Total leaf area estimation based on the total grid area measured using mobile laser scanning

Abstract

In recent years, variable-rate spraying technology has been proposed to solve the pesticide waste and environmental pollution problems caused by traditional spraying methods. Variable-rate spraying methods use sensors to measure the leaf amounts in the target canopies, which are used to adjust the amount of liquid and ensure even spraying for canopies of different sizes and densities. One of these methods uses mobile laser scanning (MLS) technologies with a two-dimensional (2D) light detection and ranging (LiDAR) sensor to capture the point cloud of the canopy, and then the total point number (TPN) is used to evaluate the total leaf area (TLA) of the canopy as a basis for variable-rate spraying. However, the moving speed and ranging distance of LiDAR often vary in the process of spraying, resulting in an inconsistent TPN and a large influence on the accuracy of the TLA measurement. In this paper, a new method for estimating the TLA of an individual tree canopy is proposed based on the total grid area (TGA) measured from the first and second echoes of a LiDAR sensor. The grid area at a single point is calculated by multiplying the LiDAR moving resolution and scanning resolution. To evaluate the proposed method, we use an artificial tree, the branches of which are pruned to generate nine canopies with different densities. Both sides of the canopies are scanned at three distances, namely, 1.0 m, 1.5 m, and 2.0 m, and at a constant moving speed of 0.104 m/s. Then, the point cloud of each canopy is sampled multiple times to simulate a variety of moving speeds ranging from 1.040 m/s to 2.080 m/s. Finally, a total of 594 test canopies are obtained. Univariate polynomial regression is adopted to evaluate the TLA by using the TPN and TGA. When the polynomial order is set to 2, the coefficient of determination (R-squared) of the TPN evaluation is 0.5759 with a 4662.4542 cm2 root mean squared error (RMSE), and the R-squared of the TGA evaluation is 0.9617 with a 1401.1652 cm2 RMSE. The experimental results show that the proposed method can effectively eliminate the effects of different LiDAR moving speeds and ranging distances during TLA measurements.