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2 Answers

How is a nautical mile defined?

Asked by: 3464 views General Aviation

Is it one minute of arc of latitude, or one minute of longitude, or both? And how does it correspond to the tick marks on the sectional chart

Simple question but I'm confused because I've seen conflicting answers from different websites. 

 

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2 Answers

  1. Best Answer


    John D Collins on Aug 31, 2018

    There is a precise definition of the nautical mile of 1852 meters. This was originally defined as one minute of latitude, but because of the shape of the earth, it is not constant and varies by about 60 feet from the equator where the earth bulges to the poles.where it is smaller. As a practical matter, one minute of latitude, or 1/60th of a degree is good enough for most uses. Sectional charts place the tic marks at each minute of latitude and longitude. The north-south or vertical north up view, these latitude tics are essentially constant and equal to one NM. One minute of longitude is zero feet at the poles and essentially the same distance as one minute of latitude at the equator and varies between these values between the equator and the pole by the Cosine of the latitude, so it is not a constant measure.

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  2. KDS on Aug 31, 2018

    John’s answer is perfect, as John’s answers always are. However, let me add this. The folks who wrote the FAA’s tests think that a nautical mile is exactly 6,000′. So, when you are doing FAA written test math, just use 6,000 as the magic number and your answers will come out the same as theirs.

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