The Foot War
Two different definitions of a foot have coexisted in United States surveying for well over a century, differing by an amount too small to notice on a tape and large enough to matter on a coordinate. Most GNSS users have never been told this, and plenty of software quietly assumes an answer without ever asking.
The U.S. survey foot and the international foot are two slightly different definitions of the same unit, differing by about two parts in a million. That difference is nearly invisible on a short measured distance and can become several feet of error on a large State Plane coordinate value if the wrong definition is assumed.
A Tiny Difference With a Long History
The international foot is defined as exactly 0.3048 meters. The U.S. survey foot, defined slightly differently in 1893 and never fully retired since, works out to about 0.3048006 meters, longer by roughly two parts in a million. That gap did not matter much for most everyday work, until State Plane coordinate systems started expressing positions as very large numbers, often in the millions of feet, at which point a fixed, tiny percentage error stops being tiny at all.
Two parts in a million sounds negligible until it is multiplied against a coordinate value in the millions, at which point it becomes a real, multi-foot error, not because the measurement was wrong, but because the wrong foot definition was assumed somewhere in the conversion.
A Transition Still in Progress
The National Institute of Standards and Technology and the National Geodetic Survey officially retired the U.S. survey foot for new work at the end of 2022, intending the international foot to be the only definition going forward. That retirement came with an explicit, important exception: the existing State Plane Coordinate Systems built on the 1983 and 1927 datums continue to use whichever foot definition each state originally adopted, unchanged. The full switch to the international foot arrives with the next generation of State Plane, tied to the broader modernization of the national reference system already discussed earlier in this series, a rollout still very much in progress rather than finished.
The U.S. survey foot being officially retired does not mean it has disappeared from active use. Existing State Plane coordinates, in the states that adopted it, continue to be expressed in the U.S. survey foot until an entirely new generation of State Plane replaces the current one, a transition still underway rather than complete.
Where This Actually Bites
A large State Plane coordinate value, converted using the wrong foot definition, produces an offset that grows with the size of the number itself. A coordinate around two million feet, converted with the wrong assumption, can end up several feet away from where it belongs, an error large enough to be obviously wrong and small enough to be genuinely confusing, especially since it often looks like nothing else this companion has covered so far. This is a units problem hiding inside a coordinate value, not a scale factor or a convergence problem.
A coordinate imported from an older dataset lands several feet away from where a fresh GNSS observation places the same point, with no scale factor or convergence explanation that fits the size of the gap. What is worth checking before anything else? Which foot definition, U.S. survey or international, each dataset actually assumes, since a mismatch there produces exactly this kind of offset on a large coordinate value.
The U.S. survey foot and the international foot differ by about two parts in a million, a gap that becomes a real, multi-foot error once applied to a large State Plane coordinate value rather than a short distance. The U.S. survey foot was officially retired for new work in 2022, but remains in active use in existing State Plane systems until the next generation replaces them, a transition still in progress.
Scale factor, convergence, and units are all distortions a project has to correct for after the fact. The next chapter looks at a growing alternative: designing the projection itself so the correction barely matters in the first place.