State Plane Coordinate Systems and the U.S. Survey Foot

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:

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

Key excerpts:

More background for those interested: