# Evaluation of Positioning Accuracy of Smartphones under Different Canopy Openness
## 5 Conclusions  
This study compares the single-point positioning accuracy and PPP post-processing positioning accuracy of three smartphones (OPPO K9, Realme GT NEO, HUAWEI P50 Pro), commonly used recreational-grade GNSS devices (UniStrong G138 BD), and survey-grade GNSS devices (HiTarget iRTK2). The influence of canopy openness on the number of satellites, signal quality, and positioning accuracy of GNSS devices is analyzed. The results show that:  
1. The number of available satellites for dual-frequency smartphones and survey-grade receivers was significantly positively correlated with canopy openness (\(p<0.05\)).  
There was a significant positive correlation between canopy openness and C/N0 (\(p<0.05\)).
2. The C/N0 of survey-grade receivers is significantly higher than that of smartphones. In addition to recreational-grade devices, the positioning accuracy of smartphones and survey-grade devices improves with the increase in canopy openness. However, there is a threshold value for improvements in the positioning accuracy of the device by openness.
3. In SPP mode, the horizontal accuracy of the smartphone is low. The positioning accuracy of survey-grade and recreational-grade devices in all directions is better than that of smartphones. In comparison, the positioning accuracy of dual-frequency smartphones in smartphones is better than that of single-frequency smartphones.
4. After PPP-mode correction, the positioning accuracy of smartphones is improved by 2-4-times. The positioning accuracy of smartphones in all directions is better than that of recreational-grade devices, while survey-grade devices still maintain the highest positioning accuracy. When the canopy openness is greater than 0.7, the smartphone can reach a positioning accuracy of about 2.5 m.  
In general, the horizontal positioning accuracy of dual-frequency smartphones under the forest is better than that of single-frequency smartphones, which can meet the accuracy of recording the approximate location of sample plots or rough navigation in the forest environment. If such work does not require high positioning accuracy, try to choose dual-frequency full-mode smartphones. After PPP-mode correction, the smartphone can achieve a horizontal positioning accuracy of about 2.5 m in the high-canopy openness environment (>0.7), meeting the accuracy requirements of forestry setting out, resetting, and forest boundary determination. However, in a low-canopy-openness environment (<0.7) and relatively complex forest location, survey-grade GNSS devices are still required to cooperate with PPP mode or real-time differential positioning technology to obtain more accurate sub-meter-level positioning data.  
Conceptualization, J.H. and Y.G.; methodology, J.H., Y.G. and X.L.; software, Y.G. and X.L.; validation, G.W.; formal analysis, J.H. and N.Z.; investigation, Y.G.; resources, X.L.; data curation, J.H. and Y.G.; writing--original draft preparation, J.H., Y.G. and N.Z.; writing--review and editing, G.W. and J.J.; supervision, N.Z.; project administration, N.Z. and G.W.; funding acquisition, G.W. All authors have read and agreed to the published version of the manuscript.  
This work was supported by APFNet (2017 SP2-UBC) and China Scholarship Council (No. 202110230003).  
Not applicable.  
The authors gratefully acknowledge [PERSON] for help during the field survey and active mapping software support. We also gratefully acknowledge the editor and reviewers for their effort and valuable comments.  
The authors declare no conflict of interest.