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    Re: Compass Checks at Sea
    From: Bill B
    Date: 2008 May 23, 01:25 -0400

    > And just to confuse the issue even further, both land and sea
    > (airplane) pilots refer to compass error as "deviation" (at least in
    > the USA, don't know if that's universal throughout the aviation world
    > or not).
    
    You seem to have it nailed, but for beginners like myself I feel I was a bit
    obtuse IMHO in my last post.  I always remember that "Dxxx" and "Vxxx" are
    different on sea and land.
    
    It is intuitively obvious, at least to me, that if I modify my compass
    reading by a "Dxxx" factor to reach magnetic, that "Dxxx" factor must be a
    magnetic distortion from from an external source(s).  Therefore "Vxxx"
    (variation) must by default be the difference between true and magnetic.
    
    Aircraft piloting/navigation also entered my thoughts, but since they have
    borrowed (stolen) almost everything (red light port, green light starboard,
    set and drift etc.) from their nautical predecessors, I assumed (and have
    noted while flying with friends in their private aircraft) that they
    followed the same traditional navigation conventions for the most part as
    their seagoing predecessors.
    
    Likewise mountain hikers that use an altimeter to assist in locating
    themselves on a topo map, while seafarers may use the depth contours on
    their charts.
    
    It also crossed my mind that for someone in the bush, influences from
    outside sources affecting their compass readings (barring a poorly placed
    flashlight, belt buckle, zipper etc.; or an iron-ore motherload or magnetic
    anomaly) are not generally taken into account.)
    
    I have made the mistake on a houseboat rental (well before GPS) of placing
    an aluminum soft drink (or adult beverage) can near the compass--which put
    me way off. Aluminum? Surprise.
    
    I also recall a cautionary tale in one of my texts of some outdoor persons
    that placed the map and compass on the hood of their vehicle at the trail
    head to orient the map and themselves, and later became hopelessly lost.  Go
    figure.
    
    Since the government put that metal plate in my head that makes me urinate
    every time mom turns on the microwave, I do have a Napier Diagram for my
    hand held compasses. (For our non-USA members, the metal plate and microwave
    are references to a National Lampoon comedy film.)
    
    Bill B.
    
    
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