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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
  
For Autodesk MapGuide, functions are extended to include edit 
capability by the SDF COM Toolkit to enable users to create, 
edit, or delete actual SDF files. From the client end, the use of a 
MapGuide server requires downloading and installing a 
proprietary plug-in. Data are in proprietary format not readily 
accessible to or useable by other applications. 
In short, over the Internet, it is essential to have a method of 
storing and representing data in an open, extendable and 
flexible way, but also in a way that is vendor-independent. 
Therefore Geography Markup Language (GML) was designed 
to help solve above problems and promote interoperability of 
spatial data. GML is an XML grammar written in XML Schema 
for the modelling, transport, and storage of geographic 
information including both spatial and non-spatial properties of 
geographic features (OGC, 2003). It is developed by Open GIS 
Consortium in order to promote spatial data interoperability and 
open standard. It is specially designed for Internet mapping 
which has the potential to change how people share mapping 
information through Internet. 
GML upholds the principle of separating geographic contents 
from its presentation. This offers more flexibility in data 
production, handling as well as visualization. GML is used as 
the standard of data exchange and then can be transformed into 
vector format such as SVG for graphic visualization so that 
downloads can be considerably shortened and even more 
important for bandwidth is the scalability inherent to vector 
lines. Once on the client browser, vectors are scalable without 
return trips to the server (Randy, 2002). The performance hence 
can be improved. It is essential for Web mapping which 
requires fast delivery time and high performance. It seems that 
GML and SVG are ideal for transporting and storing geographic 
information over Internet using advanced XML technology and 
provide potential capability to integrate different geographic 
information together. 
The aim of this paper is to investigate and develop methods for 
generalizing map online and editing vector-formatted geospatial 
data to Internet and mobile user using emerging standards — 
GML and SVG. The main purposes are: 
| To develop method and usage practices for generalizing 
the graphic representation for geospatial data in real-time. 
2 To develop data model and algorithms for testing the 
ability of editing data online. 
3 To build a prototype to demonstrate the effectiveness of 
the proposed framework. 
2. BASIC CONCEPTS 
2.1 Geographic data in GML 
GML is an XML based encoding standard for geographic 
information developed by the OpenGIS Consortium (OGC). 
GML 1.0 was released as a Recommendation Paper in February, 
2001 and GML 2.0 was approved in April 2000 as an OpenGIS 
Implementation Specification. GML 2.0 provides for XML 
Schema, complex features and feature relations, extending 
GML’s scope beyond the basic 2-dimensional vector-based 
descriptions in Simple Features. Currently, the latest version of 
GML is GML 3.0 which has been approved in January 2003. 
New additions in GML 3.0 support complex geometries, spatial 
and temporal reference systems, topology, units of measure, 
159 
metadata, gridded data, and default styles for feature and 
coverage visualization (OGC, 2003). 
The overriding goad behind the implementation of GML was 
providing a means for the transport and storage of geographic 
information, which includes both spatial and non-spatial 
properties of geographic features especially in Internet 
environment. Characteristics of GML are as following: 
Text-based, Open and Extensible: GML documents are text 
based and extensible. They can be easily edited, maintained and 
updated by any world processor. They can readily integrate 
geospatial data with other data types such as text, voice, 
graphics and video ete. Since relationships in XML elements 
are expressed in terms of parents and children, GML is 
extensible to define the content inside the elements. 
Encoding Feature Geometry and Properties: GML is based 
on the abstract model of geography developed by OGC. The 
model describes the world in terms of geographic entities called 
features. À geographic feature is “an abstraction of a real world 
phenomenon; it is a geographic feature if it is associated with a 
location relative to the Earth" (OGC, 2003). A feature consists 
of properties and geometries. Properties include usual name, 
type, value description. Geometries are composed of basic 
geometry building blocks such as points, lines, curves, surfaces 
and polygons. In the latest version, GML 3.0 is extended to 
support 3D geometry and topological relationships also. It 
works closely to the features supported by common GIS data 
formats. 
Transporting geospatial data over the Web: When the GML- 
coded geospatial data are transported, all markup elements that 
describe all spatial and nonspatial features, geometry, and 
spatial reference systems of the data are also transported to the 
recipient. So information will not get lost and distorted in the 
process of data transport and transformation. It is important for 
real time data access and transport in the Internet Environment 
(Peng, Z. R. and Tsou, M. H., 2003). 
Separating content from data display: GML is not a 
presentation language for data display. It is different from other 
GIS data formats, which separates content from data 
presentation. Like XML, GML can be transformed into other 
vector graphics formats such as SVG for data display. The 
common style engines are XSL, XSLT and CSS. This allows 
different styles applied in the same GML documents. 
Not a programming language for data processing: Like 
XML and HTML, GML is only markup language, not a 
programming language. GIS functions such as buffer analysis, 
spatial overlay, image processing and network analysis can be 
done with the aid of other programming languages such as C++, 
Visual Basic, Javascript or Java. Some other XML-based 
technologies like XSL, XSLT, Xpath and XML Query 
Language can also help to do GML data query and 
manipulation. 
2.2 GML data visualization in SVG 
Most of graphics on the Web are represented as images with a 
sequence of colored pixels such as GIF (Graphics Interchange 
Format), JPEG (Joint Photographic Experts Group) and PNG 
(Portable Network Graphics). An alternative approach is 
sending instructions for drawing features like lines or curves, 
which offers great advantages over pixel based formats.