Wedge-Filtering of Geomorphologic Terrestrial Laser Scan Data

Abstract

Terrestrial laser scanning is of increasing importance for surveying and hazard assessments. Digital terrain models are generated using the resultant data to analyze surface processes. In order to determine the terrain surface as precisely as possible, it is often necessary to filter out points that do not represent the terrain surface. Examples are vegetation, vehicles, and animals. Filtering in mountainous terrain is more difficult than in other topography types. Here, existing automatic filtering solutions are not acceptable, because they are usually designed for airborne scan data. The present article describes a method specifically suitable for filtering terrestrial laser scanning data. This method is based on the direct line of sight between the scanner and the measured point and the assumption that no other surface point can be located in the area above this connection line. This assumption is only true for terrestrial laser data, but not for airborne data. We present a comparison of the wedge filtering to a modified inverse distance filtering method (IDWMO) filtered point cloud data. Both methods use manually filtered surfaces as reference. The comparison shows that the mean error and root–mean-square-error (RSME) between the results and the manually filtered surface of the two methods are similar. A significantly higher number of points of the terrain surface could be preserved, however, using the wedge-filtering approach. Therefore, we suggest that wedge-filtering should be integrated as a further parameter into already existing filtering processes, but is not suited as a standalone solution so far.

HOVE-Wedge-Filtering of Geomorphologic Terrestrial Laser Scan Data

“…Because the section for the two angle thresholds looks like a wedge ( Figure 1A, blue dashed), we called it the wedge-method. As mentioned in our previous work [14], this filtering method is not Of course, it is rare that two connection lines have exactly the same direction. Thus, you need to define a threshold.…”

“…The work presented is based upon "Wedge-filtering of geomorphologic terrestrial laser scan data" [14], where the general procedure of the Wedge Absolute method is described. In the following, we explain our modifications that led to the Iterative Method while the basic principle of Wedge Absolute is briefly reproduced for understanding.…”

“…Because the section for the two angle thresholds looks like a wedge ( Figure 1A, blue dashed), we called it the wedge-method. As mentioned in our previous work [14], this filtering method is not a stand-alone solution for calculating a digital terrain model because it has some weaknesses. The Wedge Absolute method on its own provides satisfactory filtering results only for terrain, without steep inclines.…”

“…For the calculation of the new filter method, the VB.NET program mentioned in the article "Wedge-Filtering of Geomorphologic Terrestrial Laser Scan Data" [14] was further developed. Figure 8a shows a screenshot of the VB.Net program, in which a small section of the test area view is seen from the side (blue arrow in Figure 7).…”

“…The method of Panholzer and Prokop [14] and Puttonen et al [8] uses a priori information of the TLS configurations and scanner-centred coordinates to determine the ground level. Integrating these coordinates in the filtering process is an important additional filter parameter.…”