"John Gaquin" wrote in message
. ..

"Bruce in Alaska" wrote in message news:bruceg-

.... 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.

Actually, the earth is not a perfect sphere, and the linear distance of 1
minute of latitude does vary slightly as you progress from equator to
pole.

One ALWAYS uses the Latitude markings for
distance because they do NOT change Unit Length, EVER.....

not so....


I believe the nautical mile was at one time specifically defined as equal
to 1 minute of longitude *at the equator*. As a practical matter, you
can apply this standard to latitude as well, using a minute of latitude
as a mile indicator. This is what is taught in all the CGAux nav classes
I have taught. Of course, a minute of latitude varies a bit from one
place to another, as mentioned above, but for all practical purposes, the
error can be discarded.

oblique spheroid I believe is the correct term

--
"j" ganz @@
www.sailnow.com