The story of codifying measurement standards in the United States has often been a case of one foot forward, two feet back. Case in point: For almost 100 years the United States has actually had two different definitions for what constitutes a foot.

But after years of inaction, the federal government will finally put its best foot forward — retiring the longstanding U.S. survey foot in favor of the international foot. “It’s like an anti-standard,” says Michael Dennis, Ph.D., PE, M.ASCE, a geodesist with the National Geodetic Survey. Dennis has been spearheading the drive to retire the old standard — and making the case to skeptical surveyors. “We have these two things out there and these two versions of the foot, which creates nothing but problems. The difference is small, which is why it's so insidious.”

Indeed, the international foot is shorter than the U.S. survey foot by a vanishing 2 parts per million (or 0.01 ft every mile) — it would be like extending the east-west length of the United States by just 30 ft.

But the difference matters. As the National Oceanic and Atmospheric Administration puts it on its webpage explaining why the U.S. survey foot needs to be retired, “accidentally confusing the two types of feet can severely impact the precise coordinates and measurements used in engineering, surveying, mapping, agriculture, and other industries that depend on accurate positions.”

prototype bar
When the U.S. ordered the length of the foot to be derived from the meter in 1893, a platinum-iridium bar such as this was the physical basis for determining the length of a meter.

The change is a collaborative effort between the National Institute of Standards and Technology and the NGS, which is part of NOAA. The change will take place at the beginning of 2023, when the National Spatial Reference System is modernized, although Dennis notes the full transition may end up taking a few years longer.

The questions, of course, are: How did two standards develop and why did they persist? In a public webinar he created on the history of the two standards, Dennis explains that the need for standardized measurement dates back to the Constitution itself, which authorizes Congress to “fix the Standard of Weights and Measures” for the new country. George Washington and Thomas Jefferson were surveyors — Jefferson proposed a decimal system in 1790. Starting in 1815, the U.S. used an object called the Troughton bar, which was an exact copy of the British Imperial Yard. (The physical Imperial Yard was damaged beyond repair when Parliament burned in 1834; the replacement bar — Bronze Yard No. 11 — was 0.022 mm shorter than the Troughton bar.)

Meanwhile, the metric system, Dennis explains, had been developed in France and was gaining support. In 1866, Congress legalized use of the metric system for commerce in the U.S., and in 1893, Thomas Mendenhall, superintendent of the U.S. Coast and Geodetic Survey (the forerunner to the NGS), issued the Mendenhall Order, which based the length of the foot off the meter, where 1 ft equaled 1,200/3,937 m. So, as it turns out, the U.S. really is a metric country; it just doesn’t know it.

A more exacting definition of the foot was adopted in 1933 by the predecessor of the American National Standards Institute, the American Standards Association, to aid industry: 1 ft now equaled 0.3048 m exactly, according to Dennis. Eventually, the predecessor of NASA, the National Advisory Committee for Aeronautics, adopted this new foot standard in 1952, and in 1959, the U.S. government decided that the new foot, now called the international foot, would be, well, the foot. A giant step forward, right? Except for this: According to NIST, the 1959 redefinition of the foot still allowed geodetic surveyors to continue using the older foot standard, renamed the U.S. survey foot. The ruling mandated that the international foot replace the U.S. survey foot “upon readjustment of the geodetic control networks of the United States,” according to NIST’s website about the dueling feet.

The geodetic readjustment was completed in 1986, but reluctance among surveyors to convert to a new standard led to the federal government dragging its feet for several more decades. “The logic at the time was, ‘Well, it's no big deal,’” says Dennis. “As long as you keep track of which version of the foot, you have no problem. Well, people don't do that. So it turned into a big problem.”

Most states use the survey foot, though six states use the international foot, and a handful of states do not officially define a system of measurement. Further, in some states you might see one standard used by surveyors and another standard used on military bases or airports. As NIST explains on its dedicated website, the ambiguity of the two systems can result in “professional liability by the inadvertent violation of state law, the introduction of systematic errors in surveying and engineering projects, misreported position and location, land sale and project delays, boundary disputes, (and) additional costs associated with correcting unit mistakes” — to say nothing of the “inefficiency of managing two types of feet.”

Dennis notes that the difference in the two standards once resulted in a building in Arizona near an airport being accidentally positioned slightly within the Federal Aviation Administration’s glide path — the building had to be built with one less floor to resolve the issue.

Judging by the response from webinar presentations that Dennis has given to stakeholders, professional surveyors have bought into the changes and the deprecation will include a template for states to update their statutes to help them move off the survey foot, he says.

“We all know these kinds of processes take time,” says Elizabeth J. Benham, metric coordinator with NIST. “This is a very ingrained process in the survey industry. We wanted to do that to give people a full two years to work on it.”

After that? Maybe the country will be ready to abandon its feet for good. “I liked to see that there are a lot of people (who) were like, ‘Why don't you just go metric?’” says Benham. “Why are we just eliminating this? Why don’t we eliminate both feet?”