I only discovered the bizarre U.S. Survey foot after encountering it in a CAD file. I did the foot > metre conversion on an incoming imperial file and none of the resulting measurements made sense as were ‘a tiny bit out’. Turned out the file was actually created in U.S. Survey Feet, slightly different to normal ‘Feet’. The international foot is defined to be equal to exactly 0.3048 metres. United States survey foot are defined as exactly 1200⁄3937 meters, approximately 0.304800609601 metres. Now this historical unit has been discontinued:
“...a decision to discontinue the use of a specific measurement unit) the U.S. Survey foot…”
Sadly they are replacing it with the wrong unit…
the “U.S. survey foot” will be superseded by the “foot” (formerly known as the “international foot”), which is already in use throughout the U.S
Time to move to truly coherent system of units of measurement U.S.A, feet are for walking.