公式変換：HTDP / NADCON5
ΔX, ΔY, ΔZ = f(position, time)
For most CONUS practical work (sub-meter accuracy):
WGS84 ≈ NAD83(2011) + 1.0 to 2.0 m shift
EPSG transform: NAD83(2011) —ITRF2014 = EPSG:1188
参照テーブル
| Region | NAD83WGS84 Offset | Direction | Trend |
|---|---|---|---|
| CONUS (average) | 1.0–1.5 m | South/southwest | +1.5–2.5 cm/year |
| Hawaii | 3.0–4.0 m | Varies (Pacific plate) | +5.0–7.0 cm/year |
| Alaska | 1.5–2.5 m | North (different plate motion) | +2.0–3.0 cm/year |
| Puerto Rico | 0.5–1.5 m | Southeast | +1.5 cm/year |
工学的影響
For precision engineering, RTK drone surveys, and autonomous vehicle navigation, 1 to 2 meters far exceeds allowable tolerances. When a surveyor uses raw GNSS output (WGS84) to stake a building design drawn in NAD83(2011), the physical structure is placed 1 to 2 meters from the legal design position. This is a leading cause of:
- Boundary encroachment claims
- DOT contract non-compliance
- GPS-based construction staking disputes
よくあるご質問 (FAQ)
Does GPS output NAD83 or WGS84?
Raw GPS/GNSS receivers output coordinates in WGS84 (specifically tied to the current ITRF realization). To use these coordinates in a NAD83 engineering environment, you must apply a formal datum transformation via NGS NCAT or HTDP.
Is 1-2 meters a big deal?
For recreational use, no. For engineering, legal boundaries, and construction staking, absolutely yes. At a typical residential property line, 1 meter equals your entire setback buffer. In DOT highway work, 1 meter moves a structure from within the ROW to outside it.
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。