Read up on the Nautical system, and you'll see how much simpler it is, than the metric with regard to global, universal measurements.
This system was developed over hundreds of years of exploration. Had the metric system been easier I'm sure it would have been used, as the metric system was known in the east since baylonian times...
http://en.wikipedia.org/wiki/Nautical_mile to correct al those incorrect measurements.
In SH4 they've changed the size of the Nm to 2000m (1852m)
