Design of a Generic Virtual Measurement Workflow for Processing Archived Point Cloud of Trees and Its Implementation of Light Condition Measurements on Stems

Abstract

Virtual measurement workflow (VMW) was a generic data mining method developed in this study. It was used to extract tree information from archived point clouds under limited conditions by applying virtual measurements in virtual reality. As an example of how to use VMW for a specific topic, the VMW implementation of light condition measurement was further developed. This implementation could measure the temporal and spatial distribution of sunlight on virtual trees (stems). The output was expected as a new type of raw measurement data for tree morphology and phycological studies. At a single tree scale, it facilitated the quantitative interpretation of the growth strategy of branches. By measuring a single tree, it was found that only 4.34% of the stem surface could be illuminated throughout the day (8 h). Meanwhile, 35.87% of the stem surfaces were exposed to sunlight for less than one hour a day. A further mathematical processing of the output, i.e., γ (a ratio between relative area of triangles and relative quantities of triangles in each exposure duration group) improved the sensitivity of identifying differences in lighting conditions. Furthermore, we measured virtual trees of four species from an additional data source using a standardized setting. These include the sessile oak, gemu tree, Masson’s pine, and cherry tree. It was found that the shape of the crown was also significant for the distribution of solar energy on stems. For instance, the gemu tree had a cylindrical tree crown with narrow tree skeleton. A percentage of 10.38 of the surface on the gemu tree was illuminated throughout the day (8 h). The Masson’s pine had similar height and DBH with the gemu tree. However, the elliptical tree crown of the Masson’s pines prevented more lights. The area on the stem that was exposed to sunlight (8 h) dropped from 10.38% to 5.71%. This good differentiation of different crown structures might help this VMW implementation to continue to develop as a tool for identifying the effect of various crown shapes on radiosity for different tree species. The successful development of this VMW implementation had several practical applications for tree studies. Meanwhile, it demonstrated the overall feasibility of VMW and provided a paradigm for further development of other VMW implementations.

A reinterpretation of the gap fraction of tree crowns from the perspectives of computer graphics and porous media theory

“…For this case, the algorithm performance decreases with under- or oversegmentation issues, and spuriously large hexagonal prisms enveloping points of many leaves may be created; additionally, isolated incomplete foliage points may not be recognized, leading to detrimental deviations in the final volume-based GF vol calculation results. Fortunately, at present, many stationary terrestrial laser scanners ( Wang et al., 2021 ) of well-known brands, e.g., Leica, RIEGL and FARO, provide mature technologies to meet the density assessment requirements of most analyses. Alternatively, the user can adjust the scanning patterns by decreasing the scanning distance or aligning multiple scans in a parent coordinate system to improve the resolution of the tree point clouds.…”

Simulation on Different Patterns of Mobile Laser Scanning with Extended Application on Solar Beam Illumination for Forest Plot

“…The light quantity in the forest plot increases with the increased solar altitude angle at noon, when the layer of canopy tends to be exposed to incident light with the lowest shading disturbance, leading to a higher light transmittance of each tree species than at other times. Moreover, coniferous trees with needle-like leaves increase the amount and spatial arrangement of canopy gaps and decrease self-shading compared to deciduous trees with broader and dense leaves, with intense self-shading, which makes a difference in interspecific light transmittance [42]. Having access to tree structure data using an electromagnetic 3D digitizer [43] or other remote sensing technology [44] can allow us to directly use the mobile laser scanning simulation program to efficiently simulate solar beam illumination for forest plots, providing a basal component to establish a forest digital twin with the final goal of sustainable forest management [45,46].…”