There is a unit of measure called the "U.S. Survey Foot." It is almost exactly equal to a standard foot, but it's definition is slightly different:
- standard (or "international") foot: .3048 meters
- U.S. survey foot: 1200/3937 meters
In other words, one is defined in relation to the meter by a decimal expression, the other by a fraction. Fortunately, the difference is very small, amounting to less than a few meters over an area as large as a U.S. state. However, this small difference can cause problems in some cases, so it's advisable to always know which units are being used.
More information about how this relates to the U.S. State Plane Coordinate System is contained in this fascinating and informative document:
POLICY OF THE NATIONAL GEODETIC SURVEY CONCERNING UNITS OF MEASURE FOR THE STATE PLANE COORDINATE SYSTEM OF 1983, January 1991
- The State Plane Coordinate System of 1927 was published in a unit of measure identified as the "U.S. Survey Foot." By 1959, except for surveying and mapping applications, the United States had switched to the unit of measure identified as the "International Foot."
- (In 1983) the U.S. Survey Foot was specified in 11 states and the International Foot was specified in 6 states. In all other states the meter is the only referenced unit of measure in the SPCS 83 legislation. The remaining 19 states do not yet have any legislation concerning SPCS 83.
- Because SPCS 83 is presently published by NGS only in meters, many surveyors, mappers, engineers, and LIS users are converting these metric values to feet. [..] there have been instances where the conversions were done using the wrong conversion factor or the computation was otherwise done incorrectly.
More background for those interested: