International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
Nowadays, a number of vector formats are being used on the
Web e.g. Flash, Precision Graphics Markup Language (PGML),
Virtual Reality Markup Language (VRML) and Web Compute
Graphic Metafile (WebCGM). However, there was no widely
standard format supported by the tools for creating Web pages.
W3C therefore developed a new standard format for two
dimensional vector graphics, SVG that matches the needs of
content providers and browsers like.
SVG is an ideal candidate for displaying GIS information in a
Web browser. All typical GIS features can be readily displayed
and manipulated, and data are transferred in a compact vector
format rather than raster format. SVG knows shape types for 2D
graphics including rectangles, circles, ellipses, lines, polylines,
polygons, symbols and path elements as shown in figure |.
These graphical objects can be grouped, styled and transformed
using stylesheets such as XSLT and CSS. SVG is already
supported by several browsers, but it still requires a plug-in for
current versions of Microsoft Internet Explorer and Netscape.
Adobe offers a free SVG plug-in called Adobe SVG viewer 3.0.
Styling Engine
Figure 1 Relationship between GML and SVG
3. METHODOLOGY FOR IMPLEMENTATION
In order to test the editing capability of GML and SVG, we
propose a framework. The major target of the framework is to
design algorithms, mechanism and workflow of distributing
geographic information online with more advanced editing
functions. It focuses on examine specific problems and
approaches in using SVG as a user interface for editing
geographic data online.
3.4 Development of data model and design algorithms
GML has ability to provide a framework for modelling
application relationships. This allows application to be designed
independently yet still able to interoperate when using the same
base object model (Toon, 2001). Application framework can
depend upon one or more GML schemas by importing them
shown in Figure 2. For example, schema 1 in the application
framework imports the GML feature schema. Since this
research will focus on mobile application, mobile GIS data
model will be used. This research will based on existing mobile
GIS data model developed by Kwan (2002) and will have
further enhancement. Based on the model, the dynamic
database was created to respond to the tasks from the mobile
application. The contents in the dynamic database were
automatically updated to reflect the situation of real world
(Kwan, 2002). It is expected that further enhancements will be
performed for the model, for example, modeling the spatial
relationship, modeling spatial entities and so on. Besides, the
presented rules of mapping can be generalized easily to
translate oriented objects models, in UML or another formalism,
for files in XML Schema (Miguel and Cirano, 2002). The
relationships between features and attributes in an application
will be modelled by UML.
For graphic presentation, Scalable Vector Graphics (SVG) is
applied. SVG is an XML grammar for describing 2D graphics.
It includes elements for vector shape features, raster images,
animation and text (Randy, 2002). GML is used as a spatial
data interchange format, and SVG provides a format for
viewing spatial data. As figure 2 shows that style engine will be
used to transform GML data into SVG graphics. The figure
summarizes the work flow of data structure and transformation.
Mobile GIS model Schema 1 [|* Schema 2
framework
| Import
*
GML framework Feature < Temporal
Topology »| Geometry » XLinks
|
Y
Styling Engine
N\. Transfer to SVG
I — =
Map portrayed in SYG M :
in Web Browser
Figure 2 Overview of making vector maps on Web with GML
3.2 GML data Generation
To test the model, Hong Kong digital base map was collected
from Land Information Centre (LIC) of Survey & Mapping
Office, Lands Department for the framework. Scale 1:1000
digital topographic map (B1000) arc used. Since the native
format of the digital base map is in ArcInfo coverage format,
data conversion from coverage format to GML data is required.
The procedure of generating GML data is shown in figure 3.
Y
y
e00 "| coverage Shapefile
GML
Figure 3 The data conversion process of generating GML data
The e00 files are the export format of ArcInfo coverage. They
are first decompressed to ArcInfo coverage and then converted
to shapefiles format. The reason of converting shapefiles is that
many free tools are available to convert shapefiles to GML such
as deegree command line shape to GML converter 1.5
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