Mind Your Feet

Understanding the U.S. Survey Foot Deprecation Under SPCS2022

One of the most consequential practical changes in the NSRS modernization is a change in units. Texas surveyors have used the U.S. Survey Foot for decades. Under SPCS2022, that foot is being retired in favor of the International Foot. This guide explains the difference, why it matters, and where the real risk lies.

Foot Definitions

  • U.S. Survey Foot (national definition, used in SPCS 83, codified in Texas Natural Resources Code §21.077): 1 ft = 1200/3937 m = 0.3048006096 m

  • International Foot (used in SPCS2022): 1 ft = 0.3048 m exactly

The difference between the two is approximately 2 parts per million. On a single lot line, that difference is invisible. But it grows steadily as coordinate values increase, which is exactly the situation with large state plane coordinate values.

Why This Time is Different

The U.S. Survey Foot and International Foot have coexisted for decades, but the U.S. Survey Foot was officially deprecated effective January 1, 2023, per a Federal Register notice from NIST and NGS. NGS will no longer support the U.S. Survey Foot in the new datums. SPCS2022 supports one foot only: the International Foot.

Texas surveyors will need to get accustomed to using International feet in any new SPCS2022 work. Legacy SPCS 83 data in U.S. Survey Feet will continue to be supported, so existing records are not invalidated, but new work under the modernized system means a new unit.

The International Foot has been largely absent from Texas surveying and geospatial mapping historically. Going forward, both definitions will coexist: the U.S. Survey Foot in legacy data, and the International Foot in new SPCS2022 work.

Watch Your Step – When Feet Matter

Not every project will feel this difference the same way. The size of the error depends on your zone and the distance involved. The size of the error can be negligible, dangerous or obvious.

Different measuring tapes and scales.

Negligible

You'll never notice: A 10,000 ft measured distance shifts only by approximately 0.02 ft. When you are measuring on the ground, the 2 parts per million difference won’t present itself in a noticeable way.



A road sign with an exclamation point.

The Danger Zone

These are differences that are big enough to fail, but possibly small enough to miss. Below, find a list of approximate differences you can expect around Texas depending on the zone you use. Be cautious if you are working in low number LDP zones because their false northings and eastings are smaller, resulting in errors that matter but might be missed.

  • TX North (481001), Amarillo area: ≈ 1.1 ft

  • TX North Central (481002), DFW area: ≈ 6.9 ft

  • TX Central (481003), Austin area: ≈ 8.2 ft

  • TX South Central (481004), Houston area: ≈ 9.1 ft

  • TX South (481005), Corpus Christi area: ≈ 4.2 ft

  • TX_ATSC (482001), Choke Canyon LDP: ≈ 3.3 ft

A red flag waving in the wind.

Obvious

These errors come from very large coordinate values resulting in errors that are big enough to raise a red flag. You will likely notice if these errors pop up in your data. Most of the Low Distortion Projection (LDP) zones have large enough coordinates that result in large errors when things get mixed up.

  • Corpus Christi, TX_CRP: ≈ 20.9 ft

  • Dallas/DFW, TX_DFW: ≈ 22.9 ft

  • El Paso, TX_ELP: ≈ 28.7 ft

  • Houston, TX_HARR20: ≈ 40.8 ft

  • Amarillo, TX_POTR: ≈ 68.5 ft

  • Abilene, TX_TALR: ≈ 78.8 ft

  • San Antonio, TX_TVBX: ≈ 86.4 ft

  • Austin, TX_TVBX: ≈ 86.9 ft

Check Before You Step - Practical Advice

  • Check your unit settings on every job, every file, do not rely on default templates.

  • Use NCAT to test conversions for your specific area before you rely on it in production work. Knowing what errors to expect is your first defense in catching them.

  • Know which foot your software is producing, an update can silently change the default without any warning or error. Don’t rely on templates, always check the unit settings before working with data or importing new data.

  • Provide a clear unambiguous label for your unit of measure on every deliverable. The label should clearly differentiate which definition of foot was used. Stating the definition alongside the label removes any ambiguity.

  • Use sft for US Survey Feet and ift for International Feet and avoid generic 'ft.' labels on maps, plats, and other survey deliverables.

Reference table showing suggested labels for foot units.