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Acta Agronomica Sinica ›› 2023, Vol. 49 ›› Issue (12): 3364-3376.doi: 10.3724/SP.J.1006.2023.33001

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• TILLAGE & CULTIVATION·PHYSIOLOGY & BIOCHEMISTRY • Previous Articles     Next Articles

Comparing different machine learning methods for maize leaf area index (LAI) prediction using multispectral image from unmanned aerial vehicle (UAV)

MA Jun-Wei1,2(), CHEN Peng-Fei2,4,*(), SUN Yi3, GU Jian3, WANG Li-Juan1,*()   

  1. 1School of Geography, Geomatics and Planning, Jiangsu Normal University, Xuzhou 221116, Jiangsu, China
    2State Key Laboratory of Resource and Environmental Information System, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China
    3Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, Liaoning, China
    4Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, Jiangsu, China
  • Received:2023-01-01 Accepted:2023-04-17 Online:2023-12-12 Published:2023-05-05
  • Contact: * E-mail: pengfeichen@igsnrr.ac.cn; E-mail: wanglj2013@jsnu.edu.cn
  • Supported by:
    Strategic Priority Research Program of the Chinese Academy of Sciences(XDA28040502);National Natural Science Foundation of China(41871344);Jiangsu Normal University Graduate Research Innovation Program Project(2022XKT0070)

Abstract:

To make an accurate estimation of leaf are index (LAI) based on machine learning methods and images from UAV, we compared the several mainstream machine learning methods for maize LAI prediction, such as Artificial Neural Network method (ANN), Gaussian Process Regression method (GPR), Support Vector Regression method (SVR), and Gradient Boosting Decision Tree (GBDT). For this purpose, field experiments that considering apply of different amount of organic fertilizer, different amount of inorganic fertilizer, different amount of crop residue, and different planting density were carried out. Based on these experiments, field campaign were conducted to obtain UAV multispectral images and LAI data at different growth stages in maize. Based on above data, firstly, correlation analysis was used to select LAI-sensitive spectral indices, and then the Partial Least Squares Regression method (PLSR) and ANN, GPR, SVR, GBDT were coupled to design the LAI prediction models, respectively, and their performance for LAI prediction were compared. The results showed that the LAI prediction model constructed by PLSR+GBDT method had the highest accuracy and the best stability. The models of R2 and RMSE values were 0.90 and 0.25, and the verified R2 and RMSE values were 0.90 and 0.29 during validation, respectively. The model based on PLSR+GPR model was followed, with R2 and RMSE values of 0.86 and 0.30 during calibration, and R2 and RMSE values of 0.89 and 0.29 during validation, respectively. Besides, it had faster training speed and could give the uncertainty of the prediction. The model designed by PLSR+ANN method had R2 and RMSE values of 0.85 and 0.31 during calibration, and R2 and RMSE values of 0.89 and 0.30 during validation, respectively. The model designed by PLSR+SVR method had R2 and RMSE values of 0.86 and 0.32, and R2 and RMSE values of 0.90 and 0.33, respectively. Therefore, PLSR+GBDT method and PLSR+GPR method are recommended as the optimal methods for designing maize LAI prediction models.

Key words: LAI, machine learning, UAV, multispectral image, maize

Fig. 1

Location of the study area and distribution of field plots N1: 150 kg hm-2, N2: 180 kg hm-2, N3: 210 kg hm-2, N4: 240 kg hm-2; P1: 60 kg hm-2, P2: 75 kg hm-2, P3: 90 kg hm-2; K1: 75 kg hm-2, K2: 90 kg hm-2, K3: 105 kg hm-2; O1: 0 kg hm-2, O2: 22,500 kg hm-2, O3: 37,500 kg hm-2, O4: 45,000 kg hm-2, O5: 52,500 kg hm-2; D1: 50,000 plant hm-2, D2: 55,000 plant hm-2, D3: 60,000 plant hm-2, D4: 62,000 plant hm-2, D5: 64,000 plant hm-2; R1: 0 kg hm-2, R2: 3000 kg hm-2, R3: 4500 kg hm-2, R4: 6000 kg hm-2, R5: 7500 kg hm-2; T0: 0 kg hm-2, T1: 750 kg hm-2."

Table 1

Information of visible to near-infrared bands for Altum multispectral camera (nm)"

波段名称
Band name
中心波长
Central wavelength
波宽
Bandwidth
蓝光波段Blue band 475 20
绿光波段Green band 560 20
红光波段Red band 668 10
红边波段Red-edge band 717 10
近红外波段Near infrared band 840 40

Table 2

Spectral indices selected in this study"

