Large off-nadir scan angle of airborne LiDAR can severely affect the estimates of forest structure metrics

Liu, J, Skidmore, A, Jones, S, Wang, T, Heurich, M, Zhu, X and Shi, Y 2018, 'Large off-nadir scan angle of airborne LiDAR can severely affect the estimates of forest structure metrics', ISPRS Journal of Photogrammetry and Remote Sensing, vol. 136, pp. 13-25.


Document type: Journal Article
Collection: Journal Articles

Title Large off-nadir scan angle of airborne LiDAR can severely affect the estimates of forest structure metrics
Author(s) Liu, J
Skidmore, A
Jones, S
Wang, T
Heurich, M
Zhu, X
Shi, Y
Year 2018
Journal name ISPRS Journal of Photogrammetry and Remote Sensing
Volume number 136
Start page 13
End page 25
Total pages 13
Publisher Elsevier
Abstract Gap fraction (P-gap) and vertical gap fraction profile (vertical P-gap profile) are important forest structural metrics. Accurate estimation of P-gap and vertical P-gap profile is therefore critical for many ecological applications, including leaf area index (LAI) mapping, LAI profile estimation and wildlife habitat modelling. Although many studies estimated P-gap and vertical P-gap profile from airborne LiDAR data, the scan angle was often overlooked and a nadir view assumed. However, the scan angle can be off-nadir and highly variable in the same flight strip or across different flight strips. In this research, the impact of off-nadir scan angle on P-gap and vertical P-gap profile was evaluated, for several forest types. Airborne LiDAR data from nadir (0 degrees similar to 7 degrees), small off-nadir (7 degrees similar to 23 degrees), and large off-nadir (23 degrees similar to 38 degrees) directions were used to calculate both P-gap and vertical P-gap profile. Digital hemispherical photographs (DHP) acquired during fieldwork were used as references for validation. Our results show that angular P-gap from airborne LiDAR correlates well with angular P-gap from DHP (R-2 = 0.74, 0.87, and 0.67 for nadir, small off-nadir and large off-nadir direction). But underestimation of P-gap from LiDAR amplifies at large off-nadir scan angle. By comparing P-gap and vertical P-gap profiles retrieved from different directions, it is shown that scan angle impact on P-gap and vertical P-gap profile differs amongst different forest types. The difference is likely to be caused by different leaf angle distribution and canopy architecture in these forest types. Statistical results demonstrate that the scan angle impact is more severe for plots with discontinuous or sparse canopies. These include coniferous plots, and deciduous or mixed plots with between-crown gaps. In these discontinuous plots, P-gap and vertical P-gap profiles are maximum when observed from nadir direction, and then ra
Subject Physical Geography and Environmental Geoscience not elsewhere classified
Keyword(s) Airborne LiDAR
Gap fraction
Off-nadir scan angle
Vertical gap fraction profile
DOI - identifier 10.1016/j.isprsjprs.2017.12.004
Copyright notice © 2017 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS).
ISSN 0924-2716
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