## 4 Discussion
### Advantages of Control-Free Nap-of-the-Object Photogrammetry  
Based on the results of image processing with and without image control, a comparative analysis was conducted on the 22 June data obtained with the two photogrammetry methods. This analysis centered on image data acquisition and indoor data processing efficiency.  
In terms of data acquisition, control-free-image-based nap-of-the-object photogrammetry appears to be a straightforward, rapid, and efficient approach for obtaining image data. The equipment required only includes a UAV and flight planning software; the entire process can be executed by a single individual. Conversely, the image control technique necessitates additional equipment for ground image control points, in addition to the advance surveying of the study area for the strategic placement of these points. This method involves at least two or more personnel; therefore, it is more labor-intensive and time-consuming.  
Concerning image data processing, the process primarily comprises four steps: CGP marking, Encryption processing of aerial triangulation, DOM image processing, and DSM processing. Figure 17 illustrates the two photogrammetric processing workflows, with the specific processing times detailed in Table 7. The time required for the control-free technique is markedly less than that for the image control technique, ranging from 283.84 to 216.58 min. The DSM processing step demands the most time among the four steps, ranging from 180.05 min to 190.93 min. The complexity and technical demands of image control processing contribute to the technique's extended duration, especially in terms of control point marking, which must be meticulously executed to ensure sufficient precision. The intricate nature of this process precludes full automation, thereby increasing the workload for processing.  
Image control nap-of-the-object photogrammetry required the longest processing time among the photogrammetry methods compared in this study, 283.84 min. Control-free-image-based nap-of-the-object photogrammetry required only 216.58 min, representing the fastest processing efficiency. Control-free image data can be processed swiftly and straightforwardly with a high degree of automation. The efficiency in obtaining DOM images and DSMs is significant, streamlining the indoor processing workflow. Utilizing this technique with UAVs in the Benggang study area would facilitate close observations of the Benggang surface, yielding high-quality image data that meet the standards of fine millimeter modeling. Further, this approach would enhance the accuracy of monitoring dynamic Benggang changes pre- and post-rainfall.  
As a novel aerial survey technology, control-free-image-based nap-of-the-object photogrammetry satisfies relevant requirements in precision, non-contact operation, penetration, and rapidity, making it potentially broadly applicable. Nevertheless, in actual monitoring scenarios, it is currently limited to relatively small areas and exhibits other notable constraints. While this technology excels at capturing details of collapse walls at close range, considerations regarding safe distances limit the accurate depiction of deeper gullies. Moreover, the significant terrain drop in large-scale Benggang poses challenges in maintaining stable elevation errors in the 3D model. Further research is warranted to address the planar and elevation error challenges inherent to complex 3D models of large-scale Benggang landscapes.