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    Re: How accurate are fixes in practice?
    From: Bill B
    Date: 2008 Aug 03, 20:58 -0400

    Peter Wrote
    
    > By the way Bill, you haven't answered your own question .. I've heard that
    > Lake Michigan can work up a short nasty swell, particularly down there close
    > to the Windy City ..?
    
    My time on the big lake is limited, and often conditions are poor.  Overcast
    (shooting through clouds), poorly defined horizons, or shooting against sand
    dunes with a fuzzy horizon (about 15+' of white stuff obscuring the horizon
    one day).  Often the best I can squeeze in is a couple of rounds where I can
    manage 5 or so observations (average them on the fly then do a plot against
    the actual Hc slope later).  To say the least, my on the water experience is
    limited which is why I asked the question.
    
    As to Lake Michigan, it can get dicey very quickly.  Shallow except on the
    north end, a reasonable N/S fetch and short E/W fetch, onshore and offshore
    breezes, and large heat sinks/heat generators from Chicago and the steel
    mills south of Chicago.  I seldom experience "swells" as Bowditch would
    define them, or as I have experienced them when living/sailing on the
    Atlantic or sailing on the Pacific.  Often wind-driven "seas."  Just as
    often large chop (new/confused seas). If I recall earlier discussions
    correctly, we have waves bouncing around like a pool ball off multiple
    cushions at the southern tip of Lake Michigan.
    
    Interestingly, sailing to Chicago (288 true) on July 2 there was a major
    high at the north end of the lake, NE winds, and a 2-ft, seiche warning.
    
    Putting it in perspective, navigating the south end of Lake Michigan is easy
    when compared to the north end, Lake Superior, or the shores of New England
    etc.  Almost every dinghy racer I have met will say the same thing about
    their local pond, "If you can sail here, you can sail anywhere." 
    
    Believe it or not there are often sizeable currents which are available on
    the internet via NOAA by the hour, but not in real time or without a
    computer to the best of my knowledge.  I have, after the fact, attempted to
    factor in currents within 5 miles of shore but with no real success.
    
    I have never experience "ideal" conditions--a clear sky, crisp horizon, and
    a relatively stable platform at the same time.  I have been going through my
    logs to answer Peter's question. From an approx. 11 meter sailcraft under
    good conditions I would expect individual intercepts to be +/- 1.6 nm or
    better 66% of the time, with an average intercept (plot fitted to Hc line as
    per David Burch) better than 01.5 nm.
    
    My worst set (first on-the-water experience) was using a kit cardboard
    sextant against sand dunes with a hazy horizon and confused 8-10ft seas on a
    34' fin-keel sailboat.  My intercepts were about 5-7 nm off, all in the same
    direction.
    
    My second worst (using Astra) was a day with 4 ft seas but with the
    totally-obscured horizon mentioned above.  I looked at distance boats'
    waterlines, and adjusted with dip short to guess about where above the
    bottom and below the top of the mess the horizon might be.  The range of
    intercepts was approx. plus/minus 6'.  Using the Hc slope method the
    "averaged" outcome was almost spot on. Better lucky than good. 
    
    In my last on-the-water attempt (July 2008) clouds rolled in just as I was
    ready to shoot.  I got off 2 observations shooting through the cloud before
    the sun was obscured. 3 ft seas and a usable although not crisp horizon.  I
    had to shoot through sails/rigging so I was not able to fully rock the
    sextant to my satisfaction.  My intercepts,  0!7 and 1!9 (average 1!3) were
    both larger than the Hc. That suggests to me the sextant was not plumb.
    
    As to fixes, I cheat and do not follow established procedure. I factor
    leeway into DR and use my EP as AP. I would agree that any running fix (5
    hours) greater than 3 nm off my GPS position under average conditions would
    have me looking for a reason why.
    
    Bill B.
    
    
    
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