Laser scanning of high frequency tree movement

Abstract

Botanical trees demonstrate complicated motion patterns under wind forcing. Studying these motion patterns is relevant as tree parts may become hazardous during storms, and the movement of these trees affects the surrounding wind field, providing, for example, wind shadows. Eventually, wind enforced tree motion may even be incorporated in wind simulation models used to design liveable neighbourhoods. These motion patterns are, however, difficult to assess as in situ sensors lack coverage while video imagery lacks full 3D capacity. Recently, fast 3D laser scanners became available. For one such scanner, the Livox Avia, a workflow is presented that allows scanning tree movement at frequencies of up to 2 Hz. The scanner, possibly triggered to scan by a wind-meter, is building up a space-time array, consisting of range measurements at a give time, in a given direction. In a consecutive step, this space-time array is decomposed into single point clouds of a certain duration. The decomposition can be done over different time intervals; point clouds of shorter duration show lower coverage of measurements, but are also less affected by moving tree parts during scanning. After decomposition, motion vector fields between consecutive point clouds representing different time frames are estimated using the existing PlantMove algorithm. Applications of the workflow on different moving tree scenarios demonstrate how this novel measurement strategy is able to reveal complex and detailed spatial and temporal patterns of tree movement, with observed displacements on the order of decimeters, under wind stress. Extending the setup to e.g. three scanners will enable to capture wind induced complex tree movement at branch level, which is essential input for running reliable wind flow simulations at neighbourhood level.