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    Re: Sextant Accuracy
    From: Gordon Talge
    Date: 2003 Mar 16, 23:18 -0800

    Fred Hebard wrote:
    > The location of my house seems to be settling down to perhaps 0.3 to
    > 0.4 nautical miles north of its actual location.  All of this with an
    > artificial horizon.  One new technique I tried was to preset the
    > sextant to the future altitude of a body and then watch the body pass
    > through that altitude while listening to the time being counted out.
    > My general objective is to get these observations as accurate as
    > possible, say to within 0.1 or 0.2 miles.  I'm not sure why I have this
    > objective, but it persists.
    > Bill Murdoch mentioned he had never bothered to apply instrument
    > corrections to readings taken with his Freiberger, implying that it's
    > not necessary to achieve accuracies better than about 1 nautical mile.
    > That also seems to be the general opinion here, especially with regard
    > to observations from small yachts, where it often is not possible to
    > get more accurate.  But how about larger vessels?  Does anybody want to
    > defend trying to get accuracies closer than 1 mile?
    In my opinion, you might as well check and apply the index correction,
    since it is easy to check and no big deal to apply, but if it is small,
    considering everything else that could go wrong, it may or may not
    be worth it.
    This 0.1 - 0.4 miles stuff is just I think fooling yourself.
    If you are using the Nautical Almanac, check out section 24 in the
    Explanation. "The quantities tabulated in this Almanac are generally
    correct to the nearest 0.1'; the exception is the Sun's GHA which is
    deliberately adjusted by up to 0.15' to reduce error due to ignoring
    the correction."
    Then we got the time signal correction in section 2.
    -0.9s to -0.7s   0.2' to the east
      over to
    +0.7s to +0.9s  0.2' to the west.
    Then there are the A2 altitude and Dip correction along with the A4
    If you do get the same lat and long as your GPS you are just lucky.
    It is probably just a throw of the dice that you got that close.
    On a small pitching boat, with yourself tied to the mast hanging on
    for dear life and trying to get a shot at the crest of a wave and
    then yelling out mark to your seasick assistant, might be closer to the
    I think if you get between 1 to 3 NM of where your at you are doing
    really great. Maybe more like 1-5 might be the more like it.
    I remember reading about accounts of ships trying to find some island
    in the Pacific and then going around for days looking for it sometimes
    never finding it.
    Maybe today, that wouldn't happen with GPS and RADAR. I am not
    sure, but can't radar give an echo up to maybe 15 to 20 NM or more?
    The only way I can think of getting real accurate position is not
    with a hand held sextant, but with a theodolite, setup on land
    and the Astronomical Almanac, along with surveying equipment that
    they used before the GPS.
    -- Gordon
                                 (. .)
    | Gordon Talge WB6YKK               e-mail: gtalge@pe.net    |
    | Department of Mathematics       http://www.lbusd.k12.ca.us |
    | Wilson High School                  Long Beach, CA         |
    | (o-    Debian / GNU / Linux                                |
    | //\    The Choice of the GNU Generation                    |
    | v_/_                  .oooO                                |
    |    - E Aho Laula -     (  )   Oooo. - Wider is Better -    |
    +-------------------------\ (---(  )-------------------------+
                               \_)  ) /

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