Googleマップ上で座標がズレて表示される原因と補正方法
Verdict: It's not your GPS. It's the Map Projection.
Webメルカトル（EPSG:3857）の罠
Google Maps, Bing, and OSM use Webメルカトル (EPSG:3857). This is a visualization hack, not a geodetic standard.
Official NGA Warning
The National Geospatial-Intelligence Agency (NGA) explicitly warns:
"Web Mercator (EPSG:3857, valid -85.05° to +85.05°)... is UNACCEPTABLE for official DoD use or mission-critical operations. Deviations can reach 40km at high latitudes."
Why The Shift Happens
If you take high-precision survey coordinates (NAD83, JGD2011) and paste them into Google Maps:
- Tectonic Shift: Web Mercator (EPSG:3857, valid -85.05° to +85.05°) assumes a static WGS84. Real survey datums move with plates (2-5cm/year).
- Ellipsoid Mismatch: Web Mercator (EPSG:3857, valid -85.05° to +85.05°) uses a sphere math hack. Survey grids use the ellipsoid.
- Result: 1-3 meter shifts, making boundaries look "off".
Do Not Adjust Your Data to Match the Map!
This breaks the legal validity of your coordinates. Instead, transform the Map to your Data.
← Return to Safety: Valid Coordinate TransformerVerify transformations using PROJ 9.3 standards.
Responsibility Verification Required
Review Professional Boundary CanonReference: USGS Open-File 02-370 / Survey Act (GSI)
Responsibility Verification Required
Review Professional Boundary CanonReference: USGS Open-File 02-370 / Survey Act (GSI)
米国平面直角座標系（SPCS）変換 & ガイド
各州固有の正角投影グリッドからGPS WGS84への高精度測量・工学座標変換。
地殻変動と再施工リスク：NAD83 vs WGS84の累積誤差
北米プレートは年間約2cm移動するため、NAD83(2011)とWGS84(G1762)は現在2.2メートル以上乖離しています。 高精度なNAD83地籍杭打ちに「標準的な」GPS WGS84座標を使用したことで、基礎のやり直しやインフラの配置ミスによる5万ドルの専門家責任賠償請求が発生しています。