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    Re: Mercator vs. Great Circle Charts
    From: Derrick Young
    Date: 2001 Aug 29, 7:39 AM

    Carolyn,
    
    You pose the same type of questions that I see in most beginning navigation
    classes.  Many of these questions should be addressed in the classroom
    discussion BUT, I know from experience that they are glossed over by some
    instructors.  So here goes.
    
    1. Can true direction be determined in the same manner for all locations on
    a
    Mercator chart?  Yes - that is what the compass rose helps you with.  How
    about on a great circle chart?  Again, yes.
    
    2. How is the compass rose used on each of these charts?  The compass rose
    shows a lot of information.  It provides you with the true rose (the outer
    ring), the next ring in the compass rose provides you with the magnetic
    heading.  This is what your magnetic compass should read if there are no
    other external influences (but there are - that is why you also have a
    deviation table for the boat).  You can remember all of this by the TVMDC
    formula - True + Variation (from the nearest Compass Rose to the area you
    are working) gives Magnetic plus Deviation (unique to your boat/ship) gives
    Compass.
    
    3. If I were to draw a straight line between two points (say Tokyo and Cabo
    San Lucas) on both a Mercator chart and a great circle chart, would either
    straight line represent a great circle?  Yes - both are.
    
    4. Given the straight line drawn between Tokyo and Cabo San Lucas (see
    above), which chart, the Mercator or the great circle, would give me the
    shortest route between these two points?  By definition, a great circle
    route is the shortest sailing route.  The thing to remember here is that the
    Mercator chart is most accurate at the middle latitudes, so I would expect
    the Mercator chart to provide a longer distance than a great circle chart.
    But the sailing direction would be the same.
    
    5. Would sailing a great circle course always be the best way to travel
    across the ocean, or would ships have to take into consideration unfavorable
    
    winds, currents, or storms that might cause a delay?  Any navigator must
    apply the general prudence rule and take into account those items that will
    influence their voyage.  The thing to remember here is that there are three
    phases to navigation, planning (good old DR) the whole voyage - this is
    where you lay out the overall course and intended stops.  The next is the
    detailed planning, where you will consult the sailing directions, current
    information, weather forecasts and other external influences (like ice
    flows, etc.) and then make detail corrections that you may need to make your
    intended destination.  The last is recording what course was actually
    steered, speed actually made turns for and use this to determine where you
    actually went.  This will then require you to redo the detailed planning for
    the next step of the voyage.
    
    6. Do most large ships today use computer assistance to help them choose the
    
    best course across the ocean?  Yes they do.  They use GPS, LORAN, and some
    still have the old SATNAV system.  They also use RDF (radio direction
    finding) - but you normally see this on the older smaller ships.  They also
    use celestial navigation and the good old DR plot.
    
    
    I hope this helps, please feel free to contact me directly (
    youngd{at}hqlee.deca.mil  ) if you need further
    assistance.
    
    derrick
    

       
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