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communication links over large distances, the system must address the time differences that are inevitable between 
physical events and the corresponding received data. 
This current prototype employs the XT Communicator software from Xyphos corporation to form this communication 
backbone. This product consists of agents installed in every computer comprising the system and then forms a virtual 
private network amongst them. Using this product, the interconnection of data communications between network nodes 
and the sharing of data that is of interest to several nodes is made transparent to the application software. 
With this approach, the system interconnections are open and scaleable to any number of network node entities. 
Furthermore, the actual physical layer of the communications network is left open. Data formats for the system are also 
open and determined by the producers of data. The data communications networks used in this prototype are based on 
Internet communication and employ RF data links at the remote work site to tie mobile equipment into the network. In 
order to vary the network performance over a wider range for comparison purposes, an additional high-speed link 
employing a satellite to yield higher data rate communications between the operator station and the work site 
equipment, was used. 
    
    
    
    
   
    
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Figure 1: Block diagram of the system 
2.1 Modeling Station 
In order to locate information in a geographic information system one must first create a 3D model of the mine terrain 
and infrastructures as well as dynamic objects populating this terrain such as excavators and trucks. Many techniques 
are used to create such models but most of these models must follow some basic criteria. 
e The model must be accurate. 
e The model must be easily interpreted. 
e The model must be compatible with VR display systems. 
e Each model should have various levels of detail. 
The most common representation of a VR model is by the use of a scene graph technique [Wernecke, 1994]. Contrary 
to a pure geometrical representation where no optimization is possible, a scene graph can be optimized to hide various 
latencies of the display and communication system. This is because a scene graph is a structured representation of the 
various relationships between elements of a scene. A scene graph allows for easy propagation of properties along the 
hierarchy as well as allows the notion of level of details and context switching. 
One of the most important and fundamental problems in VR modeling is the selection of the right level of detail (LOD). 
This requirement comes from a fundamental compromise between display speed and model complexity. By using a well 
designed scene graph one can hide various latencies such as display speed and communication load using techniques 
such as geometric and communication LOD and graph culling. This is an important consideration since a complete 
mine model can be extremely large. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 93