Aircraft-borne Lidar has been used for the accurate measurement of 3D canopy structure and tree height. However, the use of Lidar is costly and is not practical for high-frequency monitoring. Unmanned aerial vehicle (UAV) is a reasonable and convenient system for remote sensing applications. In this study, we examined an effective method to reconstruct digital surface model (DSM) of larch canopy and digital terrain model (DTM) of the underlying ground surface in larch canopy from color images measured by a UAV-borne camera with different focal length lenses (28 mm, 35 mm or 50 mm). Structure from motion (SfM) and surface reconstruction methods such as inverse distance weighting (IDW) and polygon methods were used for the DSM and DTM modeling. In addition, the digital canopy height model (DCHM) was generated by subtracting DTM from DSM. It was found that the tree canopy height estimate using the IDW method and the 28 mm lens was the best with a root mean squared error (RMSE) of 0.47 m.

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     Recently, portable scanning lidars have started to be used for three-dimensional measurement of plants. A 3D scanning lidar is a laser scanner which emits laser beam in all directions and obtains the three-dimensional image around the scanner. However, the way to calculate the moving distance of lidar in order to capture the top canopy of the target trees has not been reported yet. In this study, the distance for tree height measurement was calculated and the values were validated by estimating the tree height in accordance with the calculated distance. Additionally, in case the top part of target trees cannot be captured due to the limitation of moving area, the images taken on the ground and from a high place were registrated and it was confirmed that the height of trees can be estimated by lidar by using the registration.

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     It is important to monitor three-dimensional structural parameters of forests such as tree trunk diameter, tree height and leaf area density. Structure from motion (SfM) and lidar are known as useful tools to measure plant structures. In this study, a comparison study of SfM and lidar were conducted in order to know their accuracy, measurement condition, cost and easiness to use. Tree trunk diameter, height and landscape around trees could be obtained by using SfM and lidar. Although it takes time to process data when using SfM, SfM does not necessitate expensive equipment and sophisticated skills. Additionally, it contains color information, so it is likely that color information and three-dimensional structure can be combined. As for lidar, large target site can be covered, and it does not take time to process and analyze data. Thus, SfM and lidar have advantages and disadvantages. From this reason, they should be selected as a tool of three-dimensional measurement of trees in refer to objectives and measurement conditions.

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