Combining UAV-Based Vegetation Indices and Image Classification to Estimate Flower Number in Oilseed Rape

Liang Wan,Weikang Wu,Haiyan Cen,Wenxin Yin,Dawei Sun,Hongyan Zhu,Weijun Zhou,Yijian Li,Jiangpeng Zhu,Yong He

doi:10.3390/rs10091484

Liang Wan, Weikang Wu + Show 8 more

Open Access

https://doi.org/10.3390/rs10091484

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Abstract

Remote estimation of flower number in oilseed rape under different nitrogen (N) treatments is imperative in precision agriculture and field remote sensing, which can help to predict the yield of oilseed rape. In this study, an unmanned aerial vehicle (UAV) equipped with Red Green Blue (RGB) and multispectral cameras was used to acquire a series of field images at the flowering stage, and the flower number was manually counted as a reference. Images of the rape field were first classified using K-means method based on Commission Internationale de l’Éclairage (CIE) L*a*b* space, and the result showed that classified flower coverage area (FCA) possessed a high correlation with the flower number (r2 = 0.89). The relationships between ten commonly used vegetation indices (VIs) extracted from UAV-based RGB and multispectral images and the flower number were investigated, and the VIs of Normalized Green Red Difference Index (NGRDI), Red Green Ratio Index (RGRI) and Modified Green Red Vegetation Index (MGRVI) exhibited the highest correlation to the flower number with the absolute correlation coefficient (r) of 0.91. Random forest (RF) model was developed to predict the flower number, and a good performance was achieved with all UAV variables (r2 = 0.93 and RMSEP = 16.18), while the optimal subset regression (OSR) model was further proposed to simplify the RF model, and a better result with r2 = 0.95 and RMSEP = 14.13 was obtained with the variable combination of RGRI, normalized difference spectral index (NDSI (944, 758)) and FCA. Our findings suggest that combining VIs and image classification from UAV-based RGB and multispectral images possesses the potential of estimating flower number in oilseed rape.

Highlights

Oilseed rape, which belongs to the Brassicaceae family, is one of the most important oil crops
The image classification of flowers in the rape field was conducted by the K-means clustering algorithm based on Commission Internationale de l’Éclairage (CIE) L*a*b* space, and the classified flower coverage area (FCA) was calculated
A high correlation between the FCA and ground-counted flower number was achieved with the r2 of 0.89 as presented in Figure 4, indicating that the classified FCA had a good linear relationship with the actual number of yellow flowers

Summary

Introduction

Oilseed rape, which belongs to the Brassicaceae family, is one of the most important oil crops. It is grown all around the world with the leading producers including European Union, Canada, China, India and Australia [1,2]. It is essential to measure flower number in oilseed rape under different nitrogen (N) treatments. The most commonly used method to assess flower number is by manually counting in the field, which is time-consuming and labor-intensive for researchers to conduct field measurements in a large scale. It is urgent to develop a fast, non-destructive, and reliable technique that can accurately count flower number of oilseed rape in the field

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