The efficiency of TOF technology in smartphones to estimate the diameter of some Hyrcanian forest index trees

The lack of efficient inventory tools is an old and well-known problem related to in situ forest measurements. Time of flight technology (TOF), using photogrammetric techniques and the ability to detect depth, allows the creation of 3D point clouds and the measurement of various objects, including tree stems. Therefore, this paper presents the accuracy of diameter measurement with TOF technology compared to calipers for six tree species: (Fagus orientalis Lipsky), (Quercus castanaefulia C.A.M. subsp), (Acer velutinum Boiss), (Carpinus betulus L.), (Alnus subcordata C.A.M.), (Parrotia persica C.A.Mey.), differing in stem shape and bark (20 of each species) were studied in the Darabkola Research Forest. Then, the diameters at a height of 1 and 1.30 metres of the tree trunks were measured with a vernier calliper and a point cloud was created using a Phab 2 Pro smartphone. The target diameters were measured using the CloudCompare software. The results showed that there was not much difference between the field recordings and the TOF measurement. However, Acer velutinum, Fagus orientalis and Quercus castanaefulia had fewer measurement errors than Parrotia persica, Alnus glutinosa and Carpinus betulus. Overall, the diameter RMSE at a height of 1 m and 1.30 m was 1.01 cm and 0.87 cm, respectively. According to the Bias index, the values measured with the TOF technology were higher than those measured with the caliper for most species were. Due to the results obtained and the many possibilities of TOF technology, such as the possibility to measure the diameter at different heights, the creation of RGB-3D, the high scanning speed and accuracy, low complexity, the brightness and the lower cost than other technologies, three-dimensional surveying is a good option for forest inventory verification.