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    ReRe: Level of observation accuracy
    From: Bill B
    Date: 2004 Jul 23, 16:47 -0500

    > "Did a noon sun shoot within seconds of local noon with 0 magnification sight
    > tube."
    >
    > Just curious, did you do anything to verify your calculation that it was
    > local noon? Did you check the altitude before and after to see that it was
    > lower?
    
    > You could count +/-1.5 minutes as very good.
    
    Thank you, Frank. That gives me a standard to measure against.
    
    The refraction of waves/horizon was an interesting tidbit. As a sidebar, did
    notice (looking west) after the first day's outing the horizon looked like
    the teeth of cross-cut hand saw.
    
    Figured meridian passage with arc-to-time for my EP plus 0:6:15 eqn. of time
    (July 18).
    
    I know this particular boat pretty well.  Its autohelm tends to have
    helmsman error that nulls out leeway.  By the knot meter we were about hull
    speed, but I took that down a knot for estimated SMG because of the piles of
    water we were plowing through.  Approx. two hours since the last fix, so
    felt pretty confident about my DR/EP position. In fact, checking against the
    GPS later, latitude of EP was dead-nuts on and longitude of EP was off by .4
    nm to the east (approx. 0d 00!5 = -2s time).
    
    Since I was not trying to extract longitude, the sun hangs there for minutes
    or seconds (depending on who/what you believe) and factoring in the
    conditions and instrument, it struck me that trying to determine exact LAN
    with observations was an exercise in futility.  As it turns out my
    observation was about 10 seconds past my estimated meridian passage time
    because of yaw, and waiting to reach the top of a pile of water.  All said
    and done, 8 seconds later than calculated almanac meridian passage at my
    actual position.  Shot from the windward side of the cockpit aiming between
    the backstay and leach.  I had measured the freeboard and deck (by the mast)
    the day before, and knew the leeward toe rail was about 1 foot off the
    water, so had dip down as close as was possible.
    
    While I hate to admit it, started this quest a couple of months ago with
    Celestaire's Wurzburg cardboard sextant.  Manufacturer claims it can be
    accurate to within 8 minutes of an arc.  With a little mirror tweaking and
    putting a horizontal slit across the eye side of the viewing tube, am
    getting within 2-3 minutes consistently from seated position on a stable
    platform or tripod mounted.  So I figure plus/minus 3 minutes is my range of
    instrument slop.  Humidity does affect index error. 
    
    As the plastic Davis vernier model or other plastic Davis models do not seem
    to have practice bubbles available, and the kit sextant did, it made the
    most sense at the time.  Now that I am confident I can do it and love it, I
    am starting to shop for a "real" unit.  Also a nice reason to back off the
    CRT and go look at the stars after sunset.
    
    And for the curious (and further amusement for Robert and David #s 889968
    and 990800,) LOP was 7 minutes south of my EP and a GPS fix recorded by the
    skipper at the time of the shoot.
    
    Bill
    
    
    

       
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