Insights from the Unseen - Occlusion in Forest Laser Scanning

Daniel Kükenbrink , Matthias Gassilloud, Benjamin Brede, Aline Bornand, Kim Calders, Wout Cherlet, Markus P. Eichhorn, Julian Frey, Charis Moana Gretler, Bernhard Höfle, Teja Kattenborn, Lennart Klinger, Martin Mokros, Timo Pitkänen, Ninni Saarinen, Louise Terryn, Hannah Weiser, Anna Göritz

Laser scanning is a powerful tool for assessing the structural complexity of forests and its role in ecosystem processes and functioning. However, laser scanning acquisitions are inherently affected by occlusion (where objects block laser pulses), resulting in unobserved volumes within the 3D representation of the forest. Although occlusion is a well-known and frequently discussed challenge for estimating forest structural information from laser scanning data, it is rarely quantified. 

Here, we offer a perspective on occlusion in forest laser scanning, aimed at researchers across the forest remote sensing community, ranging from point cloud specialists to ecologists relying on laser scanning derived structural products. We define and distinguish the principal types of occlusion, and examine their primary causes across multiple platforms (terrestrial [TLS], mobile [MLS], UAV-borne [ULS], and airborne [ALS] laser scanning platforms). We discuss platform-specific challenges and practical strategies to minimise occlusion across diverse forest types and acquisition scenarios. We further synthesise available tools for occlusion detection and mapping, and highlight key research opportunities, spanning uncertainty quantification, point cloud completion, virtual laser scanning, and intelligent autonomous laser scanning acquisition design. By framing occlusion as a mappable and actionable property of laser scanning data, this perspective aims to promote more rigorous assessments of point cloud suitability and inspire methodological advances in forest structural assessment.