## 4 Discussion  
The regional coordinate differences identified in this study reveal consistent spatial patterns in both magnitude and direction, despite the overall high agreement between the GNSS solutions and the KGD2002 reference frame. The residuals in the horizontal  
Figure 11: Three-dimensional scatter plots of local ENU (East–North–Up) residuals for three solution pairs: GAMIT/GLOBK vs. KGD2002 (**left**), Bernese vs. KGD2002 (**center**), and GAMIT/GLOBK vs. Bernese (**right**). Each point represents the coordinate difference at a station. Points exceeding \(\pm 1\sigma\) in any of the three components are highlighted in red to indicate potential outliers. Axis limits are fixed across all plots for consistent visual comparison. components (\(\Delta\)N, \(\Delta\)E) are generally centered near zero with standard deviations of 5-9 mm, underscoring the robustness and internal consistency of both GAMIT/GLOBK and Bernese processing outcomes. These sub-centimeter discrepancies indicate that both solutions are well-aligned with the national static datum. Nonetheless, systematic regional biases are discernible. For example, in the GAMIT-KGD2002 comparison, northeastern and southeastern provinces such as Gyeongbuk and Gyeongnam show slightly positive mean \(\Delta\)N values (up to +0.30 cm), while southwestern provinces (Jeonbuk, Jeonnam) exhibit negative \(\Delta\)N values (\(-\)0.24 to \(-\)0.37 cm). These small yet consistent offsets suggest a subtle north-south directional tendency, likely reflecting regional effects in the network geometry or processing model implementation. The east-west residuals are similarly small, with some regions such as Chungnam showing a modest eastward bias (+0.28 cm in the Bernese-KGD2002 comparison). The largest residual dispersions are observed in Jeonnam province, located at the southwestern edge of the network, where GAMIT vs. KGD2002 RMS differences reach \(\sim\)2.5 cm. This localized degradation in solution quality is likely associated with edge effects due to sparse geometric control, long baselines, or potential station-specific issues. Notably, the Bernese solution exhibits lower scatter in this region (\(\approx\)0.7 cm RMS), highlighting the sensitivity of peripheral network performance to software-specific adjustment strategies. The spatial consistency of residuals also varies between software packages. The GAMIT-Bernese comparison shows a slight east-west clustering in residual vector orientations, which may stem from differences in baseline weighting or frame alignment. Local environmental effects--such as tropospheric delay variations in mountainous or coastal provinces--could also contribute to directional biases. For instance, clustered azimuths in Gangwon and Gyeongbuk suggest region-specific modeling differences, potentially linked to variations in tropospheric mapping functions or ocean loading corrections. Moreover, a subtle southwest directional bias in the Bernese-KGD2002 residuals (azimuth \(\sim\)240-270\({}^{\circ}\)) raises the possibility of a small reference frame misalignment or common-mode error. Such effects may arise from differences in frame realization (e.g., the selection or weighting of reference stations), particularly when fitting modern GNSS data to a static datum fixed at epoch 2002.0. The limitations inherent to the KGD2002 datum likely contribute to the observed residual patterns. As a static reference frame aligned with ITRF2000 at epoch 2002.0, KGD2002 does not incorporate ongoing crustal deformation or post-seismic adjustments. Although the Korean Peninsula is considered relatively tectonically stable, minor intra-plate motions, localized subsidence, and post-seismic relaxation following moderate seismic events--such as the 2016 Gyeongju and 2017 Pohang earthquakes--can lead to discrepancies on the order of several millimeters to centimeters. These geophysical processes are not captured within the static coordinates of KGD2002 and may partially account for the regional variations observed.