# Evaluation of Positioning Accuracy of Smartphones under Different Canopy Openness
## 3 Results
### Correction of Raw Location Data of Smartphone by PPP Mode

Table 6: Positioning accuracy of each device in precise-point positioning (PPP) mode. After correcting the raw data from the three smartphones in PPP mode, compared with the pseudo-range single-point positioning mode, the plane positioning accuracy is improved by 2-4-times (Figure 12). The difference between single- and dual-frequency smartphones is not obvious under low openness (openness 0.1-0.3) and the accuracy is about 4 m; when the openness is medium and medium-high (0.3-0.7), the performance is that the single-frequency mobile phone K9 is similar to the dual-frequency mobile phone NEO. The accuracy is still about 4 m. In contrast, the dual-frequency mobile phone P50 can reach 2-3 m. This is because the dual-frequency signal and the raw data with high robustness are more conducive to the correction of PPP mode. However, the C/N0 of dual-frequency mobile phone NEO is low and the quality of the raw data obtained is poor, so the improvement effect of PPP mode is limited. Under high openness (0.7-0.9), the three smartphones can achieve horizontal accuracy of about 3 m. Compared with the data processed by the internal algorithm of the recreational-grade GNSS receiver, the horizontal accuracy of the smartphone after PPP algorithm correcting is similar in the low-openness environment (0.1-0.3) and the smartphone is slightly better, while in the medium and medium-high openness condition (0.3-0.7), the horizontal accuracy of dual-frequency smartphone P50 is significantly better than that of recreational-grade devices. In the high-openness environment (0.7-0.9), the horizontal accuracy of each smartphone is significantly better than that of recreational-grade devices. Figure 11: Horizontal DRMS of the PPP model under different openness. In general, the location availability of the post-processing mode of smartphones in a forest environment is at the same level as that of recreational-grade GNSS receivers. In the forest gap and forest edge environment, it can better complete forestry investigation, afforestation construction setting out, and determine the boundary and area of forest areas. However, when positioning in a forest with extremely low-canopy openness or a complex environment, survey GNSS devices and real-time kinematic (RTK) dynamic differential positioning technology are still needed to obtain more reliable positioning data. Select the dual-frequency smartphone P50 with high positioning accuracy among smartphones and the survey-grade device iRTK2 and compare their original positioning data at a certain test point (Figure 13A). The original data measured by iRTK2, a survey-grade device, is more concentrated and closer to the test point's accurate coordinates (0,0). The measured accuracy is 2.03 m in the north direction, 0.83 m in the east direction, and the horizontal accuracy (DRMS) is 2.20 M. The original data measured by the dual-frequency smartphone P50 is more accurate, with large dispersion of coordinates and large positioning error. The measured accuracy in the north direction is 8.40 m, the accuracy in the east direction is 12.09 m, and the horizontal accuracy is 14.72 m. There is still a large gap between smartphones and survey-grade devices. According to Figure 13B, the P50 and iRTK2 devices greatly improved the data dispersion and positioning accuracy after PPP-mode processing. iRTK2 device's data are relatively concentrated and closer to the exact coordinates of the test point (0,0) and the measurement accuracy is 1.70 m in the north direction, 0.34 m in the east direction, and 1.73 m in the horizontal accuracy (DRMS). While the smartphone P50 is relatively scattered, the positioning accuracy is significantly improved compared with the original data. The accuracy was 0.93 m in the north direction, 5.05 m in the east direction, and 5.13 m in the horizontal. The improvement in positioning accuracy of the smartphone corrected by PPP mode is greater than that of the iRTK2 device. Figure 12: Horizontal accuracy comparison between SPP model and PPP model.