# An In-Vehicle Smartphone RTK/DR Positioning Method Combined with OSM Road Network
## 3 Results
### Statistics and Analysis of Smartphone Positioning Error  
Based on eight sets of test results, RTK/DR positioning performance, in terms of direction, velocity, and position, was analysed. The DR heading angles were corrected using the map road network. Figure 10 shows the positioning results of the two smartphones in the first set of tests. The black line represents the accurate trajectory, the red one represents only RTK positioning, and the blue one represents RTK/DR positioning. Overall, the RTK \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multicolumn{2}{c}{**Heading Angle Error RMS**} & \multicolumn{2}{c}{**Horizontal Positioning Error RMS**} \\ \cline{2-5}  & \multicolumn{2}{c}{**(Degrees)**} & \multicolumn{2}{c}{**(m)**} \\ \cline{2-5}  & **DR** & **DR\_r** & **DR** & **DR\_r** \\ \hline P30-1 & 106.181 & 2.215 & 200.854 & 5.668 \\ P30-2 & 32.702 & 2.103 & 217.430 & 6.636 \\ P30-3 & 7.690 & 2.081 & 106.629 & 4.356 \\ P30-4 & 5.265 & 1.629 & 42.187 & 4.832 \\ P40-1 & 29.006 & 2.346 & 183.660 & 5.506 \\ P40-2 & 11.792 & 2.293 & 93.313 & 5.761 \\ P40-3 & 7.966 & 1.808 & 86.463 & 5.064 \\ P40-4 & 4.609 & 1.539 & 46.276 & 5.622 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Error statistics for heading angle and horizontal positioning error. Figure 9: Horizontal positioning trajectory comparison. (**a**) Smartphone Huawei P30; (**b**) Smartphone Huawei P40. and RTK/DR horizontal trajectories almost coincide with the accurate trajectories, so it is difficult to see the actual accuracy difference from the trajectory graph alone. In order to further compare the RTK/DR positioning performance to RTK, this work determines the direction error and velocity error of the two smartphones in four in-vehicle navigation tests, as shown in Table 2.  
The heading angle of RTK/DR positioning method corresponds to that of DR_r, differently from the RTK positioning result. The RMS heading angle error of RTK is between 2 and 3 degrees, while the overall change is relatively stable. Compared with RTK, the RMS heading angle error of RTK/DR decreases in each group of tests, while the heading angle error of the three groups of tests decreases to within 2 degrees. Regarding the positioning accuracy change, the heading angle accuracy of the eight tests is improved, and during five groups of tests, the accuracy is improved by more than 20%, whereas the test P30-3 records the maximum improvement, i.e., 39.3%. The combination of smartphones IMU data and OSM data provides smartphones with a more accurate heading angle. When the vehicle normally moves, there is no lateral drift and the horizontal velocity equals the forward one. The velocity measured by RTK/DR is the projection of RTK velocity on the direction of the heading angle measured by DR. The difference between the two velocities is determined by the direction difference between RTK and DR. The results of the eight tests show that the horizontal velocity measured by RTK/DR is almost the same as that measured by RTK, and most of the changes do not exceed 2%. Only the horizontal velocity of the test P40-2 has a 4.7% increase. The effect of heading angle and velocity on the position is reflected mainly in the lateral and forward directions, so the positioning error should be assessed according to the lateral and forward directions, rather than the East and North directions. Table 3 shows the positioning error of the two smartphones and the accuracy variation of RTK/DR. The