US Survey Feet vs. International Feet
The "Silent Error" that causes 4-foot shifts in State Plane projects.
The 2-Parts-Per-Million Trap Explained
In the United States, two mathematical definitions of the "foot" coexisted for over six decades:
- International Foot (1959 Agreement): Exactly 0.3048 meters. (1 meter ≈ 3.280839895013... ft)
- US Survey Foot (1893 Mendenhall Order): Exactly 1200 / 3937 meters ≈ 0.304800609601... meters. (1 meter = 39.37 inches exactly)
The fractional difference is precisely 2 parts per million (2 ppm), or 0.000002 feet per foot. Over short distances (such as a 100-foot lot boundary), the difference is negligible (0.0002 ft or 0.06 mm). However, State Plane Coordinate Systems (SPCS) use false easting values that frequently exceed 2,000,000 feet from their false origin. At 2,000,000 feet, the difference is exactly 4.00 feet (1.22 meters)—a physical offset that causes pipeline misalignments, boundary overlap lawsuits, and CAD layering failures.
Official Sunset Mandate: Federal Register Notice (85 FR 62698)
Effective December 31, 2022, the National Institute of Standards and Technology (NIST) and the National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS) officially deprecated the US Survey Foot. For all components of the modernized National Spatial Reference System (NSRS)—including SPCS2022—only the International Foot (referenced simply as the "foot") is legally authorized. However, legacy NAD83 SPCS coordinates remain in active legal use under state statutes.
State-by-State Legislative Foot Adoption (NAD83 SPCS)
Before the 2022 federal sunset, individual states enacted statutes defining which foot unit governed their official State Plane Coordinate System for NAD83. Mixing these standards in regional GIS or BIM projects creates severe coordinate drift:
| State / Jurisdiction | Statutory Foot (NAD83 SPCS) | SPCS2022 Unit | Shift at 2,000,000 ft Easting |
|---|---|---|---|
| California (CA) | US Survey Foot (ftUS) | International Foot (mandatory) | +4.00 ft (1.22 m) |
| Texas (TX) | US Survey Foot (ftUS) | International Foot (mandatory) | +4.00 ft (1.22 m) |
| Arizona (AZ) | International Foot (ft) | International Foot (mandatory) | 0.00 ft (Direct Match) |
| Michigan (MI) | International Foot (ft) | International Foot (mandatory) | 0.00 ft (Direct Match) |
| Florida (FL) | US Survey Foot (ftUS) | International Foot (mandatory) | +4.00 ft (1.22 m) |
| New York (NY) | US Survey Foot (ftUS) | International Foot (mandatory) | +4.00 ft (1.22 m) |
| Montana (MT) | International Foot (ft) | International Foot (mandatory) | 0.00 ft (Direct Match) |
| Oregon (OR) | International Foot (ft) | International Foot (mandatory) | 0.00 ft (Direct Match) |
Professional Engineering & GIS Best Practices
When working with US spatial datasets in Autodesk Civil 3D, ESRI ArcGIS Pro, QGIS, or Bentley MicroStation:
- Never rely on a generic "Feet" setting: Explicitly check EPSG code definitions. EPSG projected CRS codes distinguish units (e.g., EPSG:2227 specifies California zone III in US survey foot, while EPSG:6420 specifies the same zone in meters).
- Verify CAD Drawing Units: Inserting a shapefile defined in US Survey Feet into an AutoCAD drawing defaulted to International Feet causes a cumulative 2 ppm scaling shift.
- Document Geodetic Metadata: When exporting surveyor coordinates, explicitly state whether coordinates are NAD83(2011) epoch 2010.0 in US survey feet or modernized SPCS2022 in international feet.
Authoritative Geodetic Standards & Citations
- National Institute of Standards and Technology (NIST) and National Oceanic and Atmospheric Administration (NOAA): Deprecation of the United States Survey Foot, Federal Register Notice 85 FR 62698 (October 5, 2020).
- NOAA National Geodetic Survey: State Plane Coordinate System of 2022: Policy and Procedures, NOAA Manual NOS NGS 5.
- National Geodetic Survey: Blueprint for the Modernized NSRS, Part 1: Geometric Coordinates and Terrestrial Reference Frames, NOAA Technical Report NOS NGS 62.
- Taylor, B. N., Guide for the Use of the International System of Units (SI), NIST Special Publication 811.