Anderson Kyle E, Glenn Nancy F, Spaete Lucas P, Shinneman Douglas J, Pilliod David S, Arkle Robert S, McIlroy Susan K, Derryberry DeWayne R
Department of Geosciences, Idaho State University, Pocatello, ID, 83209, USA.
Boise Center Aerospace Laboratory, Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID, 83725, USA.
Environ Monit Assess. 2017 Oct 23;189(11):578. doi: 10.1007/s10661-017-6300-0.
Terrestrial laser scanning (TLS) provides fast collection of high-definition structural information, making it a valuable field instrument to many monitoring applications. A weakness of TLS collections, especially in vegetation, is the occurrence of unsampled regions in point clouds where the sensor's line-of-sight is blocked by intervening material. This problem, referred to as occlusion, may be mitigated by scanning target areas from several positions, increasing the chance that any given area will fall within the scanner's line-of-sight from at least one position. Because TLS collections are often employed in remote regions where the scope of sampling is limited by logistical factors such as time and battery power, it is important to design field protocols which maximize efficiency and support increased quantity and quality of the data collected. This study informs researchers and practitioners seeking to optimize TLS sampling methods for vegetation monitoring in dryland ecosystems through three analyses. First, we quantify the 2D extent of occluded regions based on the range from single scan positions. Second, we measure the efficacy of additional scan positions on the reduction of 2D occluded regions (area) using progressive configurations of scan positions in 1 ha plots. Third, we test the reproducibility of 3D sampling yielded by a 5-scan/ha sampling methodology using redundant sets of scans. Analyses were performed using measurements at analysis scales of 5 to 50 cm across the 1-ha plots, and we considered plots in grass and shrub-dominated communities separately. In grass-dominated plots, a center-scan configuration and 5 cm pixel size sampled at least 90% of the area up to 18 m away from the scanner. In shrub-dominated plots, sampling at least 90% of the area was only achieved within a distance of 12 m. We found that 3 and 5 scans/ha are needed to sample at least ~ 70% of the total area (1 ha) in the grass and shrub-dominated plots, respectively, using 5 cm pixels to measure sampling presence-absence. The reproducibility of 3D sampling provided by a 5 position scan layout across 1-ha plots was 50% (shrub) and 70% (grass) using a 5-cm voxel size, whereas at the 50-cm voxel scale, reproducibility of sampling was nearly 100% for all plot types. Future studies applying TLS in similar dryland environments for vegetation monitoring may use our results as a guide to efficiently achieve sampling coverage and reproducibility in datasets.
地面激光扫描（TLS）能够快速收集高清结构信息，使其成为许多监测应用中很有价值的野外仪器。TLS采集的一个弱点，尤其是在植被环境中，是点云数据中会出现未采样区域，即传感器的视线被中间物质遮挡。这个问题，即遮挡问题，可以通过从多个位置扫描目标区域来缓解，这样能增加任何给定区域至少从一个位置处于扫描仪视线范围内的机会。由于TLS采集通常用于偏远地区，在这些地区采样范围受时间和电池电量等后勤因素限制，因此设计能最大化效率并支持提高所采集数据数量和质量的野外方案很重要。本研究通过三项分析为寻求优化干旱地区生态系统植被监测TLS采样方法的研究人员和从业者提供参考。第一，我们根据单次扫描位置的距离量化遮挡区域的二维范围。第二，我们使用1公顷样地中扫描位置的递进配置，测量额外扫描位置对减少二维遮挡区域（面积）的效果。第三，我们使用冗余扫描集测试每公顷5次扫描的采样方法产生的三维采样的可重复性。分析是在1公顷样地中5至50厘米的分析尺度下进行测量的，并且我们分别考虑了以草和灌木为主的群落中的样地。在以草为主的样地中，中心扫描配置和5厘米像素大小能够对距离扫描仪18米以内至少90%的区域进行采样。在以灌木为主的样地中，只有在12米的距离内才能对至少90%的区域进行采样。我们发现，使用5厘米像素来测量采样存在与否时，在以草和灌木为主的样地中，分别需要每公顷3次和5次扫描才能对总面积（1公顷）的至少约70%进行采样。使用5厘米体素大小，1公顷样地中5个位置的扫描布局提供的三维采样的可重复性在灌木样地中为50%，在草样地中为70%，而在50厘米体素尺度下，所有样地类型的采样可重复性几乎为100%。未来在类似干旱地区环境中应用TLS进行植被监测的研究可以将我们的结果作为指导，以有效地实现数据集中的采样覆盖和可重复性。
