The Source Civil Engineering Magazine US plans to retire ‘survey foot’ length

US plans to retire ‘survey foot’ length

By T.R. Witcher

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.”

two crossed platinum-iridium bars against an orange background
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?”

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  1. Why half-solve the problem? Why not go fully to the metric system, as we should have when it was “mandated” in the 90s, but Clinton didn’t have the political courage to complete the transition (and neither has any other president since)? Remaining on the “English System”, which the English, being more sensible, don’t use, is absurd and causes disdain throughout the world. The metric system is a far superior decimal system, like our currency, and is fall less confusing than the English System.

    • The Federal Courthouse in Tucson AZ was the only building completely designed and constructed using the SI system. The contractor had interns take every (SI) dimension on the plans and redo them to feet/inches. It really wasn’t constructed with SI; it is a case study that shows the resistance to change.
      As an engineer, I could make the switch but my judgment (length, force) is still formed by British standard; that isn’t easily changed.

  2. Although it is traditional to use the foot (which one?), and we’ve become accustomed to using it, it’s time to change to metric. Let do it now!

    Joe Valdes
    Retired Civil Engineer

  3. The idea that the metric system and similar base 10 mensuration logic should prevail as unifying standards was endorsed in the US as far back as Thomas Jefferson and James Madison. They did a pretty good job of founding the Nation so how about we heed their wisdom here.

  4. I think the issue is not one of should we switch but rather how do we switch. It was not political courage, it was financial. First there is the cost of converting all of your old plans to metric. While most engineers only think of new designs, owner-operators continue to rely on these old plans to maintain their assets. There is a cost to updating those and training their staff so errors do not occur due to the unfamiliar units. Another financial issue is manufacturing. Think of all of the fittings, steel, wood, and piping that are based on feet and inches. Then there are all the standards that would need to be updated to reflect those changes. I owned a consulting company that helped Caltrans update their existing plans to metric in the early 90’s. One example, an existing 1″ pipe was converted to 2.54cm and the field crews were having to look up each of the pipes sizes because they were unfamiliar with metric conversion. The pipe manufactures were not converting their sizes and had existing contracts based on English. The construction crews starting making errors due to the confusion. Another concern for owner/operators, if you own thousands of miles of pipeline, when you replace or add new pipes to your network, do you put in an adaptor and switch to standard metric size pipe and have a hybrid system for decades until the old pipes get replaced? How will that change impact your analysis and engineering? Yes, these types of issues could all be mitigated but the first stab at converting to metric was not thoroughly thought through and failed. My point is that I would love to see it happen but it would require a ton of money, planning, and support to make it successful.

  5. The pleas to adopt the SI System (metric) of measurement seem to be falling on ever more hearing impared individuals including bizarrely, some engineers and scientists.
    The inch, foot (survey and international), yard and mile related by factors which surely must have been devised by some partially numerate romantic, are just the tip of the iceberg. How many people have died as a result of mistakes made when designing and building using these absurn units? One unit, the meter, would dispense with all of this nonsence.
    Speaking of icebergs, have a look at the contortions which mechanical engineers are forced to endure when designing engines and HVAC systems. Designs for relatively simple systems will include measurements of energy and work using Betty Crockeresque units such as tons of cooling, horsepower, British thermal units, quarts and teaspoons which are … wait for it…. mixed in with kilo-Watts, a compound SI unit for power! (teaspoons were added for effect. Sorry).
    Surely, budding Engineers in the US should be spared this torture. Let’s consign the learning of imperial/American/English units to their rightful place. In a history elective.


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