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Stereoscopic imaging with spherical panoramas for measuring tree distance and diameter under forest canopies

This study is the first to adapt stereoscopic imaging to measure tree distance and diameter under forest canopies with spherical panoramas. The proposed stereoscopy method requires two spherical panoramas taken at a known distance directly on top of each other. The geometry was comprehensively laid out. Twenty-three plots were randomly established in planted forests. Sample trees were selected with a small basal area factor (BAF) for diameter at breast height and horizontal distance measurement. A larger BAF was used to subsample trees for upper stem diameters. Only fully visible trees were used in the analysis. Results showed that precision decreased with increasing distance from plot centre. Tree distance could be accurately and precisely estimated up to a distance of ~10 m from plot centre. For tree diameters, better accuracy and precision was achieved when a tree was within 5 m from plot centre or its diameter was less than 20 cm. Maintaining the required vertical displacement between panoramas was challenging in the field due to difficult terrain and heavy equipment, which are likely sources of error. With continued advancement in photography equipment, shooting spherical panorama becomes easier. Our proposed method could potentially lead to a citizen science based forest resources monitoring programme.