US Survey Feet vs. International Feet
The "Silent Error" that causes 4-foot shifts in State Plane projects.
The 2-Parts-Per-Million Trap
In the United States, two definitions of the "foot" have coexisted for decades:
- International Foot: Exactly 0.3048 meters.
- US Survey Foot: Exactly 1200 / 3937 meters (~0.3048006096...).
While the difference is only 2 millionths of a foot, when applied to State Plane Coordinates (which often exceed 2,000,000 feet from the origin), the error grows to over 4 feet. This is enough to cause building foundations to be staked in the wrong lot.
Legal Warning: 2023 Retirement
As of January 1, 2023, the National Institute of Standards and Technology (NIST) and the National Geodetic Survey (NGS) have officially retired the US Survey Foot. However, trillions of dollars in legacy data and State Plane projects (NAD83) still rely on it. Always check your metadata.