International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
relationships for their documents using strict lexical and
syntactical constraints. XML documents are stored in plain test,
which introduces numerous beneficial consequences. Because
XML is human-readable, à plain text editor can be used to view
documents. It is also easily transmitted across platforms and
over the Internet. In addition, plain text is vendor-neutral, so
information that is stored in XML is not locked into a
proprietary binary format.
A popular use of XML in industry today is that of an
intermediary format to enable information sharing between
software applications. Data and content stored somewhere in a
native format can be converted to an XML format. After it is in
XML format, any other external program that understands
XML can use the data, possibly converting it back to its own
native format if required.
Text Binary |
XML
| Script DB
Fig 1. XML as a Transformation Intermediary
3. XSL IN XML TRANSFORMATION
XML uses XSL (eXtensible Stylesheet Language) to transform
contained data to other formats. XSL is a transformation and
formatting language based on XML. XSL, developed by W3C
is used to define a style for formatting or otherwise
transforming an XML document. Looked at another way, an
XML document is tree-structured and can be parsed and
transformed into another tree programmatically. XSL defines
programmatic XML transformation.
4. BENEFITS OF USING XML
Most large enterprises expand significant portions of their IT
(Information Technology) budget on developing methods to
transfer information between isolated systems. XML helps to
solve this problem by providing a flexible, platform
independent way of transferring data over standard network
protocols. Data that is being transferred between systems can be
easily translated to and from a standard XML format, by
providing a well-known, easy to use, intermediary format.
XML helps avoid the need for constantly linking different
systems to each others in ‘point-to-point’ solutions.
Some other benefits of XML are:
= Simplicity
= — Openness
= Extensibility
= Self-description
1154
= Supporting multi-lingual documents and so on.
5. WHAT IS GML?
GML is an XML-based data format for the transport and
storage of geographic information. As with other XML
variants, specific tags and attributes can be added to easily
extend GML, and GML can be validated, processed and
displayed using standard XML tools. Just as XML helps to
clearly separate content from presentation, GML aims to
separate geographic content (data) from geographic
presentation (maps and diagrams).
GML uses the W3C XML Schema Definition Language to
define and constrain the contents of its XML documents. The
GML specification defines some basic conformance
requirements for users to develop their own application
schemas. Software application attempting to process any
arbitrary GML user application schema must understand GML
and all the technologies upon which GML depends, including
W3C XML schema.
GML introduces an extraordinary flexibility by letting users
define their own application schemas suitable for their own
domains; however, this same flexibility also presents a
substantial difficulty for writing GML software applications.
Just as XML is now helping the web in general to clearly
separate content from presentation, GML does the same in the
world of geography. It is important for GML data to represent
the world in terms that are independent of any particular
visualization of that data. It is important to draw clear
distinctions between geographic data and graphic interpretation
of that data, such as maps. GML captures information about the
properties and geometry of the objects that populate the world.
Symbolizing the geographic data on map, the colors or line
weights and etc. are something quite different.
To make a map from GML, it is needed only to style the GML
elements into a form that can be interpreted for geographic
display in a web browser. Potential graphical display formats
include W3C Scalable Vector Graphics (SVG), Vector Markup
Language (VML) and the web 3D Consortium's X3D. [Lake,
2000]
6. A COMPARISON BETWEEN GML AND OTHER
FORMATS
To compare GML with other spatial data formats, three maps
were produced from a unique source. These three maps were
prepared for one of the Iranian provinces called Zanjan. First
map was generated in Esri's ArcView and included tree files
which represent a shapefile (*.shx, *.shp, *.dbf). Second map
was in GML format. This map was rendered using a GML
Viewer from eSpatial. And finally the third map was in SVG
format and included both graphic and attributes of features and
was displayed in Internet Explorer equipped with Adobe SVG
Viewer plug-in.
All three maps presented same data, but the size of occupied
memory is different. For the shape file the size is the smallest
and for GML is the biggest. The reason refers to including both
tags and data in GML file. Also text-based files are usually
larger than binary files, so is GML.
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