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基于深度学习与激光点云的橡胶林枝干重建及参数反演

Rubber tree branch modeling and property retrieval based on laser scanning data and deep learning technique

  • 摘要: 树木的几何建模在林木性状评价、森林动态经营管理与可视化研究中具有重要意义。现今,从激光雷达(Light Detection And Ranging,LiDAR)数据中重建树体三维模型并精准获取林木空间枝干结构参数是数字林业发展的必然趋势。该研究提出了一种深度学习与计算机图形学相融合的树木骨架重建与参数反演方法。该方法以PR107、CATAS 7-20-59、CATAS 8-79三个品种的橡胶树为实验对象,首先,采用背包移动激光雷达获取三个橡胶树品种的样地数据,并通过体素剖分和数据增广策略来构建橡胶树训练样本集。其次,构造由四层特征编码层和特征解码层所组成的点云分类深度学习网络,并包含优化的PointConv模块与不同尺度的特征插值模块,以实现在多尺度条件下,全面考虑点云的全局和局部优化特征,引导网络实现枝叶点云的精确分类。最后,面向分类后的枝干点云,运用计算机图形学的空间连通性算法与圆柱拟合策略,重建树木骨架模型,并自动解决叶子点云与对应的一级枝干归属问题,进而在叶团簇尺度下开展对单株树的精细描述与参数反演。通过对三块橡胶树测试样地的验证和与实测值的比对表明,该研究提出的深度学习网络枝叶分类总体准确率在90.32%以上。骨架重建与叶团簇分析结果显示,PR107品种橡胶树具有较为发散的树冠、最大的分枝夹角和叶团簇体积;CATAS 7-20-59品种橡胶树冠呈花瓶型,分枝夹角和叶团簇体积较小;而CATAS 8-79品种橡胶树尽管胸径最粗,但不耐寒害处于落叶期导致冠积最小。同时,反演得到的橡胶树一级枝干直径与实测值比对为:决定系数R2不低于0.94,均方根误差(Root Mean Square Error,RMSE)小于3.01 cm;主枝干与一级枝干的分枝角为:决定系数R2不低于0.91,均方根误差RMSE不高于4.94°。同时发现橡胶树一级枝干的直径与对应的叶团簇体积呈正相关分布。该研究将人工智能的理论模型应用于林木的激光点云数据处理中,为林木激光点云的智能化分析与处理提供了新颖的解决思路。

     

    Abstract: Abstract: Fine-scale geometric modeling of actual trees can be an essential prerequisite for forest information and phenotypic characteristics at present. It is necessary to effectively map the tree architecture, and accurately retrieve the plant growth properties from light detection and ranging (LiDAR) data in a wide range of biophysical and ecological processes. Here, a synergistic approach was proposed for the skeleton modeling and property retrieval of the rubber tree using deep learning and computer graphics. The backpack laser scanning data was also collected for three typical rubber tree clones, i.e., PR107, CATAS 7-20-59, and CATAS 8-79. Firstly, the labeled wood and leaf point samples were collected using traditional machine learning assisted by manual segmentation. The acquirement strategy was coupled with the voxelization and data augmentation to achieve a suitable and larger training data set than before. Secondly, the deep learning network of reformulation architecture was developed to comprise four features encoding and decoding layers. The improved PointConv modules were embedded to calculate the local translation-invariant point features and the interpolation modules, further propagating the features from sub-sampled point clouds to a scale-up resolution. As such, both the global and local features of the forest points were comprehensively extracted to greatly reinforce the leaf-wood point cloud classification from the forest scene geometry. Thirdly, the tree skeletons were reconstructed for the spatial connectivity and cylinder fitting scheme using the extracted branch points after the classification of forest point clouds. Finally, the affiliation relationship between foliage points and the first-order branch points was determined to simplify each tree crown with as many foliage clumps and different branch compositions. The overall accuracy of the branch and leaf classification was 90.32% using the deep learning network during the field measurements of the test samples, including three rubber tree plots, which was approximately 10% higher than before. The skeleton reconstruction results show that the average value (48.07o) of the angles between the trunk and the first-order branches for rubber tree clone PR107 was larger than that (36.31o) of clone CATAS 7-20-59. The reason was that the rubber tree clone PR107 presented a spread-out crown with a larger crown volume than the clone CATAS 7-20-59 with a vase shape tree crown. Rubber tree clone CATAS 8-79 presented the largest diameter at the breast height, but the vulnerability to chilling injury resulted in considerable defoliation, even to the smallest volume of the tree crown. Meanwhile, the high estimation accuracies of the first-order branch diameter (R2≥0.94, RMSE<3.01 cm) and the included angle between the trunk and the first-order branches (R2≥0.91, RMSE≤4.94o) were achieved for the three test rubber tree plots. In addition, there was a positive correlation between the volume of each foliage clump borne on the first-order branches and the diameter of the corresponding first-order branch. Overall, the improved 3D mapping approach can be widely expected to quantitatively characterize the structural variables of rubber trees for the stand components and the plant parameter inversion using deep learning and computer graphics. The finding can also provide a novel concept for the intelligent processing of forest point clouds.

     

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