## 3 Results
### Accuracy Assessment of GAMIT/GLOBK Adjustment Results  
The nationwide geodetic network adjustment was conducted using the GAMIT/GLOBK software suite, referencing the International Terrestrial Reference Frame 2014 (ITRF2014) at the reference epoch of 2002.0. While this epoch coincides with that of Korea's official geodetic datum (KGD2002), which is aligned with ITRF2000, the present study adopts a more recent and precise realization of the global reference system. All fixed CORS stations were referenced to ITRF2014 coordinates, ensuring enhanced positional consistency aligned with international geodetic standards. Unlike earlier adjustments in South Korea that employed region-based strategies--typically using four or five locally selected fixed stations per region--this study applied a nationally integrated adjustment framework. By fixing 17 rigorously validated permanent GNSS stations, the adjustment was simultaneously applied to all 5560 UCPs, thereby minimizing regional distortion and supporting homogeneous geodetic control across the entire territory. To evaluate the precision and internal consistency of the adjustment, residuals in the east (E) and north (N) coordinate components were analyzed. In the east direction, the root mean square error (RMSE) ranged from 0.014 cm to 9.30 cm, with an average of 0.48 cm. The differences between adjusted and previously published coordinates ranged from 0.06 cm to 23.75 cm, with a mean of 0.58 cm. The standard deviation was 0.09 cm, indicating a compact residual distribution consistent with a Gaussian model. In the north direction, RMSE values ranged from 0.017 cm to 7.20 cm, averaging 0.50 cm. Coordinate differences ranged from 0.02 cm to 60.81 cm, with a mean of 0.56 cm and a standard deviation of 0.093 cm. Figure 11 shows the histograms of residuals in both directions, overlaid with their respective normal distribution curves. The majority of residuals fall within the \(\pm 3\sigma\) range, with only 0.16% of north residuals and 0.32% of east residuals exceeding this threshold. This confirms the absence of systematic bias and supports the statistical robustness of the adjustment model. Figure 11: Histograms of residuals in the north and east coordinate directions derived from the nationwide GAMIT/GLOBK adjustment of 5560 unified control points. Solid black curves indicate best-fit normal distributions, and vertical dotted lines denote \(\pm 1\sigma\), \(\pm 2\sigma\), and \(\pm 3\sigma\) intervals (after \(3\sigma\) filtering). Most residuals fall within \(\pm 3\sigma\), with outlier rates of 0.16% (north) and 0.32% (east), 2.20% (up) confirming the internal consistency and statistical robustness of the adjustment model. While a small number of UCPs exhibited deviations exceeding 20 cm, these outliers were primarily attributed to poor GNSS data quality, including incomplete observation sessions, multipath effects, or metadata errors. Such anomalies can be mitigated through preprocessing techniques such as RINEX smoothing, antenna and radome validation, and cycle-slip detection. A statistical summary of the adjustment results is presented in Table 7. The analysis confirms that the unified adjustment framework achieves sub-centimeter RMSE in the horizontal directions while maintaining spatial consistency and statistical reliability across the national geodetic control network. Among the three coordinate components, the vertical direction (up) exhibits the largest root mean square error (RMSE) and standard deviation, which is a well-known limitation of GNSS-based height estimation due to satellite geometry constraints. In contrast, the east and north components show very small standard deviations--1.25 cm and 1.18 cm, respectively--and average residuals below 1 cm. However, all three components include exceptionally large maximum residuals, reaching 85.69 cm (east), 81.09 cm (north), and 4826.36 cm (up). These extreme values are primarily attributed to a small number of control points that were physically relocated during the observation period from 2008 to 2021.