Quality assessment of terrestrial laser scanner ecosystem observations using pulse trajectories

Considering the trajectories of pulses from terrestrial laser scanners (TLS) can provide refined models of occlusion and improve the assessment of observation quality in forests and other ecosystems. By considering the space traversed by light detection and ranging (lidar) pulses, we can separate empty regions of an ecosystem sample from unobserved regions of an ecosystem sample. We apply this method of TLS observation quality assessment, and analyze Compact Biomass Lidar 2 (CBL2) TLS observations of a single tree and of a deciduous forest stand. We show the contribution of information from each TLS scan to be inconsistent and the combination of multiple scans to have diminishing returns for new information, without guaranteeing complete coverage of a sample. We quantitatively investigate the effects of imposing information quality requirements on TLS sampling, for example, requiring minimum numbers of observations in each region or requiring regions to be observed from a minimum number of independent scans. We show empirically that rigid, predefined TLS sampling schemes, even with hypothetically dense coverage, cannot guarantee successful samples in geometrically complex systems such as forests. Through these methods, we lay the groundwork for on-the-fly assessment of observation quality according to several modeling-relevant metrics which enhance TLS ecosystem assessment. We also establish the value of flexible deployment options for TLS instruments, including the ability to deploy at a variety of heights.