Precise Lidar Data And True-Ortho Images

Precise Lidar Data And True-Ortho Images

3) Fiber Based Scanner
The Laser beam is deflected by a fixed linear array of fibers (fig. 4). Basic advantages are

Laser pulse rate is not linked to the viewing angle and flight height
Dense and regular scan pattern
No calibration required after factory setting
Forward and side looking mounting possible

Basic disadvantage is the fixed viewing angle. This concept is perfectly suitable for high precision Laser scanning and only used by TopoSys.

Figure 2: Fiber Based ALS System
EDM and airborne Lidar scanning philosophy
Airborne laser-scanning might be considered to be comparable with common distance measurements by an Electronic Distance Measurement device (EDM) but there are a some essential differences which need to be taken into account.

In a distance measurement by an EDM the requirement of a multiple measurement is normally made. The measuring duration or number of measurements is oriented to the "quality" of the reflector. The less the reflecting object (reflector, house wall, tree trunk, etc.) is known, the longer the measuring duration has to be.

Increasing the number of measurements will make the final result more precise and more reliable, as the EDM averages over the total number of individual measurements.

Airborne laser-scanning does not allow the repetition of an individual measurement, as the sensor position and the beam direction change continuously. At airborne Laser scanning, each distance measurement is unique. It cannot be verified or improved by multiple measurements. The reliability of a single measurement with a scanning laser is hence significantly less than that of a measurement with EDM.

For EDM long experience exists and guidelines are well known how to measure precisely:

No one would accept singular measurements for a reliable distance
Multiple Measurements depending on stability of reflecting surface, calculated distance is an weighted average of all measurements
With reflector:
- Required measurement duration about 1 sec
- Required number of measurements are between 100 and 1000
Without reflector:
- Required measurement duration more than 1 sec
- Required number of measurements are more than 1000

The reliability of an elevation model hence has to be attained by other means. As with EDM, an ensemble has to be found permitting dependable information. This can be accomplished by very dense measurements where it may be assumed that hardly any differences in height have occurred in the immediate vicinity. The selection of the ensemble has to be made dynamically and be adapted to the respective conditions.

Page 2 of 4
| Previous | Next |