缩写
Abbreviation
全称
Full name
公式
Formula
来源
Source
NDVI 归一化植被指数
Normalized Difference Vegetation Index
$\left( \text{NIR}-\text{R} \right)/\left( \text{NIR}+\text{R} \right)$ [22]
RVI 比值植被指数
Ratio Vegetation Index
$\text{NIR}/\text{R}$ [23]
DVI 差值植被指数
Difference Environmental Vegetation Index
$\text{NIR}-\text{R}$ [24]
EVI 增强植被指数
Enhanced Vegetation Index
$2.5\left( \text{NIR}-\text{R} \right)/\left( \text{NIR}+6\text{R}-\text{7}\text{.5B}+\text{1} \right)$ [25]
GNDVI 绿色归一化植被指数
Green Normalized Difference Vegetation Index
$\left( \text{NIR}-\text{G} \right)/\left( \text{NIR}+\text{G} \right)$ [26]
MSAVI 调整型土壤调节植被指数
Modified Soil Adjusted Vegetation Index
$\left( \text{2NIR}+1-\text{sqrt}\left( {{\left( 2\text{NIR+1} \right)}^{2}}-\text{8}\left( \text{NIR}-\text{R} \right) \right) \right)/\text{2}$ [27]
OSAVI 优化型土壤调节植被指数
Optimized Soil Adjusted Vegetation Index
$\text{1}\text{.16}\left( \text{NIR}-\text{R} \right)/\left( \text{NIR}+\text{R}+\text{0}\text{.16} \right)$ [28]
TVI 三角形植被指数
Triangular Vegetation Index
$\text{60}\left( \text{NIR}-\text{G} \right)-\text{100}\left( \text{R}-\text{G} \right)$ [29]
GRVI 绿色比值植被指数
Green Ratio Vegetation Index
$\text{NIR}/\text{G}-\text{1}$ [30]
SAVI 土壤调节植被指数
Soil Adjusted Vegetation Index
$\text{1}\text{.5}\left( \text{NIR}-\text{R} \right)/\left( \text{NIR}+\text{R}+\text{0}\text{.5} \right)$ [31]
RENDVI 红边归一化差值植被指数
Red Edge Normalized Difference Vegetation Index
$\left( \text{RE}-\text{R} \right)/\left( \text{RE}+\text{R} \right)$ [32]
RESR 红边比值植被指数
Red-Edge Simple Ratio
RE/R [33]
MCARI 改进叶绿素吸收指数
Modified Chlorophyll Absorption Ratio Index
$\left( \left( \text{RE}-\text{R} \right)-0.2\left( \text{RE}-\text{G} \right) \right)\left( \text{RE}/\text{R} \right)$ [34]
TCARI 转换叶绿素吸收指数
Transformed Chlorophyll Absorption in Reflectance Index
$3\left( \left( \text{RE}-\text{R} \right)-0.2\left( \text{RE}-\text{G} \right)\left( \text{RE}/\text{R} \right) \right)$ [35]
TCARI/OSAVI 组合植被指数
Combined Spectral Index
TCARI/OSAVI [36]
VARI 抗大气指数
Visible Atmospherically Resistant Index
$\left( \text{G}-\text{R} \right)/\left( \text{G}+\text{R}-\text{B} \right)$ [37]
RDVI 重归一化植被指数
Re-normalized Difference Vegetation Index
$\left( \text{NIR}-\text{R} \right)/\text{sqrt}\left( \text{NIR}+\text{R} \right)$ [38]
MSR 改进比值植被指数
Modified Simple Ratio
$\left( \text{NIR/R}-1 \right)/\text{sqrt}\left( \text{NIR/R}+1 \right)$ [39]
NGI 归一化绿色指数
Normalized Green Index
$G/\left( \text{NIR}+\text{G}+\text{RE} \right)$ [40]
NDRE 归一化差值红边指数
Normalized Difference Red Edge Index
$\left( \text{NIR}-\text{RE} \right)/\left( \text{NIR}+\text{RE} \right)$ [41]

Fig. 2

Flow chart of LAI prediction model designed by different methods"

Table 3

LAI statistics in maize"

生育期
Growth stage
样本数
Number of samples
最小值
Min. value
最大值
Max. value
平均值
Average value
标准差
Standard deviation
方差
Variance
变异系数
Coefficient of variation (%)
四叶期V4 stage 70 0.37 2.20 0.83 0.34 0.11 40.96
九叶期V9 stage 70 1.61 3.19 2.31 0.34 0.12 14.72

Table 4

Correlation analysis between each spectral index and LAI in maize (n = 140)"

光谱指数
Spectral index
相关系数
Correlation coefficient
光谱指数
Spectral index
相关系数
Correlation coefficient
NDVI 0.84** RENDVI 0.81**
RVI 0.85** RESR 0.80**
DVI 0.89** MCARI 0.79**
EVI 0.88** TCARI 0.67**
GNDVI 0.88** TCARI/OSAVI -0.78**
MSAVI 0.88** VARI 0.82**
OSAVI 0.87** RDVI 0.88**
TVI 0.88** MSR 0.86**
GRVI 0.89** NGI -0.88**
SAVI 0.88** NDRE 0.90**

Fig. 3

Cross validation of the PLSR+ANN model"

Fig. 4

Maize LAI prediction model designed by PLSR+ANN (a): calibration; (b): validation."

Fig. 5

Cross validation of the PLSR+GPR model"

Fig. 6

Maize LAI prediction model designed by PLSR+GPR (a): calibration; (b): validation."

Fig. 7

Cross validation of the PLSR+SVR model"

Fig. 8

Maize LAI prediction model designed by PLSR+SVR (a): calibration; (b): validation."

Fig. 9

Cross validation of the PLSR+GBDT model"

Fig. 10

Maize LAI prediction model designed by PLSR+GBDT (a): calibration; (b): validation."

Table 5

Summary of maize LAI prediction results for different methods"

模型
Models
建模Calibration 验证Validation
Rcal2 RMSEcal Rval2 RMSEval
PLSR+ANN 0.85 0.31 0.89 0.30
PLSR+GPR 0.86 0.30 0.89 0.29
PLSR+SVR 0.86 0.32 0.90 0.33
PLSR+GBDT 0.90 0.25 0.90 0.29
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