**4. Conclusions**

It is shown in this study that SfM analysis using a UAV can be used to quantify the amount of earthwork and record the procedures for repair work on forest roads. The SfM analysis with UAV was an accurate means of measuring landforms comparable to TS surveying, creating detailed 3D models, calculating precise earthwork quantities, and recording detailed area-specific work procedures at each construction stage. Since the earthwork performed at each stage of the work was closely related to the cost of construction, it should be possible to accurately estimate the cost of construction, plan the work, and predict the number of days required for the work, which are deeply related to the cost–benefit analysis of forest roads.

Studying a stable flying method under the tree canopy was necessary as a future issue. However, the drone flight plan must be thoroughly examined when the work area changes by several tens of meters daily. For example, in conditions where a small-scale forest road is being constructed. The method of capturing images must also be verified to enable highly accurate 3D modeling. The drone flight plan must be thoroughly considered, and the method of capturing images must be verified for accurate 3D modeling.

Various measurement conditions, such as light environment, and ground surface conditions, will significantly impact the measurement evaluation, and an assessment of these shooting conditions is also required. The evaluation of soil movement during the construction process will also need to be considered.

This study evaluates only one case of small-scale forest road repair work. It is expected that completely different methods or procedures will be selected for different soil types, topographies, damage conditions, and soil placement locations. Particularly in forests, UAV flight has many obstacles, such as tree trunks and branches. In addition, the GNSS reception environment is poor, making automated flights difficult. In recent years, artificial intelligence drones equipped with sensors to avoid obstacles in real time have appeared [34]. Thus, the automatic flight into forests will be possible soon. In the future, it will be possible to collect a large amount of data on various tasks at various construction sites. It will be possible to formulate optimal road network establishment plans for complex local conditions by converting this information into big data. By collecting a large amount of data under various conditions of small-scale forest road construction and repair works

using the methodology of this study, it is possible to estimate precise earthwork volume in road construction and repair works.

The disadvantage of SfM analysis is that it is computationally expensive and takes time to analyze. Still, a characteristic of SfM analysis is that it provides a point cloud with color information. Using color information for object identification is a significant advantage over other methods, such as LiDAR measurements, in forests with many different objects. SfM analysis in forests using UAVs can be used to evaluate the impact of human activities on the environment, such as vegetation change, and sediment transport.

**Author Contributions:** Conceptualization, H.H.; methodology, H.H. and A.A.S.; validation, H.H.; formal analysis, H.H. and A.A.S.; investigation, H.H., A.A.S., T.K. and K.T.; resources, H.H.; data curation, H.H. and A.A.S.; writing—original draft preparation, H.H.; writing—review and editing, A.A.S., T.K. and K.T.; visualization, H.H. and A.A.S.; supervision, H.H.; project administration, H.H.; funding acquisition, H.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by JSPS KAKENHI Grant Numbers JP21H03672 and JP19K06125.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to express our sincere appreciation to Suntory Holdings Limited, Albero Cuore Limited, Yamashiro-cho Forest Cooperative, Kitaharima Forest Cooperative, Forest Media Works Inc., Sumitomo Forestry Co., Ltd. for their great cooperation in conducting this study.

**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
