Pointwise detection of street tree crown point clouds based on mobile laser scanning

doi:10.12302/j.issn.1000-2006.202202018

JOURNAL OF NANJING FORESTRY UNIVERSITY ›› 2024, Vol. 48 ›› Issue (1): 205-213.doi: 10.12302/j.issn.1000-2006.202202018

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Pointwise detection of street tree crown point clouds based on mobile laser scanning

LI Qiujie(), LI Xiangcheng

College of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, China

Received:2022-02-21 Revised:2023-06-02 Online:2024-01-30 Published:2024-01-24

Abstract

Abstract:

【Objective】 To address the problem of online detection of street-tree crowns, this study proposes a pointwise detection method for the point cloud of street-tree crowns based on mobile laser scanning (MLS), and generates a high-performance tree-crown detector capable of online, fast and accurate detection of the point cloud of street-tree crowns to provide basic data for street-tree-targeted spraying. 【Method】 The MLS system equipped with a 2D LiDAR (light detection and ranging) was used to collect the street contour measurement data in real time, and five attributes such as three-dimensional coordinates, primary echo intensity and number of echoes were extracted; a δ spherical neighborhood search method was built to realize online and fast query of point cloud neighborhood; seven kinds of local features of point cloud such as width, depth, height, dimensionality, density, number of echoes and echo intensity were extracted from the spherical domain of the point to be identified; the supervised learning algorithm was used to fuse the local features of point cloud and train a crown detector to predict the category of the point to be identified. 【Result】 The point cloud data of a 137 m long street were collected, and four comparative experiments of neighborhood search methods, supervised learning algorithms, local features of point clouds, and pointwise crown detectors were conducted. The experimental results show that the query time of the proposed δ spherical neighborhood search method is 10.90% of that of k-D tree; among the four supervised learning algorithms of neural network (NN), support vector machine (SVM), Boosting and random forest (RF), the classification performance of the crown detector trained by RF is the best; compared with single kind of features, combined features have better generalization performance; the pointwise crown detector designed by this paper is obviously superior to the existing method in terms of detection accuracy and efficiency. When the radius δ changes within the range of 0.1-1.0 m, the F₁ score on the test set is ≥ 97.74%. 【Conclusion】 The proposed method can quickly and accurately detect street-tree crown point clouds from MLS point-cloud data collected in real time and provide a spray prescription map for targeted spraying applications on street trees.

Key words: targeted spraying, street tree, crown point cloud detection, pointwise classification, mobile laser scanning

CLC Number:

TH112
S718

LI Qiujie, LI Xiangcheng. Pointwise detection of street tree crown point clouds based on mobile laser scanning[J]. JOURNAL OF NANJING FORESTRY UNIVERSITY, 2024, 48(1): 205-213.

Figures/Tables 14

Fig. 1

Fig. 2

Fig. 3

Table 1

Seven kinds of local features of point clouds"

类别 category	特征 feature
宽度width	σ_x、Δ_x
深度depth	μ_y、σ_y、Δ_y
高度height	μ_z、σ_z、Δ_z
维度dimension	L_λ、P_λ、S_λ、O_λ
密度density	d
次数number	μ_n、σ_n、Δ_n
强度intensity	$μ I 1$ 、 $σ I 1$ 、 $Δ I 1$

Table 1

Fig. 4

Table 2

Fig. 5

Fig. 6

Table 3

Table 4

Table 5

Table 6

Fig. 7

Table 7

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类别 category	训练集 training set	测试集 test set
道路长度/m street length	33	241
行道树株数 number of street trees	6	44
点云个数 number of points	840 897	6 468 879
树冠点云个数 number of crown points	76 797	886 924
非树冠点云个数 number of non-crown points	764 100	5 581 955

δ/m	搜索时间/s search time
δ/m	本研究方法 proposed method	k-D树 k-D tree
0.1	36.09	408.98
0.2	63.46	689.03
0.3	103.39	1 073.92
0.4	148.79	1 407.73
0.5	202.82	1 924.96
0.6	266.55	2 210.29
0.7	341.11	2 954.06
0.8	428.32	3 475.83
0.9	500.04	4 362.24
1.0	569.89	5 902.45
平均average	266.05	2 440.95

