How far apart do they get?
Same number, read as US survey feet vs international feet:
| Distance | Gap | In inches |
|---|---|---|
| 1 mile | 0.0106 ft | 0.13 in |
| 100,000 ft | 0.2000 ft | 2.40 in |
| 1,000,000 ft (a common State Plane easting) | 2.0000 ft | 24.00 in |
| 2,000,000 ft | 4.0000 ft | 48.00 in |
| 10,000,000 ft (a Texas Central northing) | 20.0000 ft | 240.00 in |
So a State Plane coordinate read in the wrong foot can move a point by 2 to 20 feet.
Why are there two feet?
In 1959 the US set one foot to exactly 0.3048 meters. That is the international foot, the one on every tape measure. Surveyors were allowed to keep the older foot, 1200/3937 meters, so old maps would still match. That one is the US survey foot.
The US survey foot is longer by about 2 parts per million. That is 1/500 of an inch in 1,000 feet, but State Plane numbers are often in the millions of feet, so the gap grows to feet.
Sources: Federal Register, 85 FR 62698 (Oct 5, 2020) and NIST: U.S. survey foot; NOAA Special Publication NOS NGS 13, The State Plane Coordinate System (2018), Appendix C, Table C1.
NGS and NIST retired the US survey foot
NIST and NOAA's National Geodetic Survey (NGS) decided the US survey foot should not be used after Dec 31, 2022. From Jan 1, 2023 the "foot" means the international foot. Older data and state laws that name the US survey foot still exist for SPCS 83, so you will keep seeing it. NGS's next system, SPCS2022, will not use it.
Sources: Federal Register, 85 FR 62698 (Oct 5, 2020) and NIST: U.S. survey foot; NGS State Plane Coordinate System page.
Which one does my state use?
For SPCS 83, 40 states use the US survey foot and 6 use the international foot (Arizona, Michigan, Montana, North Dakota, Oregon, South Carolina). Some have no foot in their law. Find yours with the zone finder.
Source: NOAA Special Publication NOS NGS 13, The State Plane Coordinate System (2018), Appendix C, Table C1.
Exact numbers
| 1 US survey ft | 1200/3937 m = 0.3048006096… m |
|---|---|
| 1 international ft | 0.3048 m exactly |
| 1 m | 3.2808333… US survey ft = 3.2808399… international ft |
| Difference | 1 US survey ft = 1.000002 international ft (2 ppm) |
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