Canopy height measurement by photogrammetric analysis of aerial images: Application to buckwheat (Fagopyrum esculentum Moench) lodging evaluation

Lodging is one of the major factors contributing to crop yield and quality reduction. However, the current method for lodging rate evaluation (visual rating) lacks quantitativeness, objectivity, and precision. We have designed a low-cost, simple photogrammetric method to circumvent this problem. The efficiency of the method was evaluated in a field cultivated with a lodging-resistant variety and a standard variety of buckwheat (Fagopyrum esculentum Moench). Aerial images of the plant were acquired with a compact digital camera hung under a small balloon (2.2m in length, 0.56m^3 in volume) from a height of 50-150m. Canopy height was calculated from the aerial stereo images with photogrammetric software. The photogrammetric analysis showed that digital canopy height data compared well with ground truth data at the flowering and seed-setting stages but were underestimated at the harvesting stage. The underestimation was probably caused by the change of plant structure due to flowers and leaves withering at harvest. Digital canopy height data revealed that the lower the sowing density, the less the lodging in the standard variety. This result would not be obtained by conventional visual rating and is essential for the development of cultivation technologies to reduce buckwheat lodging. The designed photogrammetric method can effectively estimate canopy height over a wide area and analyze lodging rate quantitatively. It would also have applications in other areas, e.g., the evaluation of damage by hail, the estimation of diseased areas, and assessment for agricultural insurance application.