真实类别 actual class	预测类别 predicted class
真实类别 actual class	树冠 crown	非树冠 non-crown
树冠 crown	真正例 (T_P)	假反例 (F_N)
非树冠 non-crown	假正例 (F_P)	真反例 (T_N)

监督学习算法 supervised learning algorithm	算法超参数 algorithm hyperparameter	训练集 training set				测试集 test set
监督学习算法 supervised learning algorithm	算法超参数 algorithm hyperparameter	训练时间/s training time	查准率/% precision	查全率/% recall	F₁分数/% F₁ score	检测时间/s detection time	查准率/% precision	查全率/% recall	F₁分数/% F₁ score
神经网络 NN	hiddenSizes为10	28.69	99.90	99.94	99.92	1.04	98.46	98.62	98.54
支技向量机 SVM	线性核	2.38	87.91	83.52	85.66	0.23	92.05	82.89	87.23
Boosting	NLearn为350	138.61	100.00	100.00	100.00	95.45	99.06	98.89	98.97
随机森林 RF	NumTrees为10	12.56	100.00	100.00	100.00	8.05	99.28	99.28	99.28

特征类别 feature type	特征 feature	训练集training set			测试集test set
特征类别 feature type	特征 feature	查准率/% precision	查全率/% recall	F₁分数/% F₁ score	查准率/% precision	查全率/% recall	F₁分数/% F₁ score
单类特征 single feature	宽度 width	97.70	92.76	95.17	64.01	46.15	53.63
	深度 depth	99.86	99.61	99.73	89.78	72.76	80.38
	高度 height	99.75	98.91	99.33	84.52	77.87	81.06
	维度 dimension	99.17	96.75	97.95	77.88	63.60	70.02
	密度 density	78.01	52.80	62.98	65.51	41.15	50.55
	次数 number	97.37	89.46	93.25	83.79	73.02	78.04
	强度 intensity	99.93	99.85	99.89	78.48	84.69	81.47
组合特征 combined feature	宽度+深度+高度 width+depth+height	100.00	100.00	100.00	99.14	99.00	99.07
	宽度+深度+高度+维度 width+depth+height+dimension	100.00	100.00	100.00	99.14	99.14	99.14
	宽度+深度+高度+维度+密度 width+depth+height+dimension+density	100.00	100.00	100.00	99.11	99.11	99.11
	宽度+深度+高度+维度+密度+次数 width+depth+height+dimension+density+number	100.00	100.00	100.00	99.25	99.25	99.25
	宽度+深度+高度+维度+密度+次数+强度 width+depth+height+dimension+density+number+intensity	100.00	100.00	100.00	99.28	99.28	99.28

Pointwise detection of street tree crown point clouds based on mobile laser scanning

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方法 method	δ/m	训练集 training set				测试集 test set
方法 method	δ/m	训练时间/s training time	查准率/% precision	查全率/% recall	F₁分数/% F₁ score	平均每帧检测时间/ms average detection time per frame	查准率/% precision	查全率/% recall	F₁分数/% F₁ score
本研究方法 proposed method	0.1	57.71	99.98	99.99	99.98	22.58	98.85	98.20	98.52
	0.2	69.20	100.00	100.00	100.00	28.91	98.98	98.80	98.89
	0.3	88.79	100.00	100.00	100.00	39.56	99.13	98.68	98.90
	0.4	130.62	100.00	100.00	100.00	62.74	99.16	98.72	98.94
	0.5	192.82	100.00	100.00	100.00	96.19	99.23	98.70	98.96
	0.6	247.38	100.00	100.00	100.00	125.53	99.24	98.48	98.86
	0.7	314.85	100.00	100.00	100.00	162.10	99.24	98.00	98.62
	0.8	407.73	100.00	100.00	100.00	211.22	99.22	98.33	98.77
	0.9	466.39	100.00	100.00	100.00	243.49	99.18	97.42	98.29
	1.0	521.49	100.00	100.00	100.00	273.60	99.19	96.33	97.74
k-D树 k-D tree		1 136.03	100.00	100.00	100.00	1 609.68	94.28	88.14	91.11

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