# Tracking Sensor Location by Video Analysis in Double-Shell Tank Inspections
## 3 Results  
Most mismatches can be automatically rejected due to the unreasonable prediction of RAVIS movement. An example where this is not the case is entry 1.7639 in Table 1. This mismatch was verified manually by reviewing the keypoints in frames 4740 and 4759. To test the accuracy of the offline video analysis of the sensor movement, we collect video while the crawler moves the sensor a specified distance and compare the actual distance moved to predictions of video analysis. Data points in Figure 7 show the accumulated predictions of RAVIS movement by an analysis of the video taken in a 6-inch run starting from the entrance of the section of the air slot in our laboratory. The prediction of total distance traveled based on the accumulation of 25 separate matches of features at 5-frame intervals is 147.8 mm, which differs from the ground-truth distance of 152.4 mm by 3%. The linear fit to the data points shows that the RAVIS moved at a nearly constant speed over the test distance. The predicted distance traveled based on the empirical constant velocity is 149.3 mm, which is slightly more accurate (2% error) than the accumulation of short distance measurements. The video analysis of two more 6-inch runs predicted 152.2 and 150.0 mm based on accumulated short-distance predictions. The standard deviation of absolute error in the three predictions is 2.192 mm (1.44% of ground-truth distance). Table 2 shows data from a video analysis of five runs in the laboratory air slot mockup that simulated tracking the ultrasonic sensor from the entrance of an air slot through two turns into the next wider section of the air slot (see Figure 1). All but one of the ground-truth segments in Table 2 are 6 inches in length, which is a typical separation of data collection sites in the ultrasonic evaluation of the primary tank bottom. The notation in the first column of Table 2 shows where in the air-slot mockup the interval is located (S for initial straight section, T for turn, and W for wider section) and which camera, front (F) or back (B), is being used for the video analysis of the sensor movement. The first seven entries in Table 2 show predictions of video analysis for distance between simulated data collection sites in the straight section at the entrance to an air slot. For the first four of these, both crawlers are in the straight section with video from the front camera being analyzed. For the last three of them, the front crawler is navigating the turn into the short section of the air slot connecting the initial straight section with the wider second long section (see air slot pattern in Figure 1). The errors in predictions are less than 10%, show no distinction between front and back cameras, and occur with both positive and negative signs. \begin{table}
\begin{tabular}{c c c} \hline
**Start Frame** & **Finish Frame** & **mm Pre-Frame** \\ \hline
4700 & 4720 & 1.0174 \\
4720 & 4740 & 1.1418 \\
4740 & 4759 & 1.7639 \\
4759 & 4779 & 1.2402 \\
4779 & 4799 & 0.9720 \\ \hline \end{tabular}
\end{table} 
Table 1: Example of a small change in distance per frame due to a mismatch not automatically detected. The next five entries in Table 2 show predictions of distance between simulated data collection sites by an analysis of video from cameras moving in the short connecting section of the air slot. Among these five, the third FT prediction is for a shorter distance (3.2\({}^{\prime\prime}\)) that is sufficient to allow video from the back camera to take over the tracking of the sensor movement in the short connecting section of the air slot mockup as the front crawler navigates the second turn into the wider section of the air slot. The error in the next two BT predictions exceeds 20% (see Table 2). We believe this is due to the sluggish movement of the front crawler as it navigates the turn into the second long and wider section of the air slot. The final four predictions in Table 2 are from an analysis of video from the front camera as the front crawler moves in the wider section of the air slot. The last of these again