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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
LIDAR employs a powerful laser sensor positioned under an
aircraft. The laser sensor consists of a transmitter and receiver.
The distance from the sensor to points on the terrain below is
measured based on the location and altitude of the aircraft.
These two parameters are calculated using Global Positioning
System (GPS) and Inertial Navigation System (INS)
technologies. When a pulse hits the ground, the reflected light is
collected by a receiver. The first return may locate the top of a
tree, while the last return ideally locates the ground beneath the
tree canopy.
The spatial resolution of the DEM highly depends on the
ground cover. In a study conducted by Pereira and Janssen
(1999), it was found that the vertical accuracy of LIDAR in
open areas with hard surface is approximately 15 cm. However,
in areas with very tall and dense canopy, the photogrammetric
heights may have larger errors. Reutebuch et al. (2003)
indicated that over mature forested areas LIDAR provides
vertical accuracy of 25 cm, which is enough to generate an
accurate DEM. As one of the fastest growing remote sensing
technology, LIDAR is expected to provide even better accuracy
in the near future.
In order to assist designers in locating forest roads, various
computer-aided methods using DEMs have been introduced.
Reutebuch (1988) developed a computer program, ROUTES, to
estimate road gradient, length, and stationing along the possible
road alternatives using a DEM. In the program, the designer
could digitize the contours and use the digitizer puck to locate
the road on a large-scale contour map. Although the graphical
user interface (GUI) of ROUTES was primitive, the program
worked well and provided the designers with the ability of
quickly looking at alternative road locations at varying scales.
A computer program, PEGGER, was developed to automate
initial forest road design through the use of a Geographic
Information System (Rogers and Schiess, 2001). The
performance of the program relied on DEMs, which must
accurately represent the actual ground conditions. PEGGER
was a tool for quickly analyzing many road alternatives based
on a specified road gradient given by a user. It was not capable
of considering environmental and economic constraints such as
soil types, hydrology, property lines and slope classes.
However, forest road design using PEGGER with accurate
DEM data was expected to be more feasible and less time
consuming than the traditional road design methods.
Coulter et al. (2001) developed a method of forest road design
using high-resolution DEM data (ImxIm) from LIDAR. In the
method, road elevations were assigned to each pixel within the
road template to calculate earthwork from the difference
between road and surface elevations. This method was only
applicable to straight road segments and could not locate
horizontal or vertical curves. It also could not calculate total
road cost or consider environmental requirements. However, it
was probably the first method showing that using high-
resolution DEMs from LIDAR in forest road design may
significantly decrease design time and effort spent both in the
field and in the office.
A decision support system can be defined as an interactive
system that assists a user to conduct decision making tasks by
easily accessing decision models and data (Watson and Hill,
1983). Advances in the processing speed and real-time
rendering and viewing of three-dimensional (3D) graphics on
microcomputers permit locating a route interactively on a 3D
display of a ground surface generated by a high-resolution DEM
data. A 3-D forest road alignment optimization model,
TRACER, aided by an interactive computer system, was
developed as a decision support system. The model provides a
designer with a quick evaluation of alternative road paths to
locate the best path with minimum total road cost, while
considering design specifications and environmental
requirements. TRACER relies on a high-resolution DEM to
provide terrain data for supporting the analysis of road design
features such as ground slope, topographic aspect, and other
landform characteristics. In the model, two optimization
techniques are integrated: linear programming to minimize
earthwork allocation cost and Simulated Annealing to optimize
vertical alignment. In this paper, the features of the model were
described and an application was presented.
2. MATERIAL AND METHODS
2.1 Input Data
Input data include DEM and attribute data, road design
specifications, and environmental requirements. The DEM data
is a set of scattered metric data points (X and Y coordinates and
Z as elevation). In designing forest roads, the resolution of the
DEM should be in the range of 1.0 m to 3.0 m to represent the
actual terrain conditions (Akay, 2003). In this study, the DEM
data set of 2.0 m resolution is developed using LIDAR data set
collected from western Washington by Aerotec (1999). Bilinear
interpolarion method is used to extract ground elevations from
the DEM (Lemkow, 1977). First, the grid cell that contains the
horizontal position of the current point is determined based on
its coordinates, then elevation of this point is estimated by
interpolating between the elevations of four corners on this grid
cell. The attribute data, soil types and stream data, is
represented in the same format as the metric data points. It is
easy to incorporate other attribute data into the model if desired.
The road design specifications are geometric specifications,
local site specifications, and economic data. The geometric
specifications include road gradient, horizontal curves radius,
vertical curve length, distance between curves, and safe
stopping distance for driver safety. Road template
specifications, turnout dimensions, distance between road
sections, design speed, vehicle specifications, and traffic
volume are also included in the geometric specifications. Local
site specifications consist of swell and shrinkage factors of soils,
vegetation cover, geological data, stand data. and distance to
local resources of road construction materials. Economic data
include the unit costs for road construction, maintenance, and
transport activities. The environmental requirements considered
in this study are minimum allowable road grade for proper
drainage, minimum distance from the riparian zones and
minimum stream-crossing angle for stream protection, and
maximum height of cuts and fills for soil protection.
2.2 Displaying Terrain Image
The model uses graphics routines from NewCyber3D (2002) to
display high-resolution image of the terrain in 3D, based on
DEM data (Figure 1). The real-time 3D stereo display and
stereo image composition is also supported by NewCyber3D.
Above-below stereo display format is used to generate stereo’
scenes, which requires liquid-crystal glasses and an infrared
emitter. The graphic programming runs in full-screen mode to
be compatible with this format. At the standard sixty fields per
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