ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001_ 
ISPRS, Vol.34, Pai 
46 
developed which includes 3D visualization and operation about 
querying 3D objects and some network analysis. A test area 
about 4 km 2 is used to create 3D-city model through CWeb3D 
system. 
2.1 3D CITY BUILDING MODEL 
3D CIYT MODEL ON INTERNET 
In order to visualization the 3D city data on Internet, building 
the 3D city model on Internet is required. The city model 
includes building model and terrain model. Most 
required applications of 3D city model are related to 
buildings. With the development of modern city, there 
are much change in function and shape for buildings of 
city area, from small to large, from simple to complex in 
shape and structure. However, the terrain of city is simple. 
The GRID model is used to construct the terrain model. 
Most required visualization applications of 3D city 
model are related to buildings. Now the city buildings 
become more and more larger and complex. In order to 
describe geometrical structure and attribute of large 
buildings, layer combined model is used to construct 
building model. In this data model, the large building is 
divided into some parts and each part is also divided 
into some building layers. A building layer is defined by 
having the same function, structure or attribute, which 
is a concept layer including one or more practical layer. 
The roof is considered as an independent layer because 
of its particularity. The LC Model is illustrated as follow. 
Shape 
r Layer 1 
Attribute feature 
s' Part one (Shapel) 'S 
Layer2 
Layer3 
Attribute feature 
Building Part two (Shape2) 
v: 
Attribute feature 
Fig. 1: Layer Combined Model 
In LC Model, the buildings can be decomposed into 
many basic primitives and roofs. For the reason that 
almost all the walls of buildings are vertical, these 
primitives can be constructed by a closed polygon with 
the assumption that every edge of polygon defines a 
wall, the geometry data of the primitives can be stored 
by the point group (Xi,yi, x 2 ,y 2 ,..., x n ,y n , Zi,z 2 ) in 
clockwise or counterclockwise direction, where Zi is the 
height of bottom surface and z 2 is the height of top 
surface. At the same time, a sole symbol was assigned 
to the every wall surface to mark it. Because of the 
complexity and peculiar shape of roof, the roof can be 
depicted with lots of triangles, which were created by 
outline points and inner feature points. 
structure, containing identifiable layer (sub-object). 
Every layer has the property of geometry and attribute 
like wall, roof, color, material, texture and so on, the 
topology among layers is also well described. 
Furthermore, it can be linked with different attribute 
data or texture data to enhance the reality of buildings. 
Compared to other data model, LC model has several 
advantages: 
• Not only outer shape of buildings but also 
inner object of buildings can be depicted. 
• Multiple data source can be integrated in LC 
model such as image data, vector data, attribute 
data, and so on. 
In this model, the buildings are comprised by complex 
2.2 CITY TERRAIh 
The city terrain is r 
building model, th< 
order to improve 
Internet, we select 
CTgC, 
3. THE SYSTEM 
DATE SET 
The system ai 
visualization th« 
Web 3D data, 
Zlatanova, 199£ 
The client-serve 
communication. 
1998), our app 
interact with d< 
query respective 
efficiently man< 
server-side and 
client-side, the I 
(Gruber, 1998). 
database, the v 
presented.