Precise Lidar Data And True-Ortho Images

Precise Lidar Data And True-Ortho Images

Dipl.-Wirtschaftsing. Alexander Wiechert,
TopoSys GmbH
Obere Stegwiesen 26
88400 Biberach
a.wiechert@toposys.com

Introduction
To process an elevation model from the data acquired by an airborne laser scanner (ALS or Lidar) one needs to know four basic parameters (fig. 1):

Distance from the sensor to the ground/object
Sensor position
Sensor orientation (attitude)
Beam deflection

Figure 1: General Principle Of Airborne Laser Scanning (ALS)
The sensor systems mainly used differ very much in the technical parameter but the usability and reliability of the sensors and the derived products are closely linked to the mean of beam deflection and the scan pattern.

Laser beam deflection
IMU and GPS provide position and attitude of the sensor, but not the orientation of the laser beam. The method which is used to move the beam across the swath has got a great impact on the quality of the sensor system and the quality of the products as well as on mission restrictions and sensor efficiency.

Currently three concepts are used:

1) Oscillating mirror
The Laser beam is deflected by an oscillating mirror (fig. 2). Basic advantages are a very flexible adjustment of the required viewing angle. Basic disadvantages are

Additional errors due to the mechanics, accelerations and wear-out
Inhomogeneous scan pattern on ground
Strong reciprocal relation between viewing angle and scan frequency
Regular calibration required

Figure 2: Oscillating Mirror
This makes this concept not suitable for high precision Laser scanning but quite useful for small scale mapping. This concept is used by e.g. Optech and Leica.

2) Rotating mirror
The Laser beam is deflected by an rotating polygon mirror (fig. 3). Basic advantages are a flexible adjustment of the required viewing angle. Basic disadvantages are

Additional error due to misalignment of the surfaces of the mirror and wear-out
Inhomogeneous scan pattern on ground at wide scan angles
No practical use of the theoretically possible wide scan angle due to loss of measurements at the border of the scan
Regular calibration required

Figure 3: Rotating Mirror
This makes this concept also not suitable for high precision Laser scanning but quite useful for small scale mapping or for terrestrial scanners. This concept is used by e.g. Terrapoint or Riegl.

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