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I disagree with your assertion. The shortest distant on the  surface of the
earth is the geodesic (what you call a great circle on a spherical earth)
passing thru the two points. I believe the azimuth calculated on the ellipse
between the two points is the direction you would sail (neglecting the
variation etc.) passing from A to B. The methods you describe make an
attempt to approximate the geodesic but you cannot deny that the earth is an
ellipsoid. (I guess you can, but it flies in face of the facts). You also
cannot deny that the great circle route is a fiction and the shortest
distance between A and B is the elliptical geodesic (by definition the
shortest distance).

My comments should not be construed as meaning that great circle or rhumb
line sailing is not valid. What ever works works, but it is at best an
approximation.

Vic
__________________________________________________

Victor Fraenckel - The Windman                 vfraenc1@nycap.rr.com
KC2GUI                                                      www.windsway.com

      Home of the WindReader Electronic Theodolite
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-Count Oxenstierna (ca 1620)

----- Original Message -----
From: "Paul Hirose" 
To: 
Sent: Thursday, September 19, 2002 7:21 PM
Subject: Re: Meridional Distances


| Vic Fraenckel wrote:
| >
| > My routines give the azimuth (referenced to true north) of the second
point
| > from the first point (and the azimuth from the second point to the first
| > point - NOT recipricals on an ellipse).
|
| Yes, that's what I figured you meant. I don't even know what a
| reciprocal on an ellipse is. My point was that the Mercator methods
| yield a rhumb line, which is easier to follow. You simply keep a
| constant heading throughout the voyage. (I'm ignoring variation etc.
| for simplicity.)
|
| On the other hand, computing the azimuth from Point A to Point B
| yields the initial heading for a great circle course. However, if the
| vessel stays on that heading it will miss Point B. The miss may be
| trivial or huge, depending on distance and the lat/lon of the points.
|
| The rhumb line and great circle methods both have their place in the
| navigator's arsenal.
|

   

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