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
--
add a 2 before @

On Fri, 01 Dec 2006 20:18:10 GMT, 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

Since the earth is not a perfect sphere, the length of a minute of
latitude _does_ vary slightly with latitude - Bowditch has a table
showing the lengths of a minute of Lat and Long for various latitudes.
I vaguely recall a minute of latitude varies by 300 ft or so from
equator to pole (don't recall which end is longer), and is only a
nautical mile at about 44 degrees.

"Peter Bennett" wrote in message
news.com...
On Fri, 01 Dec 2006 20:18:10 GMT, 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

Since the earth is not a perfect sphere, the length of a minute of
latitude _does_ vary slightly with latitude - Bowditch has a table
showing the lengths of a minute of Lat and Long for various latitudes.
I vaguely recall a minute of latitude varies by 300 ft or so from
equator to pole (don't recall which end is longer), and is only a
nautical mile at about 44 degrees.

I have a feeling that small diff isn't going to hurt your transatlantic
jouneys. :-)

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

"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.

"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

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.