Intresting, seems it's a brititsh idea to change it with latitude. Copy of wikipedia:
In English usage, a
sea mile is, for any latitude, the length of one minute of latitude at that latitude. It varies from approximately 1842.9 m at the
Equator to approximately 1861.7 m at the Poles, with a mean value of 1852.3 m.
[5] The international nautical mile was chosen as the integer number of metres closest to the mean sea mile.
How ever at the top of the article it says
The
nautical mile (symbol
M,
NM,
Nm or
nmi) is a
unit of
length corresponding approximately to one
minute of arc of
latitude along any
meridian. By international agreement it is exactly 1,852
metres (approximately 6,076 feet).
So it seems the nautical mile is always 1852 metres, how ever a "sea mile" would change somewhat with placement on the earth, that seems to be an idea implemented only in the UK though, this entire subject seems to be a bit odd
