## 4 Discussion  
In this study, projection center coordinates of the obtained aerial images were determined using RTK, PPK, and PPP-AR positioning techniques. Additionally, positioning techniques were integrated with different GCP distribution models to improve the accuracy of photogrammetric products. Four different methodologies were employed in the production of UAV-based photogrammetric products. The effectiveness of these methodologies was evaluated using models featuring GCPs that varied in distribution, location, and number. Furthermore, an analysis was conducted to assess the scenario in which no GCPs were utilized. The methodology in this study proposes a more efficient approach in terms of GCP placement without reducing quality. Similar to the study by [PERSON] et al. [16], our findings indicate that the placement of GCPs with the right strategy reduces the need for more GCPs and can be a cost-effective solution for UAV photogrammetric production. The results indicate that UAV-based photogrammetric products can be achieved with positioning accuracy at the centimeter level. An analysis of the error rates across horizontal, vertical, and 3D components demonstrates that the results from each technique exhibit close correspondence. According to the results, using a large number of GCPs does not guarantee higher accuracy. It is necessary to identify the most effective balance between the number and distribution of GCPs. Similar to other studies in the literature [20], vertical accuracies are significantly improved when GCPs are integrated with RTK, PPK, and PPP-AR methods. Although Model-1, Model-2, and Model-3 are homogeneously distributed, Model-3 contains fewer GCPs and thus has less effect on the vertical RMSE. The selection of the GCP distribution model slightly affects the horizontal RMSE values. Moreover, Model-6 does not include GCP from the edge regions which negatively affects the accuracy. For this reason, the RMSE values are reduced by including GCPs from the edge regions of the study region. Direct georeferencing methods can reach centimeter level horizontal and vertical RMSE without the need for ground measurements. It is particularly suitable for photogrammetric mapping in inaccessible and disaster areas where it is not possible to install GCP. However, as reported in other studies, it gives lower accuracy to indirect georeferencing. The data presented in Table 3 indicate that, when assessing all models, the indirect approach demonstrates the lowest error in the 3D components. Due to precisely located GCPs, the calculation of external orientation parameters with high accuracy is achieved  
Figure 13: The errors in the Z-axis were evaluated for each positioning technique. in the adjustment. According to the study results, indirect georeferencing using GCPs is a useful option that provides high accuracy. Moreover, since indirect georeferencing does not require precise positioning during flight, it is especially suitable for UAVs that do not contain an RTK module. On the other hand, its accuracy is dependent on GCP measurements and distribution and requires more field work. According to Table 3, it has been observed that PPK demonstrates greater accuracy in the vertical component across all evaluated models compared to RTK. Conversely, for the accuracy in the horizontal component in all models, RTK exhibits superior performance compared to PPK. For the 3D component, PPK has better performance compared to RTK in all models, including those without GCPs. The reason for the better performance of PPK may be that the correction data may not be received continuously during the RTK procedure due to internet interruptions. Using precise orbits and clock products is another reason for improving the PPK results. Analysis of the absolute positioning-based PPP-AR technique in all models shows that the error remains at the centimeter level in all three components. PPP-AR achieved a similar precision level to that of the relative techniques.