A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lu Abel
Date: 2009 Nov 12, 09:31 -0800
Traditionally, DR is what you get simply from course steered and speed x time (= distance traveled) along the course.
An EP is a refinement based on additional "outside" knowledge -- for example, compensating for current. 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.
And an AP is any position "assumed" for the purpose of doing calculations. The nautical almanac sight reduction is an excellent example -- doesn't matter what your DR or EP is, the NASR tables work only with LHAs that are an integral number of degrees. So you need an assumed position whose longitude gives you a LHA with a whole number of degrees. And guess what? If you have two sights whose LOPs you want to cross to get a fix, you'll need two different APs to use NASR, one for each sight.
Peter Fogg wrote:
“A word means what I say it means; no more, no less” as The Red Queen put it..
Its been a while since this issue got a thrashing here. It doesn't seem to be the issue that interests John Karl although while he is being coy about what that may be, we might as well see if we can clarify what is meant by AP/DR/EP.
(Dead Reckoning) This is the basis of navigation, particularly when out of sight of land. Strictly speaking it only involves speed and direction (bearing and distance), leading to a derived position.
(Estimated Position) A DR position corrected for the estimated effects of currents, lateral movements, etc. The reason for the distinction is to allow for the separate plotting of other effects to the DR position. In practice many people adopt a modified DR position without bothering about the distinction between the terms.
(Assumed Position) Quite a few people have recently stated here what this is. A position which is almost certainly not the best estimate of position (that's the EP) but a position which is assumed for use with some tables which require entries to be made in whole degrees.Any of these can be used with the St Hilaire intercept method. The position which is likely to be closest to the actual position will tend to have the shortest intercepts, leading to a potential gain in accuracy. If the intercepts are particularly long, the position indicated by the intersecting LOPs (and let's not forget that 2 is not the ideal number) can be adopted as an improved EP and intercepts re-calculated from it, a process known as reiteration.
John, I suggest that you are only inviting the confusion you complain in advance about by adopting AP as a generic term to cover DR, EP and AP.
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