## 2 Materials and Methods
### Data Acquisition  
When acquiring data using unmanned aerial vehicle-based aerial photography, a flight plan must be established after thoroughly considering the filming conditions. Moreover, as the data quality obtained using aerial photography has a significant impact on image processing and spatial information construction, prior planning is crucial. Therefore, in this study, the flight plan was established considering flight safety and the quality of the photographed data.  
Before initiating a UAV flight, the pilot must pre-assess factors that could impact flight safety, such as the structural system of the target building, the layout of surrounding terrain and buildings, and the presence of wires near the building. Subsequently, aerial photography is conducted in a location free from potential safety concerns. When capturing images of a building's walls using a UAV, vertical flight to vary the altitude and horizontal flight to change the UAV's position are employed. For such aerial photography, a rotary-wing UAV is more suitable than a fixed-wing UAV due to its ability for hovering flight and unrestricted altitude changes during flight. Additionally, in this study, close-up photography of the target building was necessary to capture aerial images containing cracks. To manage unexpected situations with a collision risk, experienced pilots conducted manual flights.  
In obtaining aerial photographic data for estimating the location of concrete cracks, several factors must be considered, including flight stability, data quality, and overlap of  
Figure 1: Visualizing flow chart for estimating the location of cracks in the exterior walls of buildings using unmanned aerial vehicles when without the reference objects in the image: (**a**) UAV-based aerial photography; (**b**) Distorted image correction using homography matrix; (**c**) Feature point extraction using SIFT algorithm; (**d**) Construction of spatial information using image processing techniques; (**e**) Merging of spatial information layers based on point cloud technique and image stitching technique; (**f**) Crack localization and Data validation.  
acquired images. The resolution of an aerial image is influenced by the performance of the mounted camera sensor and the shooting distance. [PERSON] et al. [24] used an unmanned aerial vehicle with a resolution of 20 Megapixels (MP) to define structural cracks in the exterior walls of concrete buildings. Aerial images were acquired by setting the shooting distance to 2 m. [PERSON] et al. [25] conducted crack detection using aerial images with a resolution of 12 MP taken from a shooting distance of 5 to 10 m. However, they confirmed that the detection had low accuracy and a limited detection range. [PERSON] et al. [7] used a UAV to detect facility cracks and acquired aerial images with a resolution of 20.8 MP by setting the shooting distance to 1-2 m. [PERSON] and [PERSON] [26] used a UAV for automatic vision-based detection of cracks on concrete surfaces and set the shooting distance to 2 m to acquire aerial images with a resolution of 20 MP. Therefore, this study aimed to acquire aerial images with a resolution of 20 MP. The shooting distance was set to 2 m, considering safety.  
The image overlap has a significant impact on the construction of spatial information based on Point Cloud techniques for concrete crack location estimation [27]. [PERSON] et al. [28] constructed spatial information for crack detection in aging bridges by setting the image overlap to 60-70%. [PERSON] et al. [29] utilized aerial images with 75% image overlap for deep learning-based crack detection on roads. [PERSON] et al. [30] set the image overlap to 50% to detect defects in buildings and infrastructure. [PERSON] et al. [31] used aerial images with image overlap of more than 60% to identify cracks in aging concrete bridges using UAVs. Accordingly, this study set the image overlap to at least 65% for data acquisition to build spatial information and estimate the location of concrete cracks.