the definition of computer languages which describe the Syntax and structure of data" 
The advantages of XML are that it has the ability to describe the structure of data by itself. It can also describe data in tree formats and graphics, the data is also portable. Though the major disadvantage is that due to conversion of text it is slower to access data; also security problems.
XML provides storage (XML documents) Schemas,( DTDs, XML Schemas, Relax NG) query languages (XQuery, Xpath, XQL,XML- QL, QUILT, e.t.c). programming interfaces (SAX, DOM, JDOM)
The objective of XML is
XML database is used to store and retrieve XML documents by using the relational database system which is also used for the same purpose. There are 3 types of XML databases, these are explained below.
The structure of XML data to be processed determines the choice of database to be used for an application. There are two classes of XML documents which are: Data-centric XML documents: These are documents whose data have a symmetrical structure like a relational database table and are usually small amount of data with less complex XML tags, And Document-centric XML documents: These are not so regular in structure and the data contains a mixture of XML tags and text. The difference between these two is in their storage technology.
1 . Relational databases
These store XML documents in a relational database in the form of tables by associating each element of a table with the element of another table, there could be one or more tables that elements and attributes. The XML documents that are used for a particular database table must match the format.
2. XML-enabled databases
3. Native XML databases
Native XML databases store XML documents, though not physically and they consist of an internal logical model that is based on XML. It supports XML query languages like XPATH or XQuery.
1.3 STORAGE OF XML DATA
For data to be transferred from an XML document to a database, the XML document schema (DTD, XML Schemas, RELAX NG, etc.) has to be mapped to the database schema, the software for transferring data is then built on top of the mapping for data transfer. XML query languages such as XPath and XQuery can also be used by the software.
"Several mapping techniques have been proposed for storing XML documents in flat tables of relational databases. XML database systems store XML documents in native storage managers. These systems relieve the processing from additional layers for mapping the logical data to the physical layout, which eventually slows down query processing, subtrees are also stored in clustered physical pages, XML documents are stored in pre-order traversal in order to cluster sub-elements together".
1.4 XML Query Languages
"XML query languages can be used over any XML document. To use these with relational databases, the data in the database must be modeled as XML, thereby allowing queries over virtual XML documents. With XQuery, either a table-based mapping or an object-relational mapping can be used. If a table-based mapping is used, each table is treated as a separate document and joins between tables (documents) are specified in the query itself, as in SQL. If an object relational
mapping is used, hierarchies of tables are treated as a single document and joins are specified in the mapping.
With XPath, an object-relational mapping must be used to do queries across more than one table. This is because XPath does not support joins across documents. Thus, if the table-based mapping was used, it would be possible to query only one table at a time".
1.5 XML Schema
The XML Schema is a special XML language that describes what a correct document may contain this helps to check if a document follows certain rules or not and is a W3C recommendation. One of the advantages of XML Schema is that it provides various range of datatypes that can be used, for example byte, int and float - string types like string, token, ID - date and time types - and types for storing binary data likes hexBinary. It can also be used to declare user defined types and elements even from existing ones.
Example of XML Schema
<? xml version ="1.0" encoding ="UTF -8"?>
<xs:schema xmlns:xs =" http // www.w3.org /2001/ XMLSchema ">
<xs:element name =" article ">
< xs:complexType >
< xs:attribute name =" title " type =" xs:string "/>
< xs:attribute name =" author " type =" xs:string "/>
</ xs:complexType >
</ xs:element >
</ xs:schema >
"Augmented Reality (AR) allows the user to see the real world, with virtual objects superimposed upon or composited with the real world. Therefore, AR supplements reality, rather than completely replacing it. Ideally, it would appear to the user that the virtual and real objects coexisted in the same space." 
The main features of augmented reality are that it is interactive with real time, it registered in 3-D and lastly it combines the real and virtual.
Augmented Reality helps to improve how the user acts on real life situations and it also influences his interpretation because there is a better understanding when virtual objects that can't be seen with the physical eyes are displayed.
Augmented reality has a lot of advantages, its applications can be used in various aspects of everyday living to serve as guides for an additional accuracy to the user because it allows a realistic interaction with the images generated in 3-D and in real life.
Some examples of fields where they are applicable are in Entertainment industry, where humans are combined with virtual backgrounds, in robot path planning, in annotation and visualisation, in the mobile industry where virtual reality gives direct assistance to the user in several ways, then in the medical field where virtual reality is used by healthcare professionals for surgeries and in manufacturing and repair, where the information contained in the user guide is displayed virtually for the user and so on. The potentials of Augmented reality applications are virtually unlimited
2.3 Data management
"Any significant real-world application of mobile augmented reality will require a large model of location referenced data. While it may appear that a natural approach is to develop application specific data formats and management strategies, it has been shown that such an approach actually prevents reuse of the data and ultimately produces additional complexity in developing and maintaining the application. Therefore we developed a general model schema and a 3-tier architecture to maintain a central data store and application specific reuse through dedicated transformations".
To integrate the data from a variety of sources, structures of events and attributes can be described in XML schema, data access and distribution code can also be generated, XML can be used to describe the components of a specific scenario which are also validated against the associated XML schema. XML is a description platform that's why it is chosen, since it is an open and standardized Meta language
ideally suited for hierarchical structures.
2.4 DATA MANIPULATION
Firstly a model is defined for the data format and the next step is to feed the database by imputing data, then the data is structurally transformed and manipulated and lastly it is passed on to the user via the development of appropriate applications.
"The typical tasks is to Import or extract information from source data formats based on XML or other formats and map it to the data model.
Non-XML source formats also require the combination of an
appropriate parser with an XML generator to map the foreign"
format to XML. 
2.5 THE RELATIONSHIP
XML as the description platform for other languages, is a technology is quite flexible that is why it is practically the most commonly used for storing data especially in the relational database management systems (RDMS) which is a very effective foundation for the implementation of XML storage system.
Through the benefit of it's flexibility, the Augmented Reality 3D data models are able to directly associate with the elements of an XML structure due to the hierarchical framework that exists in 3D data models. The data definitions of Augmented Reality can then be separated by domain and individually converted without interrupting the compatibility of the application, because of the flexibility of XML technology.
The ultimate payoff of XML is self description and systematic arrangement through the use of XML namespaces and XML schemas, there are also a lot of software tools that can be used for analysing and editing XML documents.