You learn the strangest things working in radio.

Until the start of 2023, the US had two different specifications for what constituted the unit of measurement known as the foot.  On January 1st, the National Institute of Standards and Technology, or NIST, officially retired one of them, the U.S. survey foot.

Close, But Not Quite the Same

According to NIST, the length of a foot was defined by Congress back in the 1800s based on a fraction of a meter, 1200/3937 or 0.3048006 meters.  In 1959, the foot was redefined as 0.3048 of a meter exactly.  The chopped off decimals are important, as I'll show later.

The new definition was made the standard, to be applied nationwide, except for land surveying, where an exception was made allowing the continued use of the old definition.  Because it could only be used in surveying, the old standard was called the "survey foot", and the new the "international foot".

Why does this matter?

I know, I know.  Why does the 0.0000006 difference in the two matter at all?  That's nothing!  Problem is, over long distances, the difference builds.  And land surveying deals heavily in long distances.  If math isn't your thing, check out this video by NOAA that breaks the difference down.

At 2000 meters (roughly 1.25 miles), that 0.0000006 difference turns into 0.01, or one one-hundredth of a mile.  Still not a whole lot to most people, but bear with me.

At 100 miles (160934 meters), the difference is 1.05 feet.

Add another zero.  1000 miles, and there's now a discrepancy of over 10 feet between the two.

Having two different definitions for the same unit of measurement is (or was) a headache, especially if when it comes to building things like a radio or cell phone tower, where having the tower in the exact right location is a big deal.  As the saying goes, close only counts in horseshoes and hand grenades.

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