Using your figures it still works out to less than 1%
difference. Certainly close enough to approximate as
a sphere and avoid making the problem overly complex.

Jeff wrote:
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.

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


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.