# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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EP along a single LOP: WAS: AP terminology
From: Peter Hakel
Date: 2009 Nov 12, 14:05 -0800
I'd like to point out that the two-body fix techniques can also be applied to this problem.  Take the DR and the GP, and put a great circle through them.  This great circle works like the second "LOP;" it has Ho=0, and its GP is determined from the cross product between the DR and GP vectors originating from the Earth's center.  There are two such GPs, they are antipodes to each other, and either one works.  Then use your favorite direct two-body fix method to find the two intersections, and keep the one closer to the DR.

This method is marginally better than drawing that perpendicular straight line from the DR to the LOP on a Mercator chart, because it uses the actual great circle, and not its rhumb-line approximation.

Peter Hakel

From: Lu Abel <lunav@abelhome.net>
To: navlist@fer3.com
Sent: Thu, November 12, 2009 9:31:16 AM
Subject: [NavList 10592] Re: AP terminology, WAS: 2-Body Fix -- take three

Another EP situation is when a navigator gets a single line of position.  The vessel must be somewhere along that LOP, but where?   Traditionally, it's the point on the LOP closest to the vessel's DR position, which is found by drawing a line perpendicular to the LOP from the DR, the place where it crosses the LOP becomes the EP.

[parts deleted by PH]

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