WGS84とNAD83の違い：地殻変動と座標ズレの解説
Understand the 1, meter tectonic shift between WGS84 and North American Datum 1983. Learn how ignoring this offset in GIS and surveying causes structural misalignments.
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 Core Geodetic Difference
While NAD83 and WGS84 were practically identical when created in the mid-1980s, the North American tectonic plate shifts roughly 1-2 centimeters per year. Because NAD83 is tied to the North American plate (plate-fixed) and WGS84 is tied to the global center of mass (Earth-centered), they have drifted apart.
Today, the difference between NAD83(2011) and modern realizations of WGS84 (like ITRF2014) is approximately 1 to 2 meters in the continental United States. For Hawaii and the Pacific, this difference can exceed 4 meters.
2. Engineering & Surveying Implications
In high-precision surveying and civil engineering, a 1-2 meter error far exceeds allowable tolerances. Utility locators, autonomous vehicles, and drone surveys relying on RTK GNSS require centimeter-level accuracy.
3. Official Transformation Methods
To convert between NAD83 and WGS84, professionals must use time-dependent transformation parameters. The US National Geodetic Survey (NGS) mandates the use of NADCON 5 (or the NGS Coordinate Conversion and Transformation Tool - NCAT) for official NSRS horizontal datum transformations.
- WGS 84 (ITRF) to NAD83(2011): Requires a 14-parameter Helmert transformation incorporating frame drift velocities. (See EPSG: 1713 / 1727).
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。