The United States adopted the international mile and foot in 1959, but for some reason they decided to keep alongside the older units as survey mile and survey foot. These units are almost equal to the first two, but "almost" means "problem" when it comes to measuring large distances, or areas (when the units are squared), and comparing measurements. I understand that some people did not want to bother with conversion of old measurements, but that was sixty years ago and the National Geodetic Survey wasn't sleeping since then. So why do they keep both units?
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2 Answers
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Back in 1959, it was because the US had accumulated a lot of survey data using the older definition of the foot.
For most people, the 2 PPM discrepancy between the two types of “foot” went unnoticed; AFAIK, no suburban homeowner ever put up a fight over losing ~100 µm of their lawn. But if you're making a detailed coast-to-coast map of the US, the discrepancy adds up to 9 m or so, which was enough to concern the mapmakers. So the geodetic surveyors were temporarily granted permission to use the old definition. However,
It was furthermore mandated that the U.S. survey foot be replaced by the international foot upon readjustment of the geodetic control networks of the United States. Although such a readjustment was completed in 1986, use of the U.S. survey foot persisted. This situation has led to confusion and errors that continue to this day, and it is at odds with the intent of uniform standards. (Source: NIST)
So, the original reason for retaining the survey foot ended 35 years ago, but the typically slow-to-react US government just didn't bother taking the old standard off the books. Finally, in 2019 (after the OP asked the question), NIST announced the deprecation of “survey” measure, effective December 31, 2022.
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This is more of a political question than scientific, I fear. The USA is really bad at standardization. The US Navy, for example, still insists on knots, fathoms, nautical miles, etc., even though their hardware is mostly full of metric fasteners and dimensions.
So far as survey-X units go, I wouldn't be surprised to find that they are in use so that all the previous benchmarks (stones hammered into the ground, mostly) don't need to be moved. ... Peeking around, there's the "acre" and then there's the "acre (Cheshire)" which are radically different. And hectares, and rods, and furlongs....
Rather as the UK is officially on kg but people still use "stone" , I fear no culture ever gives up its semitraditional units.
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6$\begingroup$ EVERY navy uses knots and nautical miles. This has nothing to do with US. $\endgroup$ Commented Jan 30, 2019 at 0:55
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1$\begingroup$ "I fear no culture ever gives up its semitraditional units" isn't exact. But it takes time (in France, although the metric system was imposed around 1800, it has taken more than a hundred years to forget all the old units). $\endgroup$ Commented Nov 9, 2021 at 1:16