**1. Introduction**

Forest road networks can be used for timber production, recreational activities, and preventing the spread of pests and fires [1]. However, constructing a road network on sloping terrain increases the risk of slope failure due to heavy rainfall and the fragmentation of wildlife habitat [2]. For designing an optimal road network layout, it is necessary to comprehensively evaluate candidate routes from the economic and environmental perspectives by setting objective variables, such as the construction and maintenance cost, effect of incurring a minimal cost for timber harvesting and transportation, social benefits related to recreation and disaster prevention, and the risk of damaging environmental health.

The cost–benefit evaluation in designing a forest road network requires a complex analysis, as the benefits of road construction are diverse; additionally, the costs should be calculated while considering the environmental risks [3]. Several methods, such as the analytic hierarchy process, have been proposed to evaluate the optimal route planning to maximize the benefits and minimize the costs of various construction objectives [4–8]. Recently, the precise microtopographic data obtained using an airborne laser scanner (ALS) have been used for forest road construction, because the construction cost is related to the amount of earthwork. Many studies have proposed designing forest road networks with minimal earthwork using high-resolution digital elevation model (DEM), because the routes with less earthwork are economical and discharge less sediment into the environment [9–14]. Aruga et al. [15] comprehensively studied a route selection method that combines the prediction

**Citation:** Hasegawa, H.; Sujaswara, A.A.; Kanemoto, T.; Tsubota, K. Possibilities of Using UAV for Estimating Earthwork Volumes during Process of Repairing a Small-Scale Forest Road, Case Study from Kyoto Prefecture, Japan. *Forests* **2023**, *14*, 677. https://doi.org/ 10.3390/f14040677

Academic Editor: Stefano Grigolato

Received: 19 January 2023 Revised: 13 March 2023 Accepted: 21 March 2023 Published: 24 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of surface erosion and discharge of the related sediment into streams with the amount of earthwork. Ghajara et al. [16] developed a model in which the total cost of road construction may be estimated with an accuracy range of ±6.5%, assuming that the total construction cost is determined by six factors: clearing operations, embankments, pavements, gradings, culverts, and ditches. These studies have enabled us to estimate the cost of road construction precisely based on the difference in shape change between the landform before and after the forest road construction. Particularly, earthwork volume accounts for a large portion of the cost of small-scale road construction on sloping terrain [17,18], so these models have already been put to practical use as a route selection method to reduce road construction costs [19].

On the other hand, small-scale road construction operations, mainly earthwork operations, often occur on slopes, where there needs to be more space for temporary soil storage. It is necessary to repeatedly excavate, temporarily place, and fill soil in a limited area. Therefore, it is difficult to predict and quantify the amount of earthwork performed since it is difficult to ascertain all of the earthwork performed simply by comparing the shape of the soil before construction with the shape of the completed ground. It is necessary to conduct surveys during construction to grasp the precise amount of earthwork that changes with time. However, accurate quantification has been impractical.

Recently, however, with the development of unmanned aerial vehicles (UAVs), various sensors, and analysis technology, it has become possible to perform high-frequency surveying from the sky without stopping construction work [20]. The two surveying methods using UAVs are as follows: (1) obtaining a 3D model of the ground surface via structure from motion (SfM) analysis [21] based on images taken using an onboard camera and (2) measuring the distance to the ground surface using an onboard laser scanner. As the first method requires inexpensive sensors, it has recently been widely used for surveying.

UAVs are indispensable for earthwork estimation in the construction industry owing to their speed and efficiency [22]. Akgul et al. [23] compared the results of earthwork volume estimation of small-scale roads using DEMs created by fixed-wing UAV and Network real time kinematic global navigation satellite system (NRTK-GNSS). It was revealed that the UAV-based method was more cost-effective and accurate than NRTK-GNSS. Bu ˘gday [24] used UAVs and SfM analysis to create 5 cm mesh data after road construction in a forest area and showed that post-evaluation of earthwork volumes is possible. Hr ˚uza et al. [25] operated a UAV at a speed of 1 m/s and 1 frame/s from an altitude of 4–6 m to create a point cloud with an average of 3.2 points/cm2 and accuracy with a root mean square error (RMSE) of 0.0198 m for use in inspection work on asphalt-paved forest roads. Hr ˚uza et al. [26] also reported that SfM analysis was performed on asphalt paved road surfaces using photographs taken from a height of 1.5 m above the ground and that the accuracy was higher than various LiDAR methods. Many other studies have been conducted for use in the calculation of earthwork quantities at construction sites [27–29].

Although SfM analysis using UAVs is used for constructing forest road network, previous studies have used data obtained from locations higher than the tree canopy. In the SfM analysis, identifying the same object in images taken from multiple locations is necessary. The ground surface in forests cannot be photographed due to the obstruction caused by standing trees. Therefore, flying the UAV under the tree canopy is necessary for performing a detailed analysis of the changes in the amount of earthwork during construction. An automatic flight is difficult under the tree canopy, as stable global navigation satellite system (GNSS) reception and positioning accuracy cannot be ensured. Consequently, manual operation with advanced flight techniques is required in such conditions. Therefore, few studies have been conducted by flying UAVs under the canopy to obtain detailed information of forests [30,31].

In this study, we attempted to quantify the changes in topography by taking six UAV mapping and conducting an SfM analysis at a small-scale forest road repair site, where it is relatively easy to fly UAVs even under the tree canopy because earthwork is performed in a narrow area with few standing trees. At the same time, we compared the accuracy of the topography measurement with that of a total station. The obtained data were used to verify whether it was possible to estimate the amount of earthwork at each stage of the repair work. A forest road repair site was chosen for this study, instead of a forest road construction site, because the earthwork is concentrated in a small area in forest road repair sites; therefore, it is convenient to collect data and obtain accurate evaluation.
