## 2 Materials and Methods
### Digital Image Acquisition  
A camera was selected to move the UGV in a paving block adjacent to the excavation site. The camera must be lightweight, capable of continuous shooting for photogrammetry, and able to obtain stable images without shaking. An action camera (GoPro HERO7 Black) was used to obtain digital images for terrestrial photogrammetry. Action cameras specialize in recording smooth videos while moving. It may not need a gimbal, but using a gimbal makes it easy to adjust the angle of viewing and can also provide auxiliary stabilization. The GoPro HERO 7 Black weighs 116 g, and the Feiyu WG2X gimbal used weighs 238 g. If a digital single-lens reflex camera was employed for image acquisition, the total weight would be greater than 3 kg. This is approximately ten times heavier than that of the action camera system, and therefore, the UGV would be inefficient as more energy is used to move. Figure 2: Flowchart of the proposed methodology. Blocks inside the yellow box were displaced. To obtain digital images for terrestrial photogrammetry, it is necessary to evaluate the ground sample distance (GSD), which is the distance between the pixel centers measured on the ground and the field of view (FOV) according to the camera shooting height. For the employed camera, the width of the charge-coupled device was 6.17 mm with 4096 pixels, and the horizontal FOV was 86\({}^{\circ}\). As shown in Figure 3a, the GSD increased with the camera shooting height, and was less than 0.911 mm when the shooting height was lower than 2.0 m. The GSD value appeared to be sufficient to evaluate the paving block displacements adjacent to the excavation sites. The maximum shooting height was set to be 2.0 m.  
Figure 3b demonstrates the ground distances depicted in an image with a pitch angle (\(\alpha\)) while the camera is positioned at a height of 2.0 m. In this scenario, the camera is mounted in a manner where it faces downwards, and the top of the image corresponds to the front direction. Half of the vertical FOV of the camera was 35\({}^{\circ}\). When \(\alpha\) is greater than 0\({}^{\circ}\), the image becomes oblique, so it shows more of the ground in the travel direction. The ground distance PB shown in Figure 3b increases rapidly with increasing \(\alpha\), increasing the ground distance AB. However, the distance AP decreases. As a result, information far from the camera's position can be included in the image with a large \(\alpha\), making it difficult to properly create PCD. Consequently, a low-oblique image closer to a vertical image or vertical image was adopted to create PCD, rather than the high-oblique images. This is consistent with the recommendations of the SfM software manual that the camera should be kept as constant as possible over the terrain to ensure the desired GSD. The Pix4D mapper version 4.8.1 was used for the SfM software in this study. To create a PCD, the recommended frontal overlap between the images should be at least 75%. The camera employs two methods to obtain digital images: capturing a snapshot with time lapse and extracting a frame from a video. The minimum interval between images is 0.5 s for time lapse. For frame extraction from a video at 30 fps, 30 images can be obtained in 1 s; therefore, the time interval between two consecutive images is 0.033 to 1 s. Note that digital images are obtained while moving on the sidewalk; therefore, moving at a high speed is dangerous for pedestrians. Accordingly, the practical moving speed range for image acquisition is 2.0 m/s or less, which is a typical jogging speed. As can be seen in Figure 4, the frontal overlap is affected by the movement speeds and \(\alpha\). If the moving speed is fast, it may not be possible to obtain a sufficient number of images that satisfy the 75% minimum frontal overlap recommended for photogrammetry [37]. When the moving speed was 1.0 m/s, frontal overlap could be satisfied with both image acquisition methods, as shown in Figure 4a.