International Archives of the Photogrammetry, Remote Sensing 
  
  
     
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Figure 1. Displaying 3D image of the terrain 
second, scanning each image takes half the duration of an entire 
field. Using a monitor operating at 120 fields per second, each 
eye sees 60 fields of image per second, while the other 60 fields 
are prepared for the other eye. Therefore, when the left eye can 
see an image, the right eye cannot (Lipton and Meyer, 1984). 
2.3 Interactive Features 
The alternative forest road paths are selected through real time 
interaction with 3D image of the terrain. The interactive features 
of the model are provided by NewCyber3D (2002), using an 
improved 3D OpenGL accelerator. The initial trial road path is 
“traced” by locating a series of intersection points on the 
terrain, using computer cursor (Figure 2). The model provides 
the designer with the road geometry information and attribute 
data in real time to locate control points with respect to road 
design specifications and environmental requirements. If a 
candidate intersection point is not acceptable by one or more 
constraints, the model warns the designer by changing the 
colour of the line between the previously selected intersection 
point and the candidate intersection point. The designer can 
zoom, pan, rotate, and scale the area in order to search 
intersection points around the terrain. The model has various 
interactive display features such as navigation control, bird- 
view, and real-time flythrough. 
        
  
  
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Intersection 
Points 
  
  
  
  
Figure 2. Selecting intersection points of a trial road path 
and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
2.4 Calculating Horizontal and Initial Vertical Alignment 
After locating the trial path, the model automatically calculates 
the horizontal and initial vertical alignment considering road 
design specifications. Road gradient is restricted by the 
maximum allowable road grade considering the truck 
performance. The gradient is also limited by the minimum 
acceptable road grade to provide proper drainage. 
In order to determine whether any type of curve is necessary, 
the model calculates the difference between two consecutive 
road grades (A) and horizontal deflection angle (A) for each 
intersection point along the roadway. In forest roads, it is not 
necessary to locate a vertical curve if A is less than or equal to a 
specified percentage of difference between grades (Ain). which 
provides a log truck with a safe passage of the vertical curve. If 
A is greater than Ay, and A is zero, the model locates a vertical 
curve. If A is less than or equal to Amin and A is greater than 
zero, then the model locates a horizontal curve. Otherwise, à 
straight segment (tangent) is located. If there is a case where A 
is greater than Amin and A is greater than zero, the model warns 
the designer to choose a different control point to avoid 
overlapping of vertical and horizontal curves. 
To ensufe a safe roadway passage along the vertical curves, the 
model is constrained to generate a minimum adequate curve 
length, which allows a log truck to pass a curve without 
bottoming out and provides safe stopping distance for driver 
safety. To provide safe continuous operation along the 
horizontal curve, the model is constrained minimum radius, 
acceptable road grade on horizontal curve, and minimum safe 
stopping distance. 
2.5 Optimizing Vertical Alignment 
After locating the horizontal alignment and initial vertical 
alignment, the model computes the total cost of construction, 
maintenance and transportation costs. The road construction 
activities considered in this study are construction staking, 
clearing and grubbing, earthwork allocation, drainage and 
riprap, surfacing. water supply and watering, and seeding and 
mulching. Road maintenance activities include replacing the 
aggregate, grading, maintaining culverts, cleaning ditches, and 
removing brush. Transportation cost varies with vehicle 
performance, equipment costs, gradient, and curvature. The cost 
of road design activities are estimated by multiplying their 
average unit costs by the quantity of design parameters (cg m, 
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m^, m). 
The model searches for the optimum vertical alignment with 
minimum total cost among the large number of alternative 
alignments. Simulated Annealing (SA) algorithm, using a 
neighbourhood search, is employed to guide the search for the 
optimal vertical alignment. The model considers technically 
feasible grades within the specified elevation ranges of the 
intersection points. The SA algorithm was developed based on a 
metallurgical technique of annealing, in which a solid material 
is heated and cooled back slowly into an optimal state to 
produce the best product (Beasley et al., 1993). In this study, 
SA has been selected as an optimization technique because it 
generally provides a near-optimal solution and it is easy to 
implement into the model. 
For each alternative vertical alignment, the model calculates 
earthwork volumes using average end-area method, minimizes 
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