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    RV: Snellius Construction questions
    From: Andrés Ruiz
    Date: 2009 Jan 9, 09:01 +0100

    Sincerely is the first time I have heard the name of Sinellius. But the described method is very common in Coastal Navigation:

    Fix by 3 simultaneous bearings or Fix by two simultaneous horizontal angles



    is one of my favorites for its simplicity and precision.

    D: Bearing

    AH: Horizontal Angle -Ref: [NavList 2504] Re: Coastal Plotting Sheets. 28/03/2007-

    AH = ANG_H( visual1, visual2 ). From the sea, there are tree cases:

    ·         AH = 90º

    ·         AH < 90º           a = 90-AH towards sea

    ·         AH > 90º           a = AH-90 towards land



    Is independent of total correction = variation + deviation, (supposing deviation is the same for the two bearings: AH = Dv2-Dv1 = Da1+dm+Des1 - (Da2+dm+Des2) = Da2-Da1 . v:true, a:compass)

    You can use it in tree different ways:

    1.       three bearings directly

    2.       two horizontal angles directly

    3.       two horizontal angles by three bearings: AH1 = D2-D1    AH2 = D3-D2



    I usually use a sextant, or a hand- bearing compass, (HOCKEY PUCK – Celestaire)

    Aboard an 8 m sloop my precision is very similar if the sextant is used without the telescope.


    THE PLOT on a nautical chart

    I use tree methods for plotting.

    1.       The direct is to use a stationer pointer

    2.       The plotting tools like rules and compass, more complicated

    3.       The plot on fig 2. (s/v is the fix)




    For practice, when I go sailing near the coast, I usually check my position with this method against the GPS, and the results always are very good.

    Once I use a Perolus Compass aboard a cargo and compare the bearings with the hockey puck; they were the same. Also I have compared the hockey puck with other hand-bearing compasses and its superiority is clear, up to 3 degrees of error in the same conditions.



    My spanish bibliography has lot of references to this method. (a book is available off list by request, because “this is illegal”, like “The Black Crowes” song says)


    Andrés Ruiz

    Navigational Algorithms








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