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B.6 U.S. survey foot and mileThe U.S. Metric Law of 1866 gave the relationship 1 m = 39.37 in (in is the unit symbol for the inch). From 1893 until 1959, the yard was defined as being exactly equal to (3600/3937) m, and thus the foot was defined as being exactly equal to (1200/3937) m.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 m, and thus the foot has been defined as exactly equal to 0.3048 m. 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 ft = (1200/3937) m (ft is the unit symbol for the foot). 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.In Sec. B.8 and Sec. B.9, the factors given are based on the international foot unless otherwise indicated. Users of this Guide may also find the following summary of exact relationships helpful, where for convenience the symbols ft and mi, that is, ft and mi in italic type, indicate that it is the U.S. survey foot or U.S. survey mile that is meant rather than the international foot (ft) or international mile (mi), and where rd is the unit symbol for the rod and fur is the unit symbol for the furlong.
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