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Why NAD27 vs NAD83 Matters for US Surveys
The shift from NAD27 to NAD83 introduces 10 to 100+ metres of horizontal offset depending on location. CONUS shifts typically average 20—0m; Hawaii can exceed 100m. NGS's NADCON grid files (conus.las / conus.los) are the accepted national standard (EPSG method 9613) with accuracy ~0.15m.
Even NAD83 vs modern WGS84 introduces ~1—m of offset due to North American plate motion (~15mm/yr). For centimetre-grade work, NGS's HTDP tool is required to resolve epoch-specific positions in ITRF/WGS84 frames. Treating NAD83 = WGS84 is only valid at ±2m accuracy (EPSG op 1728).
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The $50,000 Geodetic Drift Liability: NAD83 vs WGS84
Because the North American Plate moves ~2cm/year, NAD83(2011) and WGS84(G1762) currently diverge by over 2.2 meters. Using a "standard" GPS WGS84 coordinate for a high-precision NAD83 cadastral staking has triggered $50,000 Professional Liability claims for foundational rework and utility misplacement.
Risk Exposure Metric:
2.2-Meter Tectonic Drift & Epoch Accumulation
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Coordinate accuracy varies by device and datum. Do not use these results for legal or construction purposes without checking:
GPS Accuracy Alert
Your phone's GPS can be off by 30 meters. This can cause critical errors in your data.
Check My AccuracyDatum Shift Risk
Using the wrong coordinate system (e.g. WGS84 vs NAD83) creates a permanent 1-meter offset.
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