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Old 05-24-2008, 02:33 AM   #1
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Does anyone know the best way to calculate the shortest route between distinations over an extended ocean voyage??
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Old 05-24-2008, 02:57 AM   #2
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Does anyone know the best way to calculate the shortest route between distinations over an extended ocean voyage??
Is this what you mean ? Click Here

Richard
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Old 05-24-2008, 08:52 AM   #3
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The shortest route between two points on the Earth's surface is a great circle. Finding the great circle route is not difficult and you can use the calculator Richard suggested but there are two issues, over and above the issue of prevailing winds, one must be aware of when sailing a great circle.

Firstly, unless sailing down a meridian (which is a semi-great circle joining the poles) or along the Equator (which is a true great circle) you will be continuously altering course as, when plotted on a Mercator projection, which is the most commonly used for navigation, the track will be a curve towards the pole.

The first issue brings us nicely to the second as the curve towards the pole brings us to higher latitudes and thus a greater risk to encounter hard weather or ice.

There is an accepted method used by large ships crossing oceans and that is to run a great circle to a predetermined "safe" latitude and then run along that parallel (parallel sailing) until coming back to the "downward" leg of the great circle track and then following it to the destination. This is called a composite great circle.

Great circle sailing can save a lot of distance on an ocean crossing but a more important issue is the weather. Merchant ships these days are weather routed rather than slavishly following a great circle.

One quick and easy way to see where a great circle will take you is to use a gnomonic projection and join the sailing and destination points with a straight line. Read off the latitude for every 5 degrees of longitude and mark those positions on your Mercator chart then "join the dots" . You then have your great circle track on a Mercator chart.

Hope this helps

Aye // Stephen
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Old 05-24-2008, 09:01 AM   #4
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Old 05-24-2008, 09:13 AM   #5
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If I'm not mistaken, the Google Earth Ruler tool will give you a very good initail idea of the length of any global route made up of virtually any number of points.

Miles or kilometres only, of course, and no good for course planning - or, indeed, anything without broadband - but a very simple and quick way to establish a starting point for more detailed passage plans.

See ya
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Old 05-24-2008, 11:33 AM   #6
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What a pity that we still have those that do not know the relationship between Degrees, Minutes & Seconds - AND Nautical Miles & Knots.

It is a distraction to constantly having to refer to :-

Miles per Hour x 0.868 = Nautical Miles per Hour (or Knots)

Kilometres per Hour x 0.540 = Nautical Miles per Hour (or Knots)
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Old 05-24-2008, 01:30 PM   #7
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Google Earth give distances in centimeters, meters, kilometers, inches, feet, yards, miles, nautical miles, and smoots. I guess that's some dry humor.
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Old 05-25-2008, 12:27 AM   #8
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While discussing Navigation with a GPS and the lat/long coordinates - remember the the following when using certain local charts :-

Horizontal Geodetic Datum: The definition of the relationship between the ellipsoid adopted as the model of the Earth's shape, and the Earth itself. Though there are hundreds of datums in use, most are only locally valid.

The WGS-84 datum is global in scope and positions obtained by satellite navigation systems are usually referred to this datum. Therefore a correction needs to be applied to a WGS-84 GPS position to agree with charts using other horizontal datums. For example to correct WGS-84 to the European datum, add 0.06'N, 0.04'E to the WGS-84 position indicated by the GPS. Fortunately, most GPS receivers may be set to display positions in several other datums besides WGS-84.

When the following Charts (amongst others) are used then the cartographic DATUM in the GPS must be entered to match the Datum of the specific chart (usually found at the bottom of the chart)

Bermuda 1957

Djakarta

Guam 1963

Indonesia 75

Indian Thailand

Indian Bangladesh

Luzon Mindanao

Hong Kong 62

Etc.......
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Old 05-25-2008, 06:24 PM   #9
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Hi All,

My old GPS automatically calculates the Great Circle track to the next waypoint, so I expect most GPS instruments would do the same. This shortest distance route does not necessarily mean that it is the quickest for a sailing vessel. I use the old sailing routes and a program called Visual Passage Planner to work out the quickest track and the smoothest passage.

Regards, Stephen
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Old 05-26-2008, 12:42 AM   #10
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Thanks Stephen for your help. I am trying to get ready for a circumnavigation and will leave from the Panama Canal headed west. My first stop might be in the Marquesas Islands so I am trying to plot waypoints at least one every 24 hour period. I'm not sure if my GPS will figure the great circle routes for me since I'm not on the boat at this time. I could use the calculator that Richard suggested but it really gives you distance and not waypoint information that I need. Any other suggestions??
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Old 05-26-2008, 04:25 AM   #11
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Hi,

If you have a good idea what speed over the ground that your boat will do on an average 24 hr period. Say 5 knots x 24 = 120 nm, then plot your Waypoint legs on your chart/s in steps of 120nm, then read off the lat/long coordinates off the chart and enter each waypoint into a route (SAY ROUTE # 1) taking up about 740 nm for each into your GPS. If you were using the Rhumb line to get to your destination of some 3,700nm - and IF you were able sail directly it would take a month. However, what you will find is that the wind will decide your course and that you will be adjusting your waypoints as you make ground towards your destination.

By the way here is a blog worth reading Radiance

Richard
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Old 05-26-2008, 08:45 AM   #12
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Quote:
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Any other suggestions??
My suggestion would be to forget the great circle / rhumb line debate. Look instead at the prevailing currents and winds. For a sailing vessel they are far more important. Even large ships consider currents important although the prevailing winds, unless very strong, or of lesser importance.

A 2 knot current will give you an extra 48 NM per day if it is with you which is very nice. On the other hand, knocking 48 NM off your days run is not so much fun and would negate the advantages of a great circle run. Add a few contrary winds to that and you will be battling to fetch your destination.

The old sailing ship routes were calculated to draw maximum advantage from both winds and currents and should be studied by any long-distance sailor but be warned, they were established by ships which, although lacking modern labour saving devices, had huge crews. A yacht crewed by, say a husband and wife team, could run into difficulties on many of the old routes due to under-manning.

In your particular case, on a voyage from Panama to the Marquesas, you will be sailing from about 8 degrees north to 9 degrees south. In other words, your departure and arrival positions are about equidistant from the equator, which is itself a great circle. Running a great circle route from a point north of the equator to one approximately the same distance south of the equator will result in a minimal saving of distance compared with a rhumb line route (Mercator sailing). In fact, on a voyage from Panama to the Marquesas you will save about 25 NM only.

My advice would be to enter your landfall position in your GPS and just head for it. Plot the rhumb line course on your chart and record your position at noon each day, expecting to be close but rarely on the course line.

Bon voyage

Aye // Stephen
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Old 05-27-2008, 06:39 AM   #13
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Any other suggestions??
Get a copy of this work - I believe it is available as a .pdf document.

Bowditch.jpg

PS. If you want more info on the PDF - PM me.
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Old 05-27-2008, 07:00 AM   #14
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Bowdithch! The definitive navigational encyclopedia.

A wonderful work which I have on a shelf at home. If you want to read it and use it for studies then take 6-months time out from everything else. It is a very solid piece of work. If you read and understand it then you will know everything you need to know and a lot more too about the theory of navigation. But this is the theory and, as someone said, "In theory, practice and theory are the same but in practice they are not." What I am saying here is that even if you possess all the knowledge of Bowditch then wielding a sextant on a pitching vessel and shooting the sun between scudding clouds is nothing one learns from the pages of a book.

Anyway, good luck with your venture but do seriously consider doing a course in navigation.

Aye // Stephen
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