# Testing Accuracy and Repeatability of UAV Blocks Oriented with GNSS-Supported Aerial Triangulation
## Results and Discussion
### Interaction between Interior and Exterior Orientation Parameters  
As far as the **12 GCP** configuration is concerned, the correlations between the principal distance and the camera heights, as can be expected, are very strong (on average 70% with maximum value 87%). This can produce restitution errors, if the scene has large differences in depth (e.g., high-rise buildings). Cross strips at different height would have limited the correlations, though the different image scale would probably limit the number of good tie-points between the sub-blocks. On the other hand, the analysis for **RTK** and **RTK+1 GCP** shows much lower correlations, respectively, 5% (with 14% maxima) and 12% (with 33% maxima). Table 5 reports the estimated camera principal distance for the three configurations and for each software package in the RTK2 flight, where the bias in elevation was remarkable with all packages. From Table 5 it can be noticed that the estimated values are remarkably different (by several micrometers) from package to package, though the standard deviation of the principal distance from the BBA varies between 0.2 and 1 \(\upmu\)m depending on the flight and on the software. These differences might be due to some extent to the different camera models (see Table A4 in Appendix A) and the estimation procedures of the three software packages; they remark, however, the already mentioned unreliability of interior orientation parameters determined using self-calibration in radial blocks. Moving the block control from ground to the projection centres (**RTK**) in the RTK2 block there is a sharp increase in principal distance (between 5 and 11 \(\upmu\)m) with every software package. Adding 1 GCP, Pix4D keeps the principal distance unchanged, PhotoScan almost reverts to the **12 GCP** value and MicMac settles for a value in between. The spatial plots of the camera location residuals (i.e., of the differences between the estimated and GNSS-measured position) of the RTK2 block are shown for **RTK** (Figure 4, left) and **RTK+1 GCP** (Figure 4, right) configurations. Table 6 reports the average of such residuals, in elevation only, for all blocks. Cross examination of Tables 3, 5 and 6 and of the projection center residual plots in Figure 4 highlights the different behaviors of the software packages when RTK-based georeferencing is constrained by the introduction of the GCP. BBA the projection centers are systematically shifted upwards; at the same time, as shown in Table 3, the principal distance is kept fixed. On the contrary, the two plots of PhotoScan in **RTK** and **RTK+IGCP** differ very little and show no systematic patterns as far as the sign of the elevation changes is concerned. It is the principal distance which changes to adapt the block to the ground constraint of the single GCP. Considering the stereo normal case, if \(z\) is the depth (vertical distance from camera centre), \(c\) the principal distance and \(m_{b}\) the image scale, a variation in principal distance produce a change in depth \(\delta z=m_{b}\delta c\). Since the average image scale is about 1:18,000, a principal distance change of 1 \(\upmu\)m is equivalent to a movement of camera height of 1.8 cm. Therefore, in RTK2, with a 3.5 \(\upmu\)m change in principal distance, a variation  
Figure 4: Plot of projection centres residuals for the RTK2 block adjusted with the three software packages: (**Left**) **RTK** configuration; and (**right**) **RTK+IGCP** configuration. The arrows depict the magnitude and direction of the horizontal coordinate changes; the diameter of the circles represents the magnitude of the elevation change, in blue and red colors, respectively, for positive and negative variations. \(\delta z\) in depth of about 6.3 cm can be expected.