# A Data-Driven Intermittent Online Coverage Path Planning Method for AUV-Based Bathymetric Mapping
## 4 Experimental Validation in Simulated Environments
### Results
Now let's look into these figures. Due to the overlap between the transects, the observation confidence of the cells in each path is high enough in general. Low confidence cells usually appear in turning points, where the consecutive transversal sonar swath is further apart at the further end. As the lawnmower path and the spiral path shown in Figure 9 both follow a series of preset waypoints, to ensure high coverage rate, their inter-distances need to be narrow enough to avoid any mapping void. On the other hand, for the proposed method, the inter-distances are able to change adaptively. In Figure 7, the path is similar to lawnmower pattern transecting only in meridional direction. This is due to the exclusion of the exploration reward. We could observe that the AUV made 6 transects through the workspace, then performed additional transect to cover the remained voids. This adaptive feature guarantees the coverage requirements that may not be achievable in lawnmower pattern planned with a wide inter-distance. Figure 6: Test results with different objective weights are presented in different color spheres. The radius of each sphere represents the runtime to reach the coverage rate of 99%. The bigger of the sphere, the lower of the corresponding coverage efficiency. Figure 8: Comparison of the results of run #5(**left one**) and run #6 (**right one**). The labeled transects reveal that run #6, which has a larger \(W_{e}\), is more likely to explore distant areas than run #5. Figure 7: Simulation results in run #2. The path is displayed with grayscale line, the colorbar indicates the overall confidence of each cell, and red rectangle bounds the workspace. Figure 8 presents the resulting path from simulation run #5 and #6, where the exploration reward are enabled. Distinguished resulting paths are found compared to that shown in Figure 7. The total traveling distances of run #5 and #6 are 10,500 m and 10,800 m respectively, which are longer than the path shown in Figure 7. However, the advantage still exists with this setup. As shown in Figure 8, the AUV explores distant areas with priority. As a result, more information will be gained if the mission was ended unexpectedly, and the obtained maps are more suitable for interpolation. The labeled transects in Figure 8 reveal that run #6, which has a larger \(W_{e}\), is more likely to explore distant areas than run #5, while the coverage efficiency of run #5, carrying a larger \(W_{c}\), is slightly higher. The results indicate that it is possible to optimize the path for different mission requirements by adjusting the weights. Figure 10 shows the comparison of coverage capabilities in different simulation runs. The height of each bar represents the runtime. The color layers indicate different coverage rate bands, 60 to 80% (blue), 80% to 90%(orange), 90% to 99% (gray) and above 99% (yellow). For each layer, the shorter its height, the higher the coverage efficiency. Totally, 12 different paths that belong to 3 different strategies are compared. As shown in Figure 10, the coverage efficiency of a spiral path is slightly lower than a lawnmower path with similar inter-distance. A problem for spiral paths is the inter-distance between zonal transects and the inter-distance between meridional transects are different if the target region is a rectangle. If the aspect ratio of the workspace is large, the coverage efficiency of the spiral path will become very low. As a result, a lawnmower path is a better choice than a spiral path if there aren't extra requirements. For both spiral paths and lawnmower paths, their performance are closely related to the inter-distance, which is hard to choose without sufficient prior knowledge of the target region. As shown in Figure 10, at a wider inter-distance, spiral and lawnmower surveys (the 8 th and the 10 th bar) both produce an low coverage rate (less than 99%). To guarantee the required coverage rate, we will have to set a smaller inter-distance in these situations.