## 2 Materials and Methods
### Multi-Scalar Realistic 3D Modeling and Data Format Conversion  
#### 2.4.1 Multi-Scalar Oblique Photogrammetry  
The high precision, high fidelity, and measurability of the real-scene 3D model produced by multi-view oblique images is an important progression in GIS research. In recent years, this method has been widely used in intelligent city management, emergency command and rescue, land surveying and mapping, and many other aspects. In the implementation of aerial survey, it is more convenient to obtain real-scene images, which greatly reduces the cost and working time. Currently, image-collecting devices used for oblique photogrammetry can be coarsely classified into two types: fixed-wing UAVs, usually equipped with an oblique camera of five tilt lenses, and multi-rotor rotatory UAVs  
Figure 4: GeoJSON format for point feature (for data protection, the integer parts of geodetic coordinates are replaced with the symbol “#”). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
\begin{tabular}{c} Extracted from \\ planned CAD \\ drawings \\ \end{tabular} & GeoJSON & MySQL database \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Brief overview of data source involved in our case study. with an on-board single or oblique camera. Fixed-wing UAVs usually have higher image acquisition efficiency due to their high flying speed and larger spatial coverage of the target area [38]. However, the exposure delay when a camera takes a picture requires a relatively long time (\(\sim\)5 ms) from focusing photosensitizing to digital storage. Therefore, the value of the ground sample distance (GSD) with fixed UAV is relatively high (usually higher than 4 cm) due to the camera's exposure time limitation interval under higher flying speed, degrading the quality of the resulting 3D real-scene model. The multi-rotor UAV is contrary to that of the fixed-wing UAV. Considering the factors discussed above and in balancing the efficiency, accuracy, and texture quality of the produced 3D realistic model, this paper proposes the following multi-scalar oblique photogrammetric scheme:  
1. Coarse-scale: For the entire study domain with a large area, we use a fixed-wing UAV equipped with an oblique camera with a five-tilt lens to collect the oblique images. This hardware setup enhances data acquisition efficiency for large-scale areas without losing the geometric accuracy required by practical applications. The coarse-scalar photogrammetry in our case study is mainly used to reconstruct the topographic scene with surface continuity and no large terrain undulations. 2. Meso-scale: The mining area's scattered villages and industrial sites are crucially concerned by mining enterprises and local government due to their social and economic importance. These regions of interest (ROIs), labeled with yellow frames in Figure 1, mainly include houses and industrial equipment above the ground surface possibly affected by mining-induced surface deformation. For this scale, a heavy-lift multi-rotor UAV equipped with a camera of five lenses can be used to obtain accurate 3D models to meet the subsequent GIS requirements. 3. Fine-scale: For some objects (e.g., tall buildings), obtaining their complete 3D models with coarse-scale or meso-scale or even by combining the above two due to flight height limitations or occlusions is challenging. Hence, the lightweight and low-cost rotatory UAV equipped with a single camera of adjusted shooting angles should be adopted to capture more multi-view photos of this type of object, enhancing the completeness and quality of the corresponding 3D photo-realistic model. The above three scales complement each other, and a high-quality 3D model can be obtained by fusing multi-scalar and multi-view images.