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Precise Lidar Data And True-Ortho Images
Scan Pattern Figure 5 shows the typical scan pattern of a mirror based ALS system. Characteristics are:
Figure 5: Mirror Based Scan Patter Clearly the scan pattern does not follow the expertise of EDM and we need to ask ourselves why anyone does rely on such a singular distance measurement? Figure 6 shows the typical scan pattern of a fiber based ALS system. Obviously the scan pattern is fully in line with the requirements of EDM. Characteristics are:
Figure 6: Fiber Based Scan Patter Obviously, depending of the scan pattern and other underlying measurement principles like beam divergence, an ALS is suitable to fulfill certain applications or not. Choosing the right ALS will prevent from being disappointed with the result. Edge Detection and Object Extraction How well or how accurately do the multitude of single measurements describe a terrain or objects? The single measurements are more or less randomly distributed over a strip. In a homogeneous terrain, the measuring distance or distribution of the measurements is of subordinate importance. If, however, objects such as trenches, embankments, roof ridges are to be recorded, then corresponding demands have to be made on distribution and measurement spacing. Figure 7 shows a scene of buildings and trees. The Lidar raw data of the fiber based scanner “Falcon II” are shown in colored elevation. Due to the wide beam, the high scan rate and the huge amount of overlapping measurements, first and last echo information is detected from all pulses which hit the edges. This allows to precisely detect edges and objects. 
Figure 7: Edge And Object Detection Capability Of Fiber Based Scanner Systems There is also a similar problem for terrain structures. Here again a very old rule applies: If a signal is scanned with equidistant increments, the step width must be less than half the smallest form one wishes to recognize. In electrical engineering this requirement is known as Shannon’s theorem and has been used for over 50 years. If one wants to recognize the 1.0 m wide retaining wall, the sampling distance must not be larger than 0.5 m. What has been explained here in simplified form, applies analogously in 3D. If the measurement spacing becomes too large, then detailed structures will be lost. The lack of capability of acquiring break lines or of object detection is not common to airborne laser scanning by itself but to certain types of laser scanner systems. |