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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The term AP
From: Henry Halboth
Date: 2015 Jan 21, 23:14 -0500
From: Henry Halboth
Date: 2015 Jan 21, 23:14 -0500
John K.,
It is necessary to note that originally the intercept, or Marc St. Hilliare, method was employed using a DR position, however, in the search for more speedy solutions and the advent of the various short tabular methods, utilizing whole numbers for Latitude and LHA, it became necessary to manipulate the vessel's actual DR position, both in Latitude and Longitude to accomodate this end - so the position "assumed" to achieve this end was named the Assumed Position, or AP. I have never had any problem understanding this as the basis of many short tabualr methods.
A review of the instructions at least to HO 208 and HO 214 should clarify this use of tha AP.
Henry
On Wed, Jan 21, 2015 at 7:11 PM, John Karl <NoReply_JohnKarl@navlist.net> wrote:
Gary L & All,
Yes, most (perhaps every) CN book that I've seen either does not clearly, or accurately, describe the role of the AP in the intercept method. Many alude to it being an approximation of some type, or even saying that "it's an approximation we wish to correct, or adjust in accuracy." Some say the AP is an approximation surrounded by uncertainty. Of course, that's true for the DR, but not for the AP. Also, some time ago when St. Hilaire was discussed at length on the NavList, a well-know member discribed it an an iterative calculation. Even my two editions of Bowditch only precriptively instruct performing the St. Hilare sight deduction with no discussion of why the AP is used.
J Karl
