# Application of a Real-Time Visualization Method of AUVs in Underwater Visual Localization
## 3 Results
### Comparison of Pose Measurement Experiments with AR-Markers  
In order to further verify the accuracy of the underwater algorithm, the algorithm was compared with an AR-marker, because there is no cumulative error and drift in the attitude information measured by an AR-marker. An AR-marker with a length of 150 mm was arranged in the experimental scene, so that the AR-marker always appeared in the field of vision of the AUV. For convenient measurement, the ARToolKitPlus [16] library was used to measure the attitude of the AR-marker. ARToolKitPlus is a software library for calculating camera position and orientation relative to physical markers in real time (see Figure 6)  
Figure 4: Stereo PL-SLAM performed in an artificial pool. (**a**) The green line represents the line features in the global map. The green quadrilateral box represents the current camera. The grey rough line represents the trajectory; (**b**) The red and black points represent the current local map points and global map points. Figure 5: Trajectory of the AUV launched by RVIZ (a 3D visualizer for the Robot Operating System (ROS) framework) released by ROS kinetic. The AUV was set to walk along a linear route. The process of the experiment is shown in Figure 7. The AR-marker recognition program and PL-SLAM were used simultaneously. The straightness of the AUV's walking trajectory as found using the PL-SLAM was analyzed and the accuracy of the pose measurement using PL-SLAM was compared with the AR-marker. The results of the experiment are shown in Figure 8. The deviation in the y,z-direction was within 5 cm. Because the AUV was not able to be strictly aligned to the center of the y,z-plane, the deviation was within the acceptable range. As shown in Figure 9, the motion trajectories of the two were approximately close, and the final error was less than 3 cm in the x-direction. Figure 8: Results of the linear motion experiments. The experimental diagram is shown in Figure 7. The red curve and the yellow curve indicate that the Z-direction and Y-direction pose changes measured by PL-SLAM were small. The blue curve indicates that the change of the X-direction pose measured by PL-SLAM was proportional to time. Figure 6: Comparison of pose measurement experiments with the AR-marker. (**a**) The red square locking AR-marker indicated that the recognition of the AR-marker was successful, which allows us to obtain the transformation matrix between the camera coordinate system and the AR-marker coordinate system. (**b**) Using the AR-marker recognition program and PL-SLAM simultaneously. Figure 7: The linear motion experiment. As shown in Figure 10, in order to analyze the cumulative error generated by the algorithm, the fixed AR-marker was arranged to appear in view of the camera at the beginning and end of the experiment, as described in Section 3.1. Thus, the accurate termination attitude of the AUV was obtained. After the algorithm finished running, the AUV's termination attitude with cumulative error was also able to be obtained. These two termination attitudes were compared and the error rates are shown in Table 1.  
According to Table 1, the attitude error was \(\sqrt{\text{Error\_x}^{2}+\text{Error\_y}^{2}+\text{Error\_z}^{2}}=0.14897\) m. The AUV walked 4.98 m during the experiment described in Section 3.1. Thus, the relative error was 2.98%. The AUV was then controlled to walk along an irregular route. The irregular route is shown in Figure 11. The accuracy of the pose measurement by PL-SLAM was then compared with the AR-marker. The results of the experiment are shown in Figure 5:0 \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Attitude\_z (m)** & **Attitude\_y (m)** & **Attitude\_x (m)** & **Attitude\_Error** \\ \hline AR-marker & \(-0.4523\) & \(0.1434\) & \(-1.056\) & \\ PL-SLAM & \(-0.3651\) & \(0.2180\) & \(-1.151\) & 2.98\% \\
**Error** & \(0.0872\) & \(0.0746\) & \(-0.095\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Error comparison of termination attitude.