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 Topic     Nautical Mile
    Title Author          Date & Time 
         International definition of the Nautical Mile John Clack 7 May 2006 06:52
           Re: International definition of the Nautical Mile John von Rosenberg 16 Aug 2007 22:19



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Title:     International definition of the Nautical Mile
Author: John Clack Posted on: 7 May 2006 06:52
 Click here to contact the author Posting ID #454
 
I understood that the NM was definined as a metric distance of 1852 m in 1929. The original definition was a British Admiralty definition of 6080 feet. Is it the case that some countries still use the British Admiralty definition? I thought the UK adopted it when UK charts adopted soundings in metres not feet.


I also thought that, perhaps pedantically, a Sea Mile was the length of one minute of latitude i.e. one minute of arc along a line of longitude (a meridian) at the point of interest, and to only measure off the latitude scale at the latitude of interest. The Sea Mile of course approximates to the Nautical Mile for most purposes of navigation. but values vary from 6,046 feet on the equator to 6,108 at the poles.
 
 



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Title:     Re: International definition of the Nautical Mile
Author: John von Rosenberg Posted on: 16 Aug 2007 22:19
 Click here to contact the author Posting ID #555
 
I usually turn to The American Practical Navigator (The Bowditch) as my authority on these things.

"For practical consideration [a nautical mile] is usually considered the length of 1 minute of any great circle of the earth, the meridian being the great circle most commonly used. Because of various lengths of the nautical mile in use throughout the world, due to differences in definition and the assumed size and shape of the earth, the International Hydrographic Bureau in 1929 proposed a standard length of 1,852 meters, which is known as the International Nautical Mile. This has been adopted by nearly all maritime nations. The U.S. Departments of Defense and Commerce adopted this value on July 1, 1954. With the yardmeter relationship then in use, the International Nautical Mile was equivalent to 6076.10333 feet, approximately. Using the yardmeter conversion factor effective July 1, 1959, (1 yard = 0.9144 meter, exactly) the International Nautical Mile is equivalent to 6076.11549 feet, approximately."

and

"sea mile - An approximate mean value of the nautical mile equal to 6,080 feet; the length of a minute of arc along the meridian at latitude 48°."