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Subject: Re: Mercator vs. Great Circle Charts
From: Young, Derrick (youngd@XXX.XXX)
Date: Wed Aug 29 2001 - 09:39:08 EDT
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@XXX.XXX> ) if you need further
assistance.
derrick
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