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    Another "emergency navigation" sight reduction method
    From: Stan K
    Date: 2015 Jul 4, 20:39 -0400
    While discussing hav-Doniol with David Burch of Starpath, David mentioned another sight reduction method that he has published, the N(x) Table.  Admittedly not competition for hav-Doniol, we felt it might be worth discussing on NavList, so David gave me permission to show and discuss it in any way I saw fit.  It has been around for almost 30 years but has received little attention.  David thought he recalled Hanno reverse-engineering it some time back, or at least trying to, as David could not recall how the table was constructed without searching his archives.

    Attached is a page from one of David's books, showing the table and instructions.  Things that are not mentioned on this particular page are that the table uses meridian angle t, not LHA, and that some applications require interpolation of the table.  (A more detailed version of the page can be seen at https://www.starpath.com/celnavbook/table_selections.pdf, on page 29.)

    Also attached is the solution for Hc and Z for the example in Greg's Ocean Navigator article.

    I looked at  what David called “the world’s shortest sight reduction tables”, and it certainly works, but I found the new hav-Doniol method easier.  Well, at least it was for me.

    Disregarding the fact that interpolation is needed to get the "accuracy" of hav-Doniol, which is easily remedied by expanding the table (something that would have to be done in order to compare apples to apples), I compared the number and types of steps required for each method.    This is what I found, just for the calculation of Hc:

    hav-Doniol N(x)
    Table entries ("forwards") 3 5 (one used twice)
    Table entries ("backwards") 1 3
    Additions and subtractions 6 7 (one used twice)
    Multiplications 1 0

    It is a matter of opinion whether the extra steps compensate for the lack of a multiplication.

    I didn't give much thought to the calculation of Z, as the Azimuth Graph can be used for any method.  That said, the N(x) table is much better than a hav-Doniol calculation of Z, at least IMHO.

    Food for though, intended for discussion, not digestion.




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