# A Hybrid Marker-Based Indoor Positioning System for Pedestrian Tracking in Subway Stations
## 4 Result
### Tracking Accuracy Test  
We compared the tracking accuracy achieved by the HMIPS with that of a model built in a supervised manner using the ground truth location labels. The error distances were measured according to the time sequences of the test data. In the tracking accuracy test, four scenarios were set, and approximately 30 markers were used for each scenario. The marker tracking experiment was conducted by dividing it as follows: the case of using only the marker; the case of using the marker and PDR together; and the case of performing error correction. In addition, for comparison between the marker- and radio-signal-based positioning, the most widely used Wi-Fi-based localization was performed so that readers could refer to it. These notations are listed in Table 1.  
Figure 11 shows the positioning accuracy of each positioning technique for each test path. In Figures 11a,b, the tester frequently changed direction. However, in Figure 11c,d, the direction changed occasionally during mainly linear movement. The y-axis of each graph of Figure 11 represents the error distance, and the number on the x-axis represents the \(n^{th}\) marker. The MP and MPE graphs show the results of positioning using PDR between consecutive markers. In each scenario, in the case of MP without inertial sensor correction, the accuracy degraded to a maximum of 2.5 m, but in the case of MPE with inertial sensor correction, the error distance did not exceed 2 m, showing an improvement of about 20%. In each graph, we can see that a red triangle rises above the blue circle.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Notation** & **Description** \\ \hline MO & Tracking with only marker \\ MP & Tracking with marker and PDR \\ MPE & Tracking with marker and corrected PDR \\ Wi-Fi only & Tracking with Wi-Fi \\ Wi-Fi + PDR & Tracking with Wi-Fi and PDR \\ \hline \hline \end{tabular}
\end{table}
Table 1: Notation summary of tracking accuracy test.  
Figure 10: Marker recognition test.  
When the direction is changed, the accuracy gradually decreased due to gyro drift, and when the tester reached the next marker, the error distance converged back to zero. Because the MPE uses distance information and location information between successive markers, the gyro drift phenomenon is alleviated, thus showing better accuracy than MP. In the case of MO using only a marker, the error distance was 0, and because PDR was not used, a black square was continuously displayed at the 0 point of the graph. The positioning using only Wi-Fi showed an average accuracy of 4.4 m, and the positioning results fluctuated and were not stable. In contrast, on using PDR and Wi-Fi together, it can be seen that the tracking accuracy converged at approximately 3.5-4 m. However, owing to the unstable Wi-Fi environment, a sudden drop in accuracy was observed in the MPE. Table 2 compares the experimental results shown in Figure 11. Note that the experimental results for MO have not been included in this table because the error distance is always 0.  
\begin{table}
\begin{tabular}{c c c} \hline
**Figure 11a** & **Mean (m)** & **Standard Deviation (m)** \\ \hline MP & 0.5046 & 1.7767 \\ MPE & 0.3362 & 1.1287 \\ Wi-Fi only & 4.4962 & 0.2944 \\ Wi-Fi + PDR & 3.6701 & 0.3088 \\ \hline
**Figure 11b** & **Mean (m)** & **Standard Deviation (m)** \\ \hline MP & 0.5050 & 1.7483 \\ MPE & 0.4186 & 1.4525 \\ Wi-Fi only & 4.0117 & 0.3112 \\ Wi-Fi + PDR & 3.1400 & 0.2994 \\ \hline \end{tabular}
\end{table}
Table 2: Comparison of experimental results shown in Figure 11.  
Figure 11: Tracking accuracy test of the HMIPS: (**a**) test path 1; (**b**) test path 2; (**c**) test path 3; (**d**) test path 4.