# An Outdoor Navigation System for Blind Pedestrians Using GPS and Tactile-Foot Feedback
## 4 Evaluation
### Experiment II: Outdoor Navigation
#### 4.2.3 Results  
The proposed environments and the paths followed by the test subjects are shown in Figure 5. Both subjects successfully completed the task with no errors. The task in environment 1 (E-1) consisted of guiding the subjects along a 380 m pathway from a random street point to a church (Figure 5a). A set of 11 directions was communicated for this purpose. The task in environment 2 (E-2) consisted of guiding the subjects along a 420 m pathway from a close road to a park's corner (Figure 5b). A set of 17 directions was communicated regarding the route. Table 2 compares the navigation times of both subjects. Note that the difference in performance is negligible. Figure 6 compares the average walking speeds of the subjects during the test. Speeds from 1.08 to 1.19 m/s were observed. The average walking speed of a normally sighted person is 1.4 m/s [47], which suggests that the navigation system does not reduce walking pace. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **E-1 (380 m)** & **E-2 (420 m)** \\ \hline Subject A & 320 s & 381 \\ Subject B & 334 s & 389 \\ Difference & +4.4\% (B) & +2.1\% (B) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Navigational times for the outdoor experiment 2. Figure 5: Examples of subject performance in the outdoor navigation task. (**a**) Subject A in E-1; (**b**) Subject B in E-2. GPS data in these plots were downsampled by a factor of 10. The corresponding walking paths and waypoints correspond to where the instructions led: (**c**) E-1 and (**d**) E-2. The subjects' navigation was performed on sidewalks except when crossing streets. Typical urban obstacles, such as street poles, trees, trash bins, and other people, were encountered and overcome using the white cane. A natural fearless walking could be observed during navigation together with firm doubtless post-instruction movements. An approximate 2-4 m resolution of the GPS longitude/latitude data provided by the smartphone was observed during the test. Its impact on navigation was quantified by calculating the path efficiency (defined as the ratio between the length of the path actually traversed versus the length of the optimal path) for each path section bounded by two waypoints where the user received directional instructions. Figure 5c,d show these waypoints for both outdoor environments, while Table 3 summarizes the calculated path efficiencies for all path sections. Note that high path efficiency rates were obtained by the subjects. Again, similar performances were observed. Despite these high scores, GPS resolution could eventually confuse users, mostly at street corners. Figure 7 illustrates this situation: when instructed to turn, users could encounter a wall. Turning would be possible a few steps beyond. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**E-1** & **s-p1** & **p1-p2** & **p2-p3** & **p3-f** \\ \hline
**Subject A** & 97.1\% & 99.4\% & 99.1\% & 96.2\% \\
**Subject B** & 96.2\% & 98\% & 99.5\% & 97.5\% \\
**Difference** & +0.9\% (A) & +1.4\% (A) & +0.4\% (B) & +1.3 (B) \\ \hline \hline
**E-2** & **s-p1** & **p1-p2** & **p2-p3** & **p3-p4** & **p4-p5** & **p5-f** \\ \hline
**Subject A** & 95.2\% & 92.3\% & 95.2\% & 98.6\% & 93.5\% & 98\% \\
**Subject B** & 94.7\% & 96.7\% & 95.4\% & 99.8\% & 98.7\% & 98.3\% \\
**Difference** & +0.5\% (A) & +4.4\% (B) & +0.2\% (B) & +1.2\% (B) & +5.2\% (B) & +0.3\% (B) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Path efficiencies for the two navigation environments. Figure 6: The participants’ average walking speeds observed during the tests. During the experiments, it was observed that subjects were initially confused by the GPS imprecisions, but with the aid of the white cane, they quickly learned to manage the situation fairly well. After the test, the subjects stated the intuitiveness of the information displayed and the low cognitive load demanded (navigational directions were displayed only when an action was required. Most of the journey the user interface did not display any information).