OR
The Case of The Double Standard
The US Metric Law of 1866 said that one meter was equal to 39.37 inches, exactly. In 1959, the relationship between inches and centimeters was redefined to be that one inch is equal to 2.54 centimeters, exactly. Maps produced by the US Coast and Geodetic Survey continued to use the old standard. To clarify which foot you are talking about, the old foot, derived from 1 meter = 39.37 inches (exactly), is referred to as the “US survey foot”. The new foot, derived from 1 inch = 2.54 cm (exactly), is referred to as the “international foot”.
The
relationship between the two is: 1 international foot is equal to 0.999 998 US
survey foot, exactly. The “error” is thus 2 parts in a
million.
Over what distance would the
error accumulate to 6 inches? The answer is 47.35 miles. In other words, at its
longest dimension, Rhode Island is about 6 inches longer than the map
says!
See: http://physics.nist.gov/Pubs/SP811/appenB.html#B6
and http://www.sfei.org/ecoatlas/GIS/MapInterpretation/ConversionConstants.html