Two feet, one word. The international foot is exactly 0.3048 m; the US survey foot is 1200/3937 m -- two parts per million longer. On a distance that is an eighth of an inch per mile. On a state-plane coordinate with an easting in the millions, it is several feet -- errors of one to six feet are typical when a file in one foot is read as the other. The survey foot was deprecated by NIST/NOAA effective December 31, 2022, and SPCS2022 drops it entirely; decades of archives are in it anyway. A PNEZD file does not say which foot it carries. The error is silent inside a file and appears only where two files meet -- so declare the unit on every deliverable, verify it on every inherited file, and inventory your archive by era. Third member of the family: the zone, the datum, and now the foot.
Two feet, one word
In 1959 the United States redefined the foot as exactly 0.3048 meters – the international foot, aligned with the rest of the English-speaking world. Surveyors were exempted. Their coordinates were tied to a national reference system built on the older definition, 1200/3937 meters, and changing the unit under a live network of monuments was judged more dangerous than keeping two feet. So the US survey foot lived on, in state law and in every state-plane coordinate system that specified it.
The two feet differ by two parts per million. The survey foot is longer by about 0.61 micrometers – an eighth of an inch across a mile. For sixty-three years, that number is exactly why nobody worried.
Why a rounding error becomes a real one
Here is the arithmetic that turns a curiosity into a liability. Two parts per million is harmless when it multiplies a distance: a 5,000-foot line differs by a hundredth of a foot, well inside anyone’s tolerance. But a state-plane coordinate is not a distance. It is a position measured from a zone origin that may be hundreds of miles away, so eastings and northings carry seven digits before the decimal.
Multiply two parts per million by an easting of 3,000,000 feet and you get six feet. Read a file of survey-foot coordinates as international feet – or the reverse – and every point shifts systematically, by an amount that depends on how far the zone origin is from the ground you are standing on. One to six feet is the typical range across state-plane zones; some are worse. The distances between your points are still right to the hundredth. Their positions are wrong by a truck width.
That is the whole trap in one sentence: the error is invisible to any check that only looks inside the file. Inverse a line, close a traverse, compare a shot to a neighbor – all perfect, because the scale error cancels within a dataset. It only appears at the seam: when your file meets someone else’s, when it lands in a GIS expecting the other foot, when a control point from a different era refuses to check by a stubborn few feet nobody can explain.
The label problem, a third time
If the pattern sounds familiar, it should. A zone is a label a coordinate does not carry. A datum realization is a label a coordinate does not carry. The foot is the same species: a PNEZD file holds numbers, and nothing in the numbers says which foot they are.
The registry knows the difference – EPSG maintains separate codes for a zone in US survey feet (ftUS) and in international feet (ft), and the whole distinction between two otherwise-identical systems is that suffix. But the label lives in a project note, a CAD unit setting, a collector configuration, or the assumption of whoever set up the job. It does not live in the file.
And the two eras are now colliding. NIST and NOAA deprecated the survey foot effective December 31, 2022. The modernized state plane system arriving with the new datums – SPCS2022 – will not use it at all. New software defaults, new agency specifications, and new projects are drifting to the international foot, while nearly every survey archive in the country is, by volume, in the old one. A firm working today straddles both – and a coordinate that was unambiguous in 2015 now needs a qualifier.
What to do
None of this requires converting anything. It requires knowing which foot every coordinate is in, and saying so:
- Declare the unit on every deliverable. Not “feet” – which feet, spelled out, next to the datum and the zone. A LandXML or a coordinate file that says NAD83(2011), Indiana West, US survey feet will never cost the recipient an afternoon.
- Verify inherited files by their seams, not their insides. Internal checks pass either way. The tell is external: a coordinate far from the zone origin, converted both ways and compared against a known monument. Off by a few feet in one direction and dead-on in the other? You just learned the unit.
- Audit the defaults. Collector settings, CAD units, GIS import assumptions – know which foot each one assumes, because the deprecation is quietly changing vendor defaults underneath you.
- Inventory the archive by era. Pre-2023 work is overwhelmingly survey feet; anything touching SPCS2022 will not be. The transition that rewrites every datum label is the natural moment to record the unit, too – while the people who set up those jobs are still down the hall.
Two parts per million was small enough to ignore for sixty-three years. Multiplied by a coordinate, it never was.
Know what every file in your archive is measured in
FieldIntel structures decades of plats, notes, and coordinate files with their labels -- datum, zone, unit -- so "which foot is this?" is answered in seconds instead of discovered at the seam. Design partners get comped onboarding and half price the first year.
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