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Comment by mdturnerphys | Hacker News Reader

It's unfortunate and potentially confusing to readers that they've used the Washington Monument and the Flatiron Building as illustrations in the article and suggested that their heights might change. The reference point for measuring a structure's height is its base, not the geoid. Unless the definition of a foot changes the height of these structures will not change.

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Curiously, the definition of a foot is changing. Or rather, one of the two definitions of a foot (the U.S. survey foot) is being deprecated in favor of the other (the international foot). The two differ by about two parts per million.

My understanding was that an inch is exactly 2.54cm, and a foot is exactly 12 inches ever since the metrification of US customary units. Is that not correct?

The foot was defined in terms of meters only in 1959, and the definition was slightly different than prior use. (The US and the Commonwealth previously used slightly different feet.) The survey foot is the definition prior to the definition in terms of meters, and is still widely used in surveying.

https://en.wikipedia.org/wiki/Foot_(unit)#Survey_foot

No, the original metric definition of US units in 1894 [0] made 1 yard = 3600/3937 metre (i.e. 1 inch = 100/3937 metre). The US survey foot is 12 of those inches.

The 1 inch = 25.4 mm definition was adopted by the British Standards Institution in 1930 and by the American Standards Association in 1933 (with the name "industrial inch"); in 1959 the US, UK, Canada, Australia, New Zealand and South Africa signed a treaty agreeing on that definition [1] (and also a definition of the pound). The international foot is twelve of those inches, but the US allowed (but recommended against) the continued use of the old foot in surveying.

[0] https://usma.org/laws-and-bills/mendenhall-order [1] https://en.wikipedia.org/wiki/International_yard_and_pound

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