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The 3rd ISPRS Workshop on Dynamic and Multi-Dimensional GIS & the 10th Annual Conference of CPGIS on Geoinformatics
Chen, Jun

1SPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS’’, Bangkok, May 23-25, 2001
participants (vendors) to collaborativeiy research, develop and
demonstrate new Standards-based Commercial technologies
that enable your web browser to seamlessly access, view and
exploit the vast, diverse and widely distributed geospatial data
holdings on the Web. The first round of the WMT was started in
May of 1999 and completed in September 1999. The Web
Mapping Testbed Phase 2 (WMT 2) expands the range of
interoperable web mapping services to meet a broader
cross-section of community needs. In addition to creating new
specifications, WMT 2 will result in more federal data coming
on-line using WMT protocols. The WMT rapidly produces
interface specifications that enable interoperability and
The Geography Markup Language (GML) is an XML
encoding for the transport and storage of geographic
information, including both the geometry and properties of
geographic features (OpenGIS Consortium, 2000). GML is
based on a common model of geography (OGC Abstract
Specification), which has been developed and agreed to by the
vast majority of all GIS vendors in the world.
GML version 1.0 is developed by OGC as a Recommendation
that was passed in May 2000, and OGC announces the release
of Geography Markup Language version 2.0 (GML) in April 4,
2001, a significant milestone in the development of
interoperable architectures for the use of spatial information
between commercial applications. The progress of GML 2.0 is
an example of the growing momentum for the acceptance and
use of OGC specifications around the world. GML represents
one of the most visible steps taken by the geospatial community
towards the vision of widespread spatial interoperability. GML
was originally implemented and tested through a series of
demonstrations that formed part of the OGC's Web Mapping
Testbed (WMT) conducted in the summer of 1999. These tests
involved GML mapping clients interacting with GML data
servers and service providers. Further exploration and
development of GML is now taking place within OGC's Web
Mapping Testbed Phase II and Geospatial Fusion Services
Testbed Phase I.
The Features of GML includes:
• GML encodes Feature Geometry and Properties
• GML encodes Spatial Reference Systems
• GML can transport geographic information and store
geographic information as well.
• GML can readily Integrate with Non-Spatial Data
• GML implements data transform between XML-to-XML.
• GML can transport behavior
Geographic data is concerned with a representation of the
world in spatial terms that is independent of any particular
visualization of that data. When we talk about geographic data,
we are trying to capture information about the properties and
geometry of the objects that populate the world about us. How
we symbolize these on a map, the colors or line weights we
choose to use is something quite different. Just as XML now
helps to clearly separate content from presentation for web
pages, so GML will do the same in the world of geography and
mapping (Ron Lake, 2000).
GML is concerned with the representation of geographic data
content. As well, we shall see we can also use GML to make
maps. This might be accomplished by developing a rendering
tool to interpret GML data. However, this would conflict with the
GML approach to standardization, and to the separation of
content and presentation.
To make a map with GML data, we must style the GML
geographical content into a suitable graphical presentation. This
involves the interpretation of the GML content using graphical
symbols, and often some sort of transformation of the geometry
of the GML data into the geometry of the visual presentation.
We refer to this interpretation process as map styling (Ron Lake,
2001). Many of the same tools and technologies, such as XSLT
(XML Transformation Language), can be used to perform this
styling operation. Potential graphical display formats include
W3C Scalable Vector Graphics (SVG), Vector Markup
Language (VML), and the Web 3D Consortium's X3D.
XSLT is well suited to XML-to-XML transformations. XSLT is
extensible. XSLT supports the concept of an extension function.
An extension function allows an XSLT engine to perform
operations that are beyond the capabilities of the XSLT
language such as geometric or coordinate transformations. In
the majority of cases we may use XSLT as a tool to style the
GML data into an XML graphical format such as SVG VML or
The process of transforming the graphical presentation into a
viewable image we shall refer to as graphical rendering. At the
present time there are a variety of graphical renders available
for each of the different XML graphical formats, some native to
the browser (e.g. Internet Explorer 5.0's built in VML processor),