America Has Two Feet. It's About to Lose One of Them

How big is a foot? In the United States, that depends on which of the two official foot measurements you are talking about. If it comes as a surprise that there are two feet, how about this: One of those feet is about to go away.

The first foot is the old U.S. survey foot from 1893. The second is the newer, shorter and slightly more exact international foot from 1959, used by nearly everybody except surveyors in some states. The two feet differ by about one hundredth of a foot (0.12672 inches) per mile — that’s two feet for every million feet — an amount so small that it only adds up for people who measure over long distances.

Surveyors are such people. For more than six decades, they have been toggling between the two units, depending on what they are measuring and where.

The toggling does not always work. Michael L. Dennis, an Arizona-based surveyor and geodesist with the National Geodetic Survey, has been cataloging mix-ups with the two feet for years and repairing errors. Last year, he had enough.

“I kept running into these problems with different versions of the foot, and I thought it was ridiculous that this thing had gone on this long,” he said. “So I had this secret desire to kill off the U.S. survey foot, and I’d been harboring that for years.”

Most states mandate the use of the old U.S. survey foot for their state coordinate systems, which allow surveyors to take into account Earth’s curvature in their measurements. A few states mandate the use of the new, international foot. A handful do not specify which of the two feet should be used. Arizona, for example, is an international-foot state, but when employees with the Federal Aviation Administration or the U.S. Army Corps of Engineers or the Park Service measure there, they use the U.S. survey foot.