Comparative analysis of extraction algorithms for crown volume and surface area using UAV tilt photogrammetry

WANG Yutang;WANG Jia;NIU Liwei;CHANG Shuping;SUN Lu;College of Forestry, Beijing Forestry University;Beijing Key Laboratory of Precision Forestry,Beijing Forestry University;

UAV tilt photogrammetry;;crown volume;;crown surface area;;point cloud density;;crown type

Canopy volume and surface area are important characteristic factors of canopy structure, which are key parameters for accurately monitoring tree growth. The use of drone tilt photogrammetry can quickly examine the three-dimensional structure of the crown. Therefore, the comparison and analysis of different extraction algorithms for crown volume and surface area can improve the extraction efficiency and accuracy of the crown volume and surface area in forestry investigations. In this study, the main street trees in Huairou District of Beijing were used as the research objects. The 3 D point cloud data of trees were obtained through the UAV tilt photogrammetry, the crown volume and surface area were calculated using the voxel method and digital elevation method, respectively, and the difference in settlement results between the two algorithms between different trees species was analyzed. The research results showed that the voxel method reflected the internal structure of the canopy, while the digital elevation method discovered that there was no void inside the canopy. Therefore, the volume and surface area of the crown obtained by the digital elevation method were significantly larger than that of the voxel method. The point cloud density and crown parameters were respectively fitted to the relative difference between the calculation results of the two algorithms. The fitting results showed that the point cloud density only affected the crown volume with a correlation coefficient R~2 of 0.443 1. While the crown parameters influenced the calculation of crown volume and surface area, with the correlation coefficients R~2 of 0.349 6 and 0.100 1, respectively. Therefore, in practical applications, although the voxel method can reflect the accuracy more realistically from the theory of the two algorithms, the calculation process is slow. In addition, it is relatively troublesome to develop the calculation tools by yourself. However, the digital elevation method can be used. Rely on the volume surface area calculation tool provided by the existing software, the crown structure factor can be quickly obtained. Therefore, for trees with small internal space and relatively regular crowns, such as arborvitae, or for studies that do not consider the internal structure of trees, such as three-dimensional green quantity, the digital elevation method still has a good application prospect. If the calculation results are required to be accurate, or there are many gaps in the canopy, the voxel method is more suitable.