According to Wikipedia, the nautical mile's derivation used to be one minute of latitude, but it's now an SI-derived unit exactly equal to 1852 m.
Also, my knit-picking about how to abbreviate nautical miles is going to come full swing here: "the symbols M, NM, Nm, and nmi are all used" to represent nautical miles. Please note that
nm is nanometers! and not nautical miles /endrant
(yes I know that I probably know what you're talking about when you use "nm" but it's a big difference in terms of distance)