## 4 Results
### Geolocation Accuracy Assessment  
To evaluate the robustness of the proposed approach, the geolocation accuracy of the extracted bottom point clouds by PGD was compared with that of LBASSD, using echo-sounding data as the ground truth. The vertical distances (AZ) between the bottom points and those from the echo-sounding data were calculated. Table 5 describes the mean error and standard deviation at each test site. The mean error of the PGD results (\(-0.315\) m) at test site 1 decreased by 0.369 m compared to that of the LBASSD results (\(-0.684\) m). On the other hand, the mean PGD error (0.109 m) at test site 2 slightly increased compared to that of the LBASSD result (0.033 m). Although the mean error is advantageous to use as a reference for measuring accuracy, it may be associated with characteristics of the Seahawk sensor system and a temporal difference between the ABL and echo-sounding data. The time differences between the acquisition of Seahawk and echo-sounding range from 3 to  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multirow{2}{*}{**Method**} & \multicolumn{2}{c}{**Total Point Cloud**} & \multicolumn{2}{c}{**Bottom Point Cloud**} \\ \cline{3-5}  & & **Number of Points** & **Point Density (pts/m\({}^{2}\))** & **Number of Points** & **Point Density (pts/m\({}^{2}\))** \\ \hline Site & LBASSD & 430,376 & 1.128 & 64,581 & 0.169 \\
1 & PGD & 979,283 & 2.568 & 76,336 & 0.200 \\ Site & LBASSD & 412,760 & 1.918 & 63,156 & 0.294 \\
2 & PGD & 545,020 & 2.533 & 71,945 & 0.334 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Point extractability evaluation of LBASSD and PGD results.  
Figure 9: Point cloud of test site 1 generated by PGD: (**a**) examples of decomposed waveforms; (**b**) cross-section of point cloud; (**c**) regional 3D point cloud; (**d**) bottom point cloud.  
4 months, during which changes in seafloor topography may have occurred. In particular, the negative mean \(\Delta Z\) value at site 1 may indicate that the seafloor height had risen between the time of the echo-sounding survey (November 2021) and the Seahawk survey (March 2022), due to the deposition of eroded soil from the coast into the near sea. The mean \(\Delta Z\) at site 2 may also be due to seafloor changes during the time gap (approximately 3 months), but the amount is negligible. From the \(\Delta Z\) results calculated for the two sites, the trend and amount of variation (standard deviation) may be more meaningful than the mean values. The standard deviation of \(\Delta Z\), which reflects the robustness of the algorithm, did not differ significantly between LBASSD and the proposed approach at both sites. The \(\Delta Z\) standard deviation of the PGD results was slightly higher than that of LBASSD at both test sites. However, considering that the number of bottom points extracted using PGD at each site increased by 18% and 14%, respectively, compared with the LBASSD results, the standard deviations of \(\Delta Z\) are essentially the same.  
Figure 10: Bottom point extraction results at test site 1: (**a**) overview of LBASSD result; (**b**,**c**): close-up views of boxes in (**a**); (**d**) overview of PGD result; (**e**,**f**): close-up views of boxes in (**d**).