What an exciting year it has been for units of measure! (Granted, it doesn’t take a lot to be an exciting year for units of measure.) In May, the kilogram got a makeover. The old standard, a piece of platinum-iridium alloy housed at the International Bureau of Weights and Measures in France since 1889, was replaced with a definition based on the fundamental constants of the universe. And this month, in a slightly more provincial matter, the U.S. has taken an important step to phase out the U.S. survey foot.

The U.S. is nearly alone in the world in resisting the metric system, but underneath the weird mishmash of binary and dozenal and other relationships the customary system uses, our measurements are secretly metric. In 1893, the foot got an official definition based on the meter: 1 foot was defined to be precisely 1200/3937 meters. 

I doubt that definition of the foot as a fractional portion of a meter—in particular, a fractional portion whose decimal representation does not terminate—was a dig at the metric system. But in my opinion it is at least ironic that the relationship between feet and meters was fractional instead of decimal.

In 1959, that irony was ironed out…almost. The foot was redefined to be precisely 0.3048 meters. The decimal representation of 1200/3937 begins 0.304800609, so the two definitions differ by only about a 10th of an inch per mile. That new foot was adopted for every purpose except land surveying. The new foot, defined as 0.3048 meters, was called the international foot, and the old definition, 1200/3937 meters, sadly was not called the fractional foot but the U.S. survey foot.

The U.S. survey foot was always intended to be a temporary measure. It was supposed to be replaced with the international foot in 1986. A few states did this, but most states still use the U.S. survey foot, leading to discrepancies between states that add up on the scales that surveying uses. A tenth of an inch per mile is not all that much, but in 1000 miles, that’s several feet. If you’ve ever had trouble with GPS directions because of minor inaccuracies or imprecisions, you can see that a few feet can make a significant difference.

The National Institute of Standards and Technology (NIST) believes that replacing the U.S. survey foot with the international foot will streamline surveying and engineering software and make map projections more accurate.

The U.S. survey foot is still here for now. The official change is not slated to occur until December 31, 2022, in tandem with a broader update of the National Spatial Reference System that year. That update will adjust official latitude, longitude, and altitude measurements at much larger scales than the foot discrepancy, so NIST anticipates that the change to the foot should be relatively minor compared to these larger changes.

Currently, NIST is soliciting comments about the change. If you have feelings about this 10th of an inch per mile, you can submit them here.