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    Re: Systematic error and its resolution
    From: Alexandre Eremenko
    Date: 2007 Apr 6, 16:54 -0400

    Ok, I understood.
    Actually this is not exactly what we call "index error"
    (at least for marine sextants)
    but "combined index+arc+bubble+prismaticity-of-filters+
    whatever systematic error".
    On Fri, 6 Apr 2007, Geoffrey Kolbe wrote:
    > Alex, the index error of a bubble sextant can be determined from an unknown
    > position.
    > By taking sightings on bodies in opposition, the separation of the
    > resultant position lines will, on average, be twice the index error.
    > In my posting #2531, I advocated taking four sightings, a North-South pair
    > and an East-West pair, on bodies at the Cardinal points. The North-South
    > pair effectively give you your latitude and the East-West pair your
    > longitude. The position lines from the four sightings should form a square
    > box if index error is the dominant error. If the box is not substantially
    > square, there is a non-systematic error somewhere which is of the same
    > order or greater than the index error.
    > See http://www.pisces-press.com/desert/14thMarch.jpg for an example of what
    > I mean. In this example, the box is substantially square, giving us
    > confidence that the dominant error is indeed index error. The size of the
    > box is about 10' on a side, from which we deduce that the uncorrected index
    > error was 5'. The position lines are all 'away from' the body in question,
    > so the sextant altitudes are 5' too small. I need to add 5' to my altitudes
    > to correct the index error.
    > A subsequent round of sights at a different location a few days later show
    > the results when this 5' is factored in. See
    > http://www.pisces-press.com/desert/19thMarch.jpg
    > Geoffrey Kolbe
    > At 20:24 06/04/2007, you wrote:
    > >Geoffrey,
    > >
    > > > It is easy to determine the Index Error of a marine
    > > > sextant, but not of bubble sextants.
    > >
    > >Easy or not, but I infer from your message that you
    > >can find the IC of your bubble sextant SOMEHOW.
    > >
    > >I don't see any way to do this at all,
    > >except taking an altitude from a known position.
    > >
    > >This can be done once in a while, but certainly not
    > >before or after an observation, if the purpose of
    > >the observation is to find this position.
    > >
    > >Alex.
    > >
    > >
    > >
    > >
    > >
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