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    Re: Manual calculation of compass variation
    From: Bill B
    Date: 2004 Oct 17, 17:10 -0500

    > Richard,
    > I was surprised that a vessel needs to be going 5-10 km/hr to get a GPS
    > track accurate to better than one degree, and am wondering what the
    > accuracy of the track is for low speeds, which might easily be
    > encountered in a sailing vessel; I suppose this information is included
    > in GPS manuals, but I don't have one.
    
    A couple of points:
    
    When steering by a compass and fixes, the navigator can calculate set,
    drift, leeway and wind-induced surface current.  The GPS shows COG and SOG,
    therefore the navigator has to work backwards to determine what direction
    the lubber line was headed.
    
    If in doubt, point your nose north and walk laterally to your right.  The
    GPS will tell you you are going east. In fact you are, but your nose is
    pointing north.  It is not a compass.
    
    As to speed vs., accuracy, my Garmin 76 gives no such warnings.  In walking
    and sailing tests I have not found it to be off due to low speed.
    
    I do find it to be off in other areas:
    
    1. When measuring course and distance between points, my manual tells me it
    is giving me the distance on the great circle route as opposed to along the
    rhomb line.
    
    2.  I have calculated short courses (30-60 nm) using a polyconic-projection
    chart (Great Lakes), various sailings, and trig.  Also with my GPS.  I found
    that usually all the traditional calculations agreed to the tenth of a
    degree.  My unit reads out in whole degrees.  What surprised me was that the
    GPS unit uniformly rounded DOWN to the nearest whole degree, even with
    calculations of ddd.6 to.9.  Even on a 30-60 nm run, that is pretty far of
    course.
    
    Practically speaking, who is going to steer perfect ddd.d  in a sailboat
    with chop?  Still, if you took the GPS information without verification,
    hardly optimal.
    
    My other problem is that friends with boats new to them who have made GPS
    point-and-shoot their standard, and now leave their paper charts stowed.
    They have no idea their new wing keel has different leeway charactersitics
    than their old fin keel boat.  On the Great lakes set and drift are
    generally not a consideration.  Because of short fetch and ever-changing
    wind patterns, wind-induced surface currents are neglibable unless (Lake
    Michigan) it has been blowing out of the north for a day or more.  So a
    perfect testing ground for determining leeway under various conditions and
    points of sail.  But they either no longer care or don't know how to work
    the it out "backwards."
    
    Sad.  A student learning to fly does not start out under the hood, but must
    become proficient in VFR first.  E-nav, IMHO, starts too many boaters out
    under the hood, or seduces skippers who should know better.
    
    Bill
    
    
    

       
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