Nautical Kilometers:

[Brian D. McCary]

If the intent of nautical miles is to relate the size of the mile to the 
size of the earth (ie, 1 arc minute at sea level is approximately 
one nautical mile, resulting in 360*60 = 21600 nautical miles of circumferance,
give or take) then the kilometer is, in a similar sense, a nautical
kilometer, automatically.  The meter distance was set up so that there 
would be 10,000 kilometers from the equator to the pole.  Again, this
is approximate, given all the eccentricities of the earth's shape.  Here,
the arc minute has been abandoned in favor of the grad: there are 100 grads
in a 90 degree arc, so that 1 grad = 0.9 degrees.  Thus, there are 100
kilometers in 1 grad at sea level.  Nobody except perhaps artilery men or 
civil engineres use grads, but if you check any scientific calculators, there
will be keys marked DRG, which will allow you to convert between degrees,
radians, and grads.  What a drag.

Brian McCary