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    Re: GPS shortcomings.
    From: Lu Abel
    Date: 2005 Jun 9, 07:36 -0700

    Peter Fogg wrote:
    >>>>On Behalf Of Lu Abel
    >>Do you speak from experience or just theory about "plastic?" [sextants]
    > When I've had the chance to examine them I've found them disconcerting to
    > use - for example, the index error was different each time I checked on one
    > instrument, before and after taking a sight. It didn't inspire confidance. I
    > know and accept that they have been used to cross oceans with, and possibly
    > they are ideal for the task of teaching students.
    These were Davis sextants or one of the others?   (Although I don't
    believe there are any other brands of plastic sextants being
    manufactured these days, I recollect there was a British one that came
    out about the same time as the Davis sextants).
    I can just speak from experience with my Davis.  I think most of us were
    taught to check Index Error every time we take a round of sights.  I
    check my IE both before and again after taking a round and have never
    found a difference.  One thing I have noticed with my Davis is that
    there is a bit of backlash error on the micrometer drum (maybe on the
    order of 0.2').  Again, I was taught to always bring a body to the
    horizon in the same direction (ie, always bring it down or always bring
    it up) to avoid backlash error, so I consider this an annoyance (if I
    overshoot bringing a body down I can't just bring it back up, I have to
    go well back and try to bring the body down again).
    And, let's not forget the original premise:  We are speaking of a BACKUP
    system for navigation, not the primary.
    Lu Abel

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