OSGB36とWGS84の違い：英国座標系の変換と精度
Explore the massive 100+ meter differences between raw WGS84 GPS coordinates and the British National Grid (OSGB36). Essential for UK surveying and GIS.
Warning: Raw GPS to CAD Coordinate Discrepancy
Combining uncorrected WGS84 drone data with NAD83 site plans creates a structural shift of 1-2 meters. Review the massive legal implications of this error.
Explore Boundary Dispute Liability1. The British National Grid Offset
A frequent and costly mistake in UK mapping is attempting to directly overlay raw WGS84 GPS coordinates onto Ordnance Survey (OS) maps utilizing the OSGB36 datum. Due to fundamentally different mathematical ellipsoids (Airy 1830 vs. WGS84), the raw uncorrected coordinates differ by roughly 100 to 120 meters.
2. ETRS89 and the Plate-Fixed Solution
In Europe, the tectonic plate drifts relative to the global center of mass. To prevent GPS coordinates from changing year-over-year, Europe utilizes ETRS89 (a plate-fixed realization locked to ITRF in 1989). Today, ETRS89 differs from current WGS84 by several decimeters.
3. The OSTN15 Transformation Grid
The Ordnance Survey expressly forbids using generic 7-parameter Helmert transformations for mission-critical or property boundary work. Instead, professionals must use the OSTN15 transformation grid.
- OSTN15 transforms ETRS89 directly to OSGB36 National Grid eastings and northings with a reported RMS accuracy of ≈ .009 meters.
- Using older transformation methods (or no transformation) can introduce up to 3 meters of error on land, which the OS defines as completely unacceptable for survey-grade applications.
誤った測地系（データム）の指定や、グリッド・地上補正（Grid-to-Ground）を適用しない座標変換は、1〜400mの位置ズレを引き起こします。これは測量過失請求や施工紛争の代表的な原因です。