When the holiday season is over and life returns to normal in January, 2023, the US survey foot will have officially been retired – or “deprecated” – by the National Institute of Standards and Technology (NIST). In its place will be a unit of measurement known as the international foot.
As NIST states, after December 31, 2022, “the deprecated measurement unit known as the U.S. survey foot will be deemed obsolete, its use is to be avoided, and it will be retained for historical purposes and legacy applications only.”
Here is a bit of history from NIST itself:
The U.S. Metric Law of 1866 gave the relationship 1 meter = 39.37 inches. From 1893 until 1959…the foot was defined as being exactly equal to (1200/3937) meter… In 1959 the definition of the yard was changed to bring the U.S. yard and the yard used in other countries into agreement… Since then the yard has been defined as exactly equal to 0.9144 meter, and thus the foot has been defined as exactly equal to 0.3048 meter. At the same time it was decided that any data expressed in feet derived from geodetic surveys within the United States would continue to bear the relationship as defined in 1893, namely, 1 foot = (1200/3937) meter. The name of this foot is “U.S. survey foot,” while the name of the new foot defined in 1959 is “international foot.” The two are related to each other through the expression 1 international foot = 0.999 998 U.S. survey foot exactly.
While the difference between the US survey foot and international foot is very small (several parts per million), NIST notes that significance differences in distances and coordinate values can occur, including measurements involving rectangular coordinates such as those used in the State Plane Coordinate System (SPCS).
The shift to the international foot was originally intended to be part of a larger suite of changes associated with the modernization of the National Spatial Reference System (NSRS) by the National Geodetic Survey (NGS). At the state level, the modernization effort will result in an updated version of the state plane system known as SPCS2022. However, due to delays in the modernization effort – which is now slated for 2025 or later – the change to the international foot will precede other NSRS modernization changes. When NSRS modernization was delayed, NGS decided to remain on its original schedule for survey foot deprecation, due to the difficulty of going through the deprecation process and because it gives people more time to prepare for the change when NSRS modernization occur.
Why is NIST enacting this change? According to NIST, “Discontinuing the use of the U.S. survey foot in surveying, mapping, and engineering is a practical solution to a long-term problem. Ending this will eliminate confusion and unnecessary costs. Improved measurement uniformity and accuracy will benefit stakeholders, including the States and professionals in the surveying, mapping, and engineering fields.”
Skeptics do exist, however. Following an NGS webinar on Nov. 10, 2022, one participant asked:
We are eliminating the U.S. Survey foot in favor of the International foot when the vast majority of DOT’s and surveyors use the U.S. Survey Foot. The only three countries which still use Imperial measurements are the U.S., Liberia and Myanmar. Therefore, why are we changing to a standard that only exists for a smaller population instead of holding the U.S. Foot which is in the majority use? … Since the NGS is metric, what difference does it make to you as to which foot is being used?
The NGS response was as follows:
The main reason for deprecating the U.S. survey foot (sft) and adopting the international foot (ift) nationwide is that the ift was adopted as the official version of the foot in 1959 by the U.S. (and several other countries at about the same time). The previous foot was named the sft at that time and was allowed to persist temporarily for one and only one reason: for continued use in geodetic survey applications (such as State Plane). It was not intended for use in boundary surveys or any other application, and its use was supposed to end when NAD 83 was adopted in 1986. But that did not occur, and an unintended outcome was the continued use of two nearly identical versions of the foot. It has nothing to do with the ift being “better” or having an exact metric equivalent; it is simply an effort to standardize a unit of measurement. And although most states use the sft for surveying applications, surveying represents only a small fraction of overall usage. Every other part of the U.S. society (and the world) that uses the U.S. customary system uses the ift. The difference it makes to NGS, and to the National Institute of Science and Technology (NIST), is that having two versions of the foot in concurrent is at odds with the idea of standards, and it causes errors and confusion.
It may sound counter-intuitive, but NIST and NGS actually recommend continuing to use the US survey foot in conjunction with NAD 83 after the end of 2022. Specifically, you should not make the switch to the international foot while using SPCS 83 or another coordinate system based on NAD 83. This includes WISCRS (Wisconsin Coordinate Reference Systems). States like Wisconsin currently using the survey foot with NAD 83 should continue to do so. The international foot will be used for SPCS2022 and other components of the modernized NSRS, but that will not be until at least 2025. NGS will continue to support the survey foot even after NSRS modernization is complete.
For example, consider projects in SPCS 83 using U.S. survey feet that are already underway when the 2022 NSRS modernization occurs. In some cases, the most cost-effective and efficient strategy will be to maintain the project in the legacy coordinate system and units. In other cases, it will be better to migrate the entire project to the new system during execution. The most appropriate choice for managing the change will depend on a variety of factors, including the type of project, its size, complexity, duration, and status, as well as the ability, willingness, and preparedness of the organization making (or mandating) the change. But in all cases, the part of the change due to elimination of the U.S. survey foot will be a very small part of the overall change.
Existing legal statutes specifying the US survey foot for SPCS 83 are also not a problem, since NGS will maintain SPCS 83 in legacy definitions.
Another point of possible confusion is that, as of January 1, 2023, the term “foot” as used in surveying and mapping will officially refer to the international foot. Any legacy data that uses the term “foot” to mean “US survey foot” will be out of synch with official nomenclature. It will be important to ensure metadata reflects updated terminology. Users will need to be especially diligent when dealing with legacy data that was developed before the US survey foot was deprecated. Otherwise, users could be inappropriately mixing US survey feet and international feet without realizing it.
As NIST notes, “The most important way to prepare is simply being aware that the change will occur and maintaining documentation (metadata).”
Another concern is software. It is unclear how or when software companies will accommodate the change, since NIST and NGS have no control over private software developers and cannot say definitively how they will handle deprecation. There are several concerns listed here and here:
- Instances where software or electronic surveying devices default to one or the other foot definitions, but users incorrectly assume the actual unit of measure in use.
- Making sure that software supports the international foot definition. (One way to do this is by checking large standard values used in surveying, such as the semi-major axis of the GRS 80 ellipsoid.)
- NGS will provide SPCS2022 definitions to EPSG, but this will not be until 2025 or after.
Until details are completely worked out, software users should exercise caution when dealing with “feet” in their projects.
And in Conclusion…
After almost 130 years, the US survey foot may finally be on the path to retirement, but even NIST and NGS admit it will still be with us for some time. In the not-too-distant future the survey foot will cease to be actively used in surveying and mapping, but its use in legacy systems and data could easily continue for decades. So rather than saying “Farewell” to the survey foot, perhaps just say, “See you next year!”