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DR sight-reduction methods endorsed

Jan 1, 2003

I think that John Cattuna Jr.'s recent evaluation of the most popular methods of sight reduction was excellent ("Sight reduction methods compared," Issue No. 92, Sept./Oct. 1998). I especially liked his argument that DR-based reduction methods are "a little easier, if only from a plotting perspective." I could not agree more. Assumed position-based reductions are nice for the rec room chart table purist but lose any advantages they might have in the small, dark, wet, and constantly gyrating cubby hole that is most boat's nav station.

With GPS rapidly becoming accepted as the primary navigation tool used by most navigators, celestial back-up should be as easy as possible; first, to encourage the navigator to use it regularly; and, second, to reduce the chance of errors due to cumbersome methodology. In my opinion these arguments alone fully justify the use of reduction methods that start from a DR position over the mental and plotting gymnastics required of the AP schemes. In this day of cheap scientific calculators, by far the best reduction methods, as a GPS backup, are the Law of Co-sines formulas described in Bowditch (1984), Appendix T. A programmable calculator can produce accurate results in seconds. A dual-powered (battery and solar) calculator in a tightly sealed plastic bag is part of my anti-Murphy emergency kit.

In case Murphy does strike, I also carry a copy of Allan E. Bayless's Compact Sight Reduction Table. This table is an excellent modification of the Ageton Method; however, like the S-table, it is only nine pages long. Dr Bayless's booklet contains outstanding explanations of the uses of his methods and includes Sadler's Technique, which, I can assure you, completely negates Mr. Cattuna's warning regarding the forbidden zone of when intermediate values of K are near 90°. Sadler's Technique allows reductions up to and including K = 90° with no loss of precision.

Thus, my recommendations for today's navigation practice is GPS as the primary, backed up with celestial cross-checks reduced by calculator using GPS as DR positions. Modified Ageton (Bayless) may then be used as the cross-check for the calculator reduction.