Fruit tree canopy segmentation from UAV orthophoto maps based on a lightweight improved U-Net

Segmenting fruit tree canopies from drone remote sensing images is a prerequisite for achieving accurate agricultural monitoring and precision aerial spraying at the individual tree (instance) level. However, current instance segmentation algorithms for canopy segmentation require repeated sampling of digital orthophoto maps (DOMs), resulting in high computational complexity and poor performance in densely planted orchards. Therefore, we propose a canopy labeling method suitable for U-Net and a lightweight segmentation network, where a lightweight backbone network, an attention mechanism, and a focal loss function are introduced to improve the U-Net decoder, greatly reducing the computational complexity required for large-scale canopy segmentation. The feasibility and effectiveness of the proposed method were validated using datasets from two seasons of two different lychee orchards. The average recognition rate of the improved U-Net was 90.98 %, 28.59 % higher than the basic model U-Net, and the floating-point operations (FLOPs) amounted to 50.86 GFLOPS, 27.67 % lower than the basic model U-Net. Under the same experimental conditions, the proposed model outperformed mainstream semantic segmentation models such as Deeplabv3 + and ResNet50-U-Net, and it was more efficient than previous instance segmentation methods based on YOLACT, as it does not require repeated sampling of the same region. For the same area, the number of sampled small images decreased from 194 to 78, resulting in a 148 % overall efficiency improvement while achieving better segmentation results. Thus, the proposed model can be used to extract and locate the crown of a lychee orchard, aiding in accurate management of a lychee orchard and in a differentiated agricultural analysis and decision-making for individual lychee trees.