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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 02-04-2008
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Question Great circle/rhumb line

to calculate a series of rhumb lines to get a great circle. What is the standard method?
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Old 02-04-2008
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The easiest way is to plot the course on a gnomonic chart and then transfer the coordinates to a mercator chart.

The mathematical way can be calculated in many places on the web.
Links:
http://www.geocities.com/CapeCanaveral/2265/gcsail.htm
http://andrew-gray.com/dist/

I would/do use the gnomonic chart method. Using pure calculations and not plotting the rhumblines on a chart runs the risk of sailing over something you don't want to sail over.

Gaz
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Old 02-04-2008
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You can use pure calculations for the Great Circle routing as long as you plot the results on the mercator charts. Thus you can see if you need to truncate the route or not.
We would use general charts and plot the Great Circle Course for every five degrees across the Pacific Ocean. This works quite well for my navigation.
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It doesn't matter what kind of chart you plot the results of your calculations on as long as the dangers are marked. It is easier on a mercator because rhumb lines are straight and you can draw them with a ruler.

Gaz
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I ran my numbers at the web page link...now can I plot it on a blank plotting sheet just to test the curve?
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It depends on what the projection of the plotting sheet is. If it is a mercator projection you should see a series of rhumb lines approximating a curve. If it is a gnomonic chart you should get a straight line.

Plotting sheets are normally mercator sheets.

Short answer - yes.

Gaz
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I've never seen a gnomonic chart of a scale suitable for plotting points and then transferring to a plotting sheet, for a transoceanic passage. My advise is to calculate your Great Circle via nav. calculator or h.o. 229 for changes of course every 5 degrees of longitude and, once you go off your coastal chart, plott them on a small scale plotting sheet. You're unlikely to encounter any obstructions where using a G.C. is advantageous.
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Quote:
Originally Posted by sailaway21 View Post
You're unlikely to encounter any obstructions where using a G.C. is advantageous.
Just don't use the hurricane tracking mercator chart of the North Atlantic, like the skipper of this J-boat did. His GCR took him right over Sable Island, which wasn't on the chart:



A while later:

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Sable Island is not a good place... basically a ship's graveyard in the North Atlantic.
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Quote:
Originally Posted by sailaway21 View Post
I've never seen a gnomonic chart of a scale suitable for plotting points and then transferring to a plotting sheet, for a transoceanic passage. My advise is to calculate your Great Circle via nav. calculator or h.o. 229 for changes of course every 5 degrees of longitude and, once you go off your coastal chart, plott them on a small scale plotting sheet. You're unlikely to encounter any obstructions where using a G.C. is advantageous.
Here is a link to Admiralty Charts for plotting Great Circles:
http://www.sailgb.com/p/admiralty_ch..._gnomonic_135/
The idea is you just draw a straight line between the start and the end of your great circle route and pull the waypoints off it and plot them on a navigation chart.

As for obstructions, suit yourself.
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There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217
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