## 3 Precision Analysis of GNSS-Derived elevations  
### Test Regions  
Since the GNSS-derived height determination was introduced in 2014, only some studies have been performed in Korea [17; 18; 19; 20]. Most of the test regions were located inland. Here, however, the aim was to analyze the precision of GNSS-derived elevation in the coastal regions and islands and verify the applicability of GNSS-derived height determination for unifying the inland and island areas. Therefore, the GNSS-derived elevation was determined by fixing two near control stations located inland, considering the actual surveying environment. The five test regions for this study are located in the western and southern parts of Korea, where there are many islands. The original GNSS data were obtained by National Geographic Information Institute (NGI) research projects [21; 22; 23].  
Figure 2 shows the geometric distribution of the fixed and unknown stations; the stations are also summarized in Table 2. In the case of USS04B, USS13B, TBM1, and TBM2, which are located in regions one and five, the elevations were computed by both GNSS-derived height determination and sea-crossing leveling. The fixed or unknown stations  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Target Precision** & **3 cm** & **5 cm** \\ \hline Observation time per day & 4 h & 2 h \\ \hline Repeated observation & Y (2 days) & N \\ \hline Data logging interval & Shorter than 30 s & Shorter than 15 s \\ \hline Survey type & Static, differential \\ \hline Receiver & & Dual-frequency \\ \hline Antenna & & Calibration of phase center variation is available \\ \hline  & \(\bullet\) & Unknown stations should be surrounded at least \\  & 3 fixed stations \\  & \(\bullet\) & Maximum baseline distance between fixed stations should be shorter than 20 km \\ Geometric distribution & & \\ of fixed station— & & \\ unknown station & & \\ \hline Exception & \(\bullet\) & Two fixed stations could be applied when unknown stations \\  & & are located in coastal regions or islands \\ \hline Check the consistency & Smaller than 3 cm between 4 h \\ of results & & solutions of the first and second days & Smaller than 3 cm between \\  & & second days & 1 h solutions \\ \hline Geoid model & Most recent local geoid model (currently KNGeoid18) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Surveying conditions for GNSS-derived height determination in Korea [10].  
with names starting with \"U\" are newly established 3D control points called unified control points (UCPs). These UCPs have official ellipsoidal heights and elevations. Stations ending with \"B\" are supplement points of UCPs. UCPs installed from 2008 to 2010 use names in the form of \"U\" + number, whereas those installed after 2011 use \"U\" + region name + number. Thus, USS04 is a UCP installed in the Seo-San (SS) region. 14-04-24-06 and 14-00-29-02 are the spirit benchmarks, and TBM is a temporarily installed point for sea-crossing leveling. Through the distribution illustrated in Figure 2, it was found that the unknown stations were not surrounded; instead, only two fixed stations located inland were connected with unknown stations.  
Figure 2: Surveying stations and their distribution in the five test regions.