# Optimization of Number of GCPs and Placement Strategy for UAV-Based Orthophoto Production
## 6 Conclusions  
In this study, we performed a comparative analysis of the accuracy of orthophoto production with UAVs by setting different scenarios according to the number of GCPs and their distribution. Furthermore, we presented the optimal number and distribution of GCPs in urban areas. The findings of this study can be summarized as follows.  
First, through an analysis of previous studies, the margin of error for the geolocation of 2D GCPs based on a map scale from 1/500 to 1/600 was analyzed according to the Aerial Photogrammetry Work Regulations of Korea. For a comparative evaluation of accuracy according to the number and distribution of GCPs, the area around Gyeongsangbuk-do Province in South Korea was selected as the study site, UAV-based aerial photography was performed, and 473 photos were collected as image data. Using the acquired aerial imagery data, 3D modeling was performed for each scenario based on a point cloud model, and an orthophoto was produced.  
To meet the margin of error specified in the Aerial Photogrammetry Work Regulations for an area of approximately 1,000,000 m\({}^{2}\) and an average altitude of 100 m, the optimal strategy was found to be installing a minimum of three GCPs and distributing them evenly across the entire study area. This configuration allowed the desired accuracy requirements to be satisfied while minimizing the number of GCPs needed. However, it is important to note that the optimal number and spatial arrangement of GCPs may vary depending on the characteristics of the target site, such as its size, terrain complexity, and land cover features.  
Regardless of the GCP distribution status, the analyses showed that at least three GCPs would be needed to secure the desired accuracy. The optimal number and distribution of GCPs may vary depending on the nature of the project, characteristics of the target site, and techniques used. However, to establish an optimal strategy that satisfies the goals of individual projects, it is necessary to determine the optimal number of GCPs and develop a strategy for efficient placement.  
This study provides important insights into the optimal number and placement strategy of GCPs for UAV-based orthophoto production. Through system validation, we were able to determine the minimum required number of GCPs to maximize the accuracy of orthophotos. Furthermore, the discussion on the generalization capability of the system to various applications demonstrates the potential for widespread applicability of this technology in real-world scenarios.  
In conclusion, this study offers practical guidelines for the efficient utilization of GCPs in UAV-based orthophoto production. These findings will prove valuable for both researchers and practitioners in the field. We anticipate that this study will contribute to the advancement of UAV-based surveying and related domains.  
Conceptualization, D.-M.S. and W.-J.N.; methodology, D.-M.S. and H.S.; software, H.-J.W.; validation, D.-M.S. and W.-H.H.; formal analysis, W.-J.N.; investigation, D.-M.S.; resources, H.S.; data curation, W.-H.H. and H.-J.W.; writing--original draft preparation, D.-M.S.; writing--review and editing, W.-J.N.; visualization, D.-M.S.; supervision, W.-J.N. and H.S.; project administration, W.-H.H.; funding acquisition, W.-J.N. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Education) (NRF2020R11A3068406).  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author/s.  
The authors declare no conflicts of interest.