Bruce in Alaska wrote:
In article ,
krj wrote:

Peter Bennett wrote:
On Fri, 01 Dec 2006 14:12:52 GMT, keith_nuttle
wrote:


Since the nautical mile is defined as one minute of latitude, I doubt it
will ever disappear in earth based navigation.


The Nautical Mile was originally based on a minute of latitude, but
the length of a minute of latitude varies with latitude, so an
International Nautical Mile is now defined as 1852 metres exactly, or
6076.11549 US feet, approximately, according to Bowditch.



I believe that the nautical mile is based on the minute of LONGITUDE not
latitude. Longitude doesn't vary with changes in latitude (or attitude).
That's why when you are using paper charts (remember those) you measure
the distance with your dividers on the longitude scale on the east or
west side of the chart.
krj

Hmmmmm..... I think you have your terms "Wrapped around the Axle"
here.... Nautical Miles is based on "Minute of Latitude" which does
NOT change since the distance from the Equater to the Poles is ALWAYS
the SAME, from any place on the EQUATER. Therefor, when dividing that
distance in Degrees, MInutes, and Seconds, will ALWAYS produce the same
Unit Lengths. Where as Longitude is the cicumfrence of the earth at
the equater divided into Degrees, Minutes, and Seconds, and that DOES
change Unit length as you move closer to the Poles. Longitude is marked
on the bottom and top of Maps, where Latitude is marked on the right and
left had edges of Maps. One ALWAYS uses the Latitude markings for
distance because they do NOT change Unit Length, EVER.....

Bruce in alaska

Bruce has this right, but do watch out for very large area charts,
like, say, the North Atlantic (do they still print this?). To measure
nautical miles, you have to use the latitude scale near the latitude
you are measuring because the printed "length" of a minute of latitude
changes up and down the chart. This is an oddness of Mercator
projections. Similarly the shortest distance between two points on a
large area Mercator chart is usually NOT a straight line (see great
circle courses). But the Earth is round, and a real minute of Latitude
is always a nautical mile.