Michael A. Covington wrote:
A nautical mile is an arc-minute
on the globe of the earth.

However, the earth is not a true sphere. It is flattened like a
spinning top and has bumps and hollows like a potato. Thus there is no
constant relationship between angle at the core and distance at the
surface. A minute of arc at one point can be tens of metres different
to a minute of arc at another.

This uncertainty became unacceptable decades ago as navigation became
more precise. The superceded definition remains a convenience, but the
current definition no longer refers to angle. It is a fixed distance.

1 nautical mile = 1852 metres precisely
http://www.bipm.org/en/si/si_brochur...r4/table8.html