Since 1959, there have been two official versions of the foot used in land surveying – the U.S. survey foot and the international foot. In 2022 the U.S,. survey foot will be officially replaced by the international foot – ending a time of confusion and errors.
History of the Two Feet
In 1893, through the Mendenhall Order, the U.S. created a definition for the foot – later called the U.S. survey foot — to support U.S industry and international trade by aligning with the international metric standard. This new foot was defined by the meter – essentially making the U.S. a metric country even if it has never felt like one. The definition of the U.S. survey foot is equal to 1200/3937 meters, around 0.3048006096 meters.
After 1901 a new, more exact definition of the foot started to be used internationally. Under this new definition, one foot equaled exactly 0.3048 meters. This became known as the international foot. The international foot was adopted by the American National Standards Institute’s predecessor in 2033 and the National Advisory Committee for Aeronautics (NASA’s predecessor) in 1952.
In 1959, the international foot was fully adopted by the U.S. and was meant to be the only definition of the foot. However, a temporary allowance was given for surveying and mapping. This allowance was due to the large amount of existing survey data that used the U.S. survey foot, and because the difference was not noticeable when surveying small parcels of land.
The U.S. survey foot should have been replaced with the international foot in 1986, when the geodetic datum was changed. This didn’t happen. In fact, both definitions are still used by some states and government agencies, regardless of statute. The difference between the two definitions is small – 0.01 ft/mile – but has still caused major problems.
The Announcement of the End
The National Institute of Standards and Technology (NIST), National Geodetic Survey (NGS), National Ocean Service (NOS), National Oceanic and Atmospheric Administration (NOAA) worked together to phase out the U.S. survey foot. This change will happen in 2022 as part of the modernization of the National Spatial Reference System to minimize disruption.
The Federal Register issued a notice on October 17, 2019 announcing the end of the U.S. survey foot. After December 31, 2022, the U.S. survey foot will no longer be used in surveying and mapping. The international foot will be known simply as a foot. The Federal Register Notice invited public comments until December 2, 2019, in order to make the transition as smooth as possible and make sure nothing related to the transition was overlooked.
What Comes Next?
In June 30, 2020, the final ruling will be made. A second Federal Register Notice will be published sometime around that date to officially and formally announce the change. The Notice will also describe the final details and how the change will happen.
The Bureau of Land Management and other government agencies will support and adapt to these changes. The NIST “Guide for the Use of the SI” unit of measurements, currently only based on the U.S. survey foot – like acre – will be updated with the new foot definitions.
NGS will help with the transition by automatically supporting older survey data that uses the U.S survey foot and still giving the “correct” foot for SPCS 83 and SPCS 27. SPCS 22 will only use the international foot.