NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: AP terminology, WAS: 2-Body Fix -- take three
From: Jeremy C
Date: 2009 Nov 12, 10:27 EST
From: Jeremy C
Date: 2009 Nov 12, 10:27 EST
A DR is a position based on course and speed. An AP is a position
used for calculations.
I never use my DR for the AP for navigation at sea. When doing
249/229 reductions I use an AP chosen for each sight to give me whole degrees of
Latitude and LHA. If I am using a calculator or computer, I choose a
convenient GPS fix (nearest half hour) as the AP.
The only time I can think of that you wouldn't need an AP is when scribing
circles of equal altitude because you are not really doing any trig
computations. This can be done on computer easily enough, and in certain
circumstances on paper, but without an idea of where you are in the
world, you need at least 3 LOP's to deal with the fact that two circles will
offer two intersections. When shooting high-altitude circles, having a DR
still may not help as I have seen several times when shooting them.
JCA
In a message dated 11/11/2009 11:01:13 P.M. Eastern Standard Time,
jhkarl@att.net writes:
No, Peter, I am not.
The term AP is the location used to solve the navigation triangle in
the St. Hilaire method. This location might be the DR, it might be an
EP (estimated position), it might be a recent fix of any kind, it
might be a GPS position (used to check the GPS, of course). Again,
the DR is a particular choice for the AP. And there should be no
confusion between the AP and a DR.
John
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