Next message: Trevor J. Kenchington: "Significance of azimuth errors, was : Principles and Being Practical"
Is Ageton's method described in Bennett's book ? If not, where is the
best exegesis of it, please ?
Thank you.
Dr. Geoffrey Kolbe wrote:
>George Huxtable has pointed up a potential problem with the azimuth tables
>in George Bennett's book "The Complete On-board Celestial Navigator". He
>has shown that there can be errors in computed azimuth of (at least) 15
>degrees where the celestial body is that sort of distance away from the
>prime vertical.
>
>Peter Fogg tells us that this is "nit-picking" and that in any case, the
>book tells us that, "In extreme cases the table should be interpolated when
>observations have been made in the vicinity of the prime vertical."
>
>I do not have the second edition, only the 1999-2003 edition where this
>phrase is not present. Perhaps Peter can tell us just what "extreme" means
>in this context? When do we know we are in an extreme case? George also
>posed some other pertinent questions to Peter and I too would be interested
>to see the answers...
>
>I also wonder just how much of a problem it would cause having your
>near-prime-vertical azimuths off by around 15 degrees? For a cluster of
>star sights, say, a prudent navigator would also be taking sights from
>objects far away from the prime vertical (to get useful angular separation)
>and this would tend to mitigate any problems due to bad near-prime-vertical
>azimuths. The inaccuracy of the tables near the prime vertical are also
>mitigated by being able to assess independently (in many cases) in which
>azimuth quadrant the celestial object sits.
>
>If your estimated position is pretty close (say, within 10 nautical miles)
>to your actual position then I cannot think of any circumstances where it
>would significantly affect the sort of accuracy we would expect from CN in
>a small boat at sea, which is the sort of user the book was aimed at in the
>first place. I have not thought deeply on this problem and I would
>appreciate the thoughts of other listers who will have greater insight on
>this problem than I.
>
>The "short" method of sight reduction used by Bennett is popular because
>the computed altitude can be arrived at quite quickly. But a different
>procedure is required to calculate an azimuth and this rather takes the
>gilt off this method. Ageton's method, by contrast, requires more steps to
>get to the calculated altitude, but the azimuth then drops out very quickly
>and is accurate. Azimuth quadrant ambiguities are also easily resolved.
>Too, only one set of tables is required for the Ageton method.
>
>Geoffrey Kolbe
>
>
>-------------------8<---------------------
>From: George Huxtable
>The problem with these azimuth tables ...
>is not in their ambiguity, but in their inaccuracy, and that inaccuracy is
>exactly what I have complained about. And there is not one word, not even a
>hint, in the book that major errors in azimuth can occur, for certain
>observations in a VERY wide swathe around East or West.
>-------------------8<---------------------
>>From Peter Fogg
>
>Inserted in second edition is . "In extreme cases the table should be
>interpolated when observations have been made in the vicinity of the prime
>vertical and/or LHA, declination and latitude require substantial rounding
>off before using the table. When in doubt use the Weir diagrams.
>
>In practice you could happily sail across an ocean and never notice this
>supposed problem, particularly by following the common sense approach
>outlined previously. With nav. it it often a case of one system checking
>another. In fact taking sights and working out a fix is a check on the
>basic tool of running a DR.
>
>If the whole book has been subjected to the same searching criticism and
>this rather inconsequential nit-pick is the only flaw found, then it is
>really a back-handed compliment to the book as a whole. A ferocious critic
>seems to think the rest works just fine.
>
>Border Barrels Ltd., Newcastleton, Roxburghshire, TD9 0SN, Scotland.
>Tel. +44 (0)13873 76253 Fax. +44 (0)13873 76214.
>
>
--
Courtney Thomas
s/v Mutiny
lying Oriental, NC
