You're being a bit free and easy with the math. Although the
curvature is small, the length of a minute of latitude varies from
1842.9 meters at the equator and 1861.57 at the poles. For the
nautical mile, many countries had their own version, the current
convention is 1852 meters. This is not enough of a discrepancy
(usually) to ruin your day, but with a GPS is should be easily noticeable.

The land based mile varied considerably throughout the "civilized"
world, much to the confusion of ancient navigators and modern
historians. For example, the Italian mile was about 4800 feet, but
the length of the Arab mile was 4000 cubits, which, depending on what
you took for a cubit (there were several versions) is about 6545 feet.



wrote:
wrote:

Bart Senior wrote:
How long is a minute of Longitude--at your Latitude?
And what is the formula to calcuate it?

There are 1440 minutes to a day, regardless of what latitude or
longitude you live in. Divide that by 360 (degrees) you get 4 (mins).
Every degree of longitude you have 4 mins.

Rgds
Ravi

Incorrect Ravi. Perhaps I should have been more clear. I am looking
for a distance not a time measurement.

One minute of arc is 1 nautical mile where ever you are. The earth is
not a sphere, there is about a 1% difference between the values at the
poles. Close enough for us to call in a sphere.

This works out nicely because for latitude, the spacing between
lines of latitude is constant. This was defined as a nautical mile
because it was fairly close the the distance used on land, the
statute mile. It is easier to use the nautical mile to avoid
constantly
converting from one unit to the other.

So one minute of latitude is one nautical mile and one can read
this measurement off the edge of a chart as well as the distance
scale on the bottom of the chart and be equally accurate.

For longitude lines which converge at the poles things are different.
At the poles a minute of longitude is impossibly short and approaches
zero as you approach the pole. Moving away from either of the poles
to the equator, one minute of arc approaches 1 [1 nautical mile].

There is a formula for this:

1 minute of longitude = the cosine of your latitude in nautical miles.


I live at 41.265 North The cosine of which is: .752.

So 1 minute of longitude at my location can be approximated
as 3/4 of a nautical mile. It is a handy thing for me to know.

BTW, thank for the invite. Do you have any pictures of your part of
the world, and your boats you can show us?

Bart