ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001 
225 
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yiodel (DEM) 
sreograph by 
VirtuoZoNT digital photogrammetry system of the supresoft inc.. 
Spatial data of this DEM then transformed into VRML format. 
The vector information of geographic objects such as buildings 
was extracted based the DEM, and also transformed into the 
VRML file. 
The orthograph of the dalian port area and the surface images of 
other geo-objects act as texture. All these images are stored in a 
certain directory, ImageTexture node of VRML mapping them to 
relative geo-objects. When VRML browser creating virtual scene, 
the texture image will be obtained from the certain directory and 
mapped to relative modeling, making the scene look more real. 
Object in VRML file with more details looks more real, but on the 
other hand, VRML browser would take more time to display it. A 
Level-of-Detaii (LOD) grouping is one way to optimize the 
rendering performance and the display speed of VRML world. 
Create LOD that contain multiple copies of the object, called 
levels, modeled to various degrees of complexity. There are two 
types of LOD, distance and performance. A distance LOD 
specifies the distance at which each level is displayed. In the 
VRML file of digital harbor, we create four versions of a 
geo-object. The close-up is fully detailed. As moving away from 
the object, a distance LOD replaces the fully detailed object with 
a simpler model, as moving more far away, the model might be a 
simple bounding box with color. For the farthest distance, a 
empty level make the object disappear. Add these four versions 
to a LOD grouping so the browser displays a different model 
from the group depending on distance from current viewpoint. 
The range value of every level is set according to the attribute 
grade of the object. As a concomitant of LOD grouping, multiple 
scales spatial information of geo-objects is formed, based on 
which multi-scale display is available. 
3.2 INFORMATION QUERY 
In the digital harbor system, bidirectional information query 
based on spatial feature and based on attribute are available. 
3.2.1 SPATIALFEATHER BASED INFORMATION QUERY 
In VRML file, geographic objects are designed as child of anchor 
node, and the URL field of anchor node link to an ASP 
programme that would finish the required information query. In 
the virtual scene, when user touch a modeling in which he has 
interest, the ID of the correlative geographic object would be 
transported to the ASP programme at the apply-server, then a 
SQL indict was send through ODBC driver to the SQL server 
data-base, desired query started. The attribute information of 
object would be extracted from attribute database and 
downloaded to the browser and displayed as form. Fig2 is the 
illustration of the process. 
3.2.2 ATTRIBUTE BASED INFORMATION QUERY 
In the digital harbor system, an attribute query page is offered, in 
which user can organize the query sentence according to 
attribute field of object. Data records meeting with the query 
conditions would be downloaded by the ASP programme from 
SQL server database to browser and displayed as form. When a 
record was selected, the correlative geographic object would be 
displayed in the virtual scene as conspicuous sight. Fig3 
illustrate the process. 
Fig3. Illustration of attribute based information query 
External Authoring Interface (EAI) technology and Viewpoint 
node of VRML were used to finish such attribute based 
information query. 
For communication between a VRML world and its external 
environment an interface between the two is needed, this 
interface is call an External Authoring Interface and it defines the 
set of functions on the VRML browser that the external 
environment can perform to affect the VRML world. The EAI 
allows Java applets embedded on an HTML page to 
communicate with a VRML scene on that same page. A Java 
applet communicate with a VRML world by first obtaining an