公式変換：地心NTv2グリッド
Method 1 (Conformal only):
X_GDA2020 = X_GDA94 + ΔX (from Helmert)
Shift ≈ 1.8 m NE in most of Australia
Method 2 (Conformal + Distortion, recommended):
Applies 1.8m + localized distortion corrections
EPSG: GDA94, GDA2020 = EPSG:8447 (NTv2 grid method)
参照テーブル
| Location | Approx. Shift (metres) | Primary Direction | Note |
|---|---|---|---|
| Sydney, NSW | 1.80 m | North-northeast | Matches national average |
| Melbourne, VIC | 1.79 m | North-northeast | Matches national average |
| Perth, WA | 1.82 m | North-northeast | Fastest-moving region |
| Darwin, NT | 1.78 m | North-northeast | Slightly different orientation |
| Hobart, TAS | 1.78 m | North-northeast | Southern extent slowest |
工学的影響
Mixing GDA94 and GDA2020 spatial datasets without transformation causes systematic 1.8 m misplacement across all mapped features. In practice this means:
- Cadastral parcel boundaries appear 1.8 m from their actual position
- Infrastructure (pipelines, roads, utilities) registered in GDA94 will not align with new GDA2020 surveys
- Modern autonomous agriculture equipment operating from GDA2020 GPS will have systematic errors relative to GDA94 farm plans
- Regulatory non-compliance for projects submitted to agencies requiring GDA2020 since it became mandatory
よくあるご質問 (FAQ)
Is GDA2020 now mandatory in Australia?
Yes. Most Australian state jurisdictions have mandated GDA2020 for new cadastral surveys and government spatial data submissions. NSW and WA both have transitional policies requiring data providers to migrate and flag the datum used.
Do I always need the full NTv2 grid transformation?
For most professional survey work, yes. The conformal-plus-distortion NTv2 grid is the recommended method. A simple 7-parameter Helmert captures the bulk 1.8 m shift but misses localized network distortions inherited from the GDA94 adjustment, which matter at the cm level.
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。