US survey foot
Lengthexact by definition
The US survey foot is exactly 1200/3937 of a metre, about 0.30480061 m. It is two parts per million longer than the international foot of 0.3048 m, a difference that survives because the United States defined the foot against the metre in 1893 and did not adopt the 1959 international value for geodetic work.
Watch out: Two parts per million is nothing on a building and everything on a coordinate. A state plane northing of 2,000,000 ft shifts by about 4 ft between the two definitions, which is a wrong-side-of-the-property-line error. NIST and NGS retired the US survey foot on 31 December 2022, so all new work uses the international foot, but legacy datasets carry the old one and must be labelled.
| 1 ftUS | 0.30480061 m |
The 1959 international yard and pound agreement redefined the foot as exactly 0.3048 m, but American geodesy had already published state plane coordinates in the older foot across the whole country. Rather than reissue every coordinate, the survey foot was kept for that purpose and named separately.