## 3 Small Scale Experimental Test
### Three-Dimensional Modeling  
The image taken as a single photo of the temporary structure using UAVs has low resolution, so it is difficult to accurately inspect the status of the temporary structure. To solve this problem, the temporary structure was photographed from various sides and angles and 3D modeling was applied. Three-dimensional modeling naturally connects the entire shooting area as a single image based on the characteristics of the overlapping areas to overcome the limitations of cameras with a narrow view as well as the contrast of the area photographed. The point-cloud transformation of a two-dimensional image captured by the UAV is a set of identifiable reference points in the captured image. Point cloud-based 3D data conversion is a 3D mesh construction process that expresses point cloud data in 3D using triangles of various sizes.  
For 3D modeling, the Pix4D mapper program was used, which is an image processing software based on a technique for automatically finding common points between images of the target structures. The features of this program include the identification of a number of feature points in an image and their connection between images. In this study, after performing image processing on the 2D images acquired by an UAV, 3D modeling is carried out in the following steps: initial matching of images (step 1); point cloud and mesh generation (step 2); Digital Surface Model (DSM) Digital Terrain Model (DTM), Digital Elevation Model (DEM), and orthoimage creation (step 3). The 3D modeling process is shown in Figure 9. Figure 10 presents the results of steps 1 and 2, which include the shooting angle of the UAVs, point cloud of the experimental temporary structures, and triangular mesh for considered temporary structure. A ground control point (GCP) survey was conducted to convert the coordinate values of the captured images into ground coordinates. Three GCPs representing the highest resolution and positioning accuracy were set.  
Figure 8: Comparison of the original and improved images: (**a**) Original image (Step 1); (**b**) Improved image (Step 6).  
Image distortion must be taken into consideration in 3D modeling using an UAV. The main causes of image distortion in 3D modeling include errors in the shooting process (flight path, camera angle, lack of overlap) and the image processing process (alignment errors, lack of feature points). In order to mitigate distortion, the planned flight path was maintained when shooting with an UAV and data density was increased by securing 70-90% overlap and shooting at various altitudes. Additional methods to reduce distortion include improving camera image quality and adding GCP to improve location accuracy.  
To quantitatively evaluate the 3D modeling results, GSD (Ground Sampling Distance) and RMSE (Root Mean Square Error) were evaluated. GSD refers to the distance that one pixel occupies on the ground. The smaller the value, the higher the resolution, and the more precise the 3D model is created. RMSE represents the average error between predicted coordinates and actual coordinates, and the smaller the value, the more consistent it is with the actual structure. The following represent the GSD and RMSE evaluation equations.  
\[GSD=\frac{H\times S}{F}\, \tag{2}\]  
\[RMSE=\frac{1}{n}\sqrt{\sum_{i=1}^{n}\left(X_{i}-X_{i}^{\prime}\right)^{2}+ \left(Y_{i}-Y_{i}^{\prime}\right)^{2}+\left(Z_{i}-Z_{i}^{\prime}\right)^{2}}\, \tag{3}\]  
where H is the UAV's flight altitude (m), S is the pixel size of the sensor (mm), and F is the focal length of the lens. X, Y, Z are the actual coordinates and X\({}^{\prime}\), Y\({}^{\prime}\), Z\({}^{\prime}\) are the coordinates  
Figure 10: Three-dimensional modeling: (**a**) Shooting angle; (**c**) Point cloud creation; (**c**) Triangular mesh creation.  
Figure 9: Three-dimensional modeling process.  
predicted by the model. n is the number of data points. The GSD and RMSE evaluation results for 3D modeling of the considered experimental temporary structure are shown in Table 2.