Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: sight reduction tables
    From: John Karl
    Date: 2007 Sep 30, 09:22 -0700

    Sure, there are many cases where 1' accuracy (or even less) is not
    relevant, such as open ocean navigation, beginning practice or
    instruction, and celestial body identification.  But there are several
    cases where it is:  landfall to a low island from a small boat (low
    eye level), avoidance of a shallow reef, and an evaluation our CN
    HO 249's tabulation to 1' implies a accuracy of  +or- 0.5'.  But I've
    never seen that accuracy claimed.  Now HO 229 is tablulated to 0.1",
    but it only claims accuracy to +or- 0.2'.  And when the double second-
    difference interpolation is indicated, HO 229 accuracy drops to +or-
    0.31', which is not much better than the presumed accuracy of HO 249.
    These accuracies are consistent with my own comparisons using 10-digit
    Now consider reducing a sight using HO 249 by working all seven math
    operations to only 1' (i.e., + or - 0.5').  The round off error then
    ranges from zero to +or- 3.5' from the sight reduction alone.  It
    seems to me, that whether navigating in serious situations or just
    evaluating our CN skills, we would like to be assured that all the
    error is due to sources outside of arithmetic, such as Almanac
    (accurate to 0.2' to 0.3', depending on the body), observational,
    abnormal refraction, and sextant error.
    So, when I want the comfort of eliminating math round off error, I
    like to do all math to 0.1' accuracy, neither HO 249 or HO 229 will do
    -- only a calculator (or computer) will.  And in other cases, I might
    as well use HO 249 for its organizational convenience.  This leaves HO
    229 on my bookshelf.
    And my confidence in HO 214 is shaken by the corrections that have
    appeared since its 1936 publication.  BTW, since John Napier's first
    tabulation of logarithms in 1614, how were these hand calculations
    checked for errors?  In our age of digital computers, I find the labor
    that must have gone into these calculations unimaginable.
    (Incidentally, I am old enough to remember when the word "computer"
    referred to a person).
    ....  John Karl
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site