# Node-Based Optimization of GNSS Tomography with a Minimum Bounding Box Algorithm
## 5 Conclusions  
A new node-based tomographic (NNT) approach is proposed to define the tomographic model at each epoch dynamically. This contains two parts: (1) determination of the tomographic region, and (2) determination of the position of the tomographic node at each tomographic epoch. First, the convex hull algorithm and the minimum bounding box algorithm are used to dynamically adjust the tomographic region, thus ensuring the region consistent with the distribution of the GNSS signals. Second, the tomographic node position is defined based on the expected intervals at each tomographic plane, which are used to match the density of the GNSS signals at different tomographic planes.  
NNT is compared against the common node-based tomographic approach (i.e., CNT) for accuracy verification. Radiosonde data from King's Park Meteorological Station were introduced for assessing the quality of NNT and CNT technique. Overall, daily and monthly statistics (at UTC 12 every day) of  
Figure 10: The scatter plots of GNSS tomographic deviations from CNT (**a**) and NNT (**b**) during May 2015.  
the NNT and CNT results throughout the entire month of May 2015 indicated that the NNT technique outperformed the CNT technique, except for three days (i.e., DOY 127, 131 and 141). However, the difference (9 percent in RMSE) between the NNT and CNT results can be neglected compared with that of the other days (above the average of 38 percent in RMSE). NNT also makes about a 43 percent improvement in SD. NNT was shown to have a smaller systematic error, evident by the bias statistic (in Table 3) and the plots in Figure 10b.  
In the future, the work will be focusing on improving the mesh generation of the GNSS tomographic region, which is extremely important for GNSS tomography.  
Data curation, N.D., X.Y., Y.W. and W.Z.; Formal analysis, N.D.; Funding acquisition, N.D., S.Z., X.W. and K.Z.; Investigation, X.Y., X.W., W.Z. and K.Z.; Methodology, N.D., Y.W. and X.L.; Project administration, N.D., S.Z. and K.Z.; Resources, X.Y.; Software, X.W., Y.W. and W.Z.; Supervision, S.Z. and K.Z.; Validation, Y.Z. and K.Z.; Visualization, X.W. and X.L.; Writing--original draft, N.D., S.W., Y.Z. and L.H.; Writing--review & editing, N.D., S.W., Y.Z. and L.H. All authors have read and agreed to the published version of the manuscript.  
This study is funded by the National Natural Science Foundation of China (41904013), National Natural Science Foundation of China (No. 41774026), the National Natural Science Foundation of China (No. 41730109), Beijing Natural Science Foundation (8184092), Foundation of Jiangsu Normal University (9212619103), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and CAS Pioneer Hundred Talents Program.  
The authors acknowledge the Survey and Mapping Office (SMO) of Lands Department, Hong Kong for the provision of GNSS data from the Hong Kong Satellite Positioning Reference Station Network (SatRef). We also thank King's Park Observatory for the provision of radiosonde data and the Department of Earth Atmospheric and Planetary Sciences, MIT for the GAMIT/GLOBK software. The editor and reviewer team are highly appreciated for their valuable comments, which makes great improvements in the paper's quality. GNSS data in the RINEX format used for this study can be downloaded from the website ([[http://www.geodetic.gov.hk/smo/gsi/programs/en/GSS/satref/satref.htm](http://www.geodetic.gov.hk/smo/gsi/programs/en/GSS/satref/satref.htm)]([http://www.geodetic.gov.hk/smo/gsi/programs/en/GSS/satref/satref.htm](http://www.geodetic.gov.hk/smo/gsi/programs/en/GSS/satref/satref.htm))). Radiosonde data of King's Park Meteorological Station can be downloaded from the website ([[http://weather.uwyo.edu/upperair/sounding.html](http://weather.uwyo.edu/upperair/sounding.html)]([http://weather.uwyo.edu/upperair/sounding.html](http://weather.uwyo.edu/upperair/sounding.html))). The authors do not have any possible conflicts of interest.  
The authors declare no conflict of interest.