The influence of flying altitude, beam divergence, and pulse repetition frequency on laser pulse return intensity and canopy frequency distribution

Abstract

Eight airborne light detection and ranging (lidar) data collections were carried out over a forested and agricultural study site in Nova Scotia during 2005. The influences of flying altitude, beam divergence, and pulse repetition frequency on laser pulse return intensities and vertical frequency distributions within vegetated environments were investigated. Experimental control was maintained by varying each survey configuration setting independently while keeping all other settings constant. The land covers investigated were divided into highway, tall vegetation (mature and immature mixed wood regeneration stands), and short vegetation (hay field and potato crop). Laser pulse return data for 24 tall and 18 short vegetation plots were extracted, and the quartile heights of each vegetation profile were compared for each configuration. Observed laser pulse intensity values were found to be linearly related (coefficient of determination r2 = 0.98) to the peak pulse power concentration. A simple routine was developed to allow intensity data to be normalized and made comparable across datasets. By comparing the intensity and laser pulse return profiles it was found that reducing the peak pulse power concentration by widening the beam, increasing the flying altitude, or increasing the pulse repetition frequency tends to lead to (i) slightly reduced penetration into short canopy foliage by up to 4 cm, and (ii) increased penetration into tall canopy foliage (i.e., reduced maximum canopy return heights) by 15-61 cm. It is believed that a reduction in peak pulse power concentration delays pulse triggering within vegetation (i.e., increases penetration of the pulse into foliage) due to the need for increased surface area backscatter to raise the return pulse energy above some minimal threshold within the timing electronics of the sensor. Exceptions to these general observations were found in the high pulse repetition frequency data, where increased sample point density results in (i) increased noise and height range in the lidar distribution data, and (ii) increased likelihood of ground returns in the tall canopies sampled due to increased probability of pulses encountering canopy gaps. The implications of these results are that (i) laser pulse peak power concentration is the largest determinant of pulse return intensity and survey configuration based variations in canopy frequency distribution, and (ii) laser pulse height- and intensity-based models developed for vegetation structural or biomass assessment could be improved if they accounted for variations in peak power concentration.