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*From*: Hugh Haskell <hhaskell@MINDSPRING.COM>*Date*: Thu, 19 Aug 2004 13:19:01 -0400

At 8:56 AM -0500 8/19/04, RAUBER, JOEL wrote:

If memory of my days doing navigation while in the Navy serves, There

As a side note, the airlines and mariners have long been interested in the

solution to this problem. And certain map projections have been designed to

facilitate the solution.

is not a single heading one can take to follow a great circle route

from point A to point B, since the great circle route crosses each

meridian at a different angle, and it is that angle that determines

the compass heading one takes at that point. The straight line path

on a Mercator projection map (called a "rhumb line") does have a

constant heading, but it is not the great circle path.

Given that, I don't think that ships or aircraft would be interested

in a single heading to follow for an extended trip--one for which a

great circle path is significantly different from the rhumb-line

path. As a practical matter, on a trans-oceanic trip, a ship's

navigator will calculate the great circle path (and I have forgotten

the method used to do that on a Mercator projection), and then

approximate that path with a series of rhumb-line segments, so that

the ship's heading will change periodically during the trip. I recall

vaguely that the rhumb-line segments typically were about 5 degrees

of longitude, each, or some 300 nautical miles or less--roughly one

day's steaming.

Things may have changed dramatically with the advent of GPS

navigation, which occurred long after I got out of the navigation

business. I would expect that computerized nagivation systems will

automatically calculate the heading on a great circle route, leaving

the navigator with precious little to do, once a chip is clear of

harbor restrictions.

Hugh

--

Hugh Haskell

<mailto:haskell@ncssm.edu>

<mailto:hhaskell@mindspring.com>

(919) 467-7610

Never ask someone what computer they use. If they use a Mac, they

will tell you. If not, why embarrass them?

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