一般的な測地系不一致の大きさ
The severity of using the wrong datum depends on which datums were confused. The Earth is not a perfect sphere, and different regions use different ellipsoids to model the ground.
| Wrong Datum / Target Datum | Typical Shift Magnitude | Risk Level |
|---|---|---|
| NAD83(2011) vs WGS84 (Current) | 1.0 to 2.0 meters | High Construction Risk |
| NAD27 vs NAD83 (North America) | 10 to 100 meters | Severe Boundary / Legal Risk |
| Tokyo Datum vs JGD2011 (Japan) | ~400 meters | Exceeds All Operational Tolerances |
| ED50 vs WGS84 (Europe/Offshore) | 100 to 150 meters | Critical Leasing / Positioning Risk |
実務上の影響
A "datum shift" is not simply a rounding error. It physically displaces all geographic data in the real world.
- Infrastructure Hits ($1M+ Liability): In documented legal cases, horizontal directional drilling (HDD) crews utilizing WGS84 GPS to locate a pipeline mapped in NAD83 have struck the active gas line, resulting in massive explosions, fatalities, and EPA fines.
- Offshore Drilling Disputes: The North Sea oil industry saw severe legal disputes when boundary leases were recorded in ED50 but navigated via WGS84 GPS. A 100-meter shift meant drilling occurred illegally in a competitor's lease block.
- Aerial / Drone Survey Rejection: High-end drone surveys submitted in WGS84 to a State DOT requiring NAD83(2011) Epoch 2010.00 will be instantly rejected. The deliverables are geometrically incompatible with the state's CAD system, forcing a complete redo at the surveyor's expense.
よくあるご質問 (FAQ)
What happens if I use WGS84 instead of NAD83?
Your positions will appear perfectly valid on screen but will be physically shifted by 1 to 2 meters (in North America). If used for construction staking or boundary layouts, this shift will result in physical encroachment or utility conflict.
How big is a datum error?
It depends on the datums confused. Mixing NAD83(1986) with NAD83(2011) creates a 0.5 to 1.5 meter error. Mixing modern WGS84 with local historical datums like Tokyo Datum or ED50 can cause errors exceeding 100 to 400 meters.
Are datum errors covered by Professional Liability Insurance?
Often yes, under Errors & Omissions (E&O). However, standard Commercial General Liability (CGL) policies explicitly exclude geographic/surveying measurement errors. Unlicensed contractors attempting to do their own GPS staking face uncovered personal liability.
Using the wrong datum or applying coordinates without grid-to-ground correction can cause 1-400 metre positional errors —a leading cause of surveying negligence claims and contract disputes.