Abstract
The transmittance of laser pulses through the forest canopy was studied as a function of forest attributes (inventory parameters) and the scanning angle from the point of view of elevation modeling. Here transmittance is defined as the ratio of the number of pulses within a threshold of the detected elevation model to the total number of transmitted pulses. Airborne laser scanning (ALS) using a Leica ALS50-II scanner took place on 25 July 2009 in the Evo test area in Southern Finland. The total number of circular field test plots with a radius of 10 meters was 246. Several of the test plots were observed from two different flight lines, and this resulted in 454 observations. Multiple regression analysis was applied to calculate statistical parameters for the scanning angle and the forest attributes. The canopy layer is an important factor that influences the number of ground hits. We found that the characteristics of the trees determine the number of transmitted pulses penetrating down to the ground level. When using scanning angles between 0 to 15 degrees in forested areas, the results showed that the scanning angle did not have a statistically significant effect on the vegetation penetration nor on the number of ground hits. It appears to be feasible to increase the scanning angle for boreal forest elevation modeling if some degree of local shadowing can be accepted in the data. By increasing the scanning angle, it is also possible to perform laser scanning and digital aerial photography simultaneously even over forested areas. Nationwide laser scanning in Finland and Sweden is carried out with scanning angles of ±20 degrees, but further studies are needed to assess the results when using even larger scanning angles.
Highlights
When a laser beam hits the forest canopy non-vertically, the other side of the canopy objects is partly or totally shadowed and there are areas on the ground receiving no hits or only a small number of hits
When looking at the relationship between the transmittance and the scanning angle, Figures 1–3 depict how the transmittance of laser pulses through the canopy of a boreal forest occurs in relation to the scanning angle
We investigated airborne laser scanner (ALS) transmittance as a function of scanning angles in a boreal forest area
Summary
When a laser beam hits the forest canopy non-vertically, the other side of the canopy objects is partly or totally shadowed and there are areas on the ground receiving no hits or only a small number of hits (shadowing effect). In urban mapping with airborne laser scanner (ALS), the facades of buildings in built-up environments and ground elevations cannot be obtained from shadowed areas. At the time of the advent of airborne laser scanning, the shadowing problem was considered to be a serious challenge. The TopoSys airborne laser scanner was designed to include a scanning angle of ±7.1 degrees off-nadir in order to minimize shadow formation [1]. Laser scanning has been applied to tasks such as the creation and updating of nationwide elevation models and standwise forest inventories where data covering large areas need to be collected cost-efficiently. Scanning angles of ±15 degrees have been generally accepted in operational work, but larger scanning angles are applied, e.g., in Finnish and Swedish nationwide airborne laser scanning, the corresponding scanning angle is ±20 degrees
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