Fahmy Asal 
  
states that, an accuracy of 3 to 4 centimeters can be obtained if the aircraft flies within 30 to 40 kilometres from a 
ground base. If more than one ground base can be established, this is useful in strengthening the quality of the laser 
scanning process. Moreover, installing more than one receiver on the aircraft improves the positioning accuracy. 
However, this increases the final cost of the project. 
For sometime in the past, only one GPS observation could be obtained per second, now this has been increased to be 
potentially greater than 10. Unfortunately, this is not sufficient for airborne laser scanning processes, since a positional 
measurement is essential at every time of pulsing as stated above. This means that about 2000- 5000 (depends on the 
pulsing rate of the system) positional measurements may be needed every second, which is outside the capability of 
direct measurement using present GPS technology alone. In order to help solve this problem, an INS is used which also 
has the ability to measure the attitude of the platform to enable the direction of the scanning pulse to be determined. 
3.2.3 Attitudes of the Laser Sensor Unit. Attitudes of the laser sensor are described by the rotation angles about the 
. main axes of movement and called; roll, pitch and yaw where (figure: 1): 
a) Roll: is the rotation angle of the aircraft about the horizontal longitudinal axis (X), which takes the main direction 
of movement. It can also be expressed as the angle of rotation of the aircraft, which causes a wing-up or wing 
down movement. 
b) Pitch: is the rotation angle of the aircraft about an axis (Y), which is normal to the longitudinal axis (X). Also, it 
may be expressed as the angle of rotation, which cause a nose-up or nose down movement. 
C) Yaw:is the rotation angle of the aircraft about its vertical axis (Z) so that the longitudinal axis deviates to left or to 
right from the flight line. 
If the laser system is rigidly fixed to the fuselage of the aircraft then these rotations correspond to the aircraft 
movements. 
The Inertial Navigation System (INS) is a device that has the capability of determining the aircraft attitudes with high 
accuracy over short periods of time. The drop off in accuracy is due to the INS values drifting which can be overcome 
by integrating with GPS. The INS has gyroscopes for measuring the attitudes and accelerometers for determining the 
flying accelerations. This allows the aircraft position (X, Y, Z) to be determined with high frequency by applying 
double integration to the measured acceleration. Synchronization of measurements (timing) is important between the 
INS, GPS and the laser pulsing time. 
3.24 Data Storage and Processing Unit. Another important component of the airborne laser scanner is a PC unit 
with large RAM and large hard disk capacity. The PC is connected to the different units of the system and controls their 
operation. 
33 Quality of the LiDAR DSM in Different Applications 
Due to continuous improvements in the airborne laser scanning system and due to decreasing cost of such new 
technology, there are a number of commercial companies offering a LiDAR service. This section will review a few 
projects that have been undertaken to show the variety of potential applications. Vaughn et al. (1996) states that a laser 
altimeter, profiling system can measure the ellipsoidal height with accuracy better than 10cm, if careful calibration of the 
laser scanner, accurate attitude measurements, precise determination of the platform trajectory by using GPS and 
relatively low flying height are applied. Also, Csatho et al. (1996) describes the use of NASA's laser system, which 
covers a swath of 130 to 200m in width for studies of the polar region. The aim of these studies was to obtain a digital 
elevation model (DEM) and topographic maps from airborne laser altimetry data. A dense distribution of measured 
points to an accuracy from +10 to +20 cm was obtained and used to create the DEM. Another DEM was created of the 
same area using traditional aerial photogrammetric methods. A comparison between DEMs indicated a strong 
agreement. 
Flood and Gutellius, (1997) states that, airborne laser scanning surveys are more economical and more productive 
specially in areas where traditional surveying methods such as aerial photogrammetry and terrestrial surveying methods 
are impractical or impossible. The airborne laser scanning system is found beneficial if used in areas such as coastlines, 
wetlands, forests, power lines and transportation corridors. Also, it can be used successfully in topographic mapping 
and in mapping areas of high dynamic environmental changes due to episodic disturbances such as earthquake and 
hurricane regions. Wright et al. (1996) explains that an airborne laser scanner called airborne topographic mapper 
(ATM-ID, developed by NASA has the capability to produce high resolution maps, was used in mapping and profiling 
arctic ice in Greenland, Svalbard, and Iceland in October 1996. This instrument was also used to survey a 600km 
  
48 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 
  
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