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Old 13-10-2010, 19:37   #31
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Always plot in true,,, then convert,,, your deviation can change on every trip depending on what type of vessel you are in,,, steel hull for instance,,,,,,,
plot true, then convert,,,,,to ships compass
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Old 14-10-2010, 09:44   #32
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That's OK in the Coast Guard and in any other situation when the crew are well trained and everybody speaks the same nautical language.

On an average sailing boat the crew can be a mixed batch of more or less educated sailors and plotting BOTH will make the chart a bit cluttered. Two lines x 3 crew = 6 interpretations ;-).

I never plot the magnetic - I find it just a stage in finding the compass or whatever I need to pass on to the driver.

If one plots the magnetic, why do they not plot the compass?

I say one line on a chart is enough. It can bear a written info along it giving all other details that a particular skipper finds necessary. But the line plotted should be, IMHO, true.

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Old 14-10-2010, 10:08   #33
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Thanks for all the comments. My initial question was how you plan a trip on paper, before you depart on your trip, which rose do you use, mag or true.

My extended question was why the plotter only shows the routes (in the route planner only) in true...and not even given a choice to have it show magnetic to match my paper.

I guess you could convert both to 'compass', but I just wanted to have both my paper and gps plotter routes to match. ie, I wanted to double-check my route planning skills with the plotter.

The rest of the posts are great..nice to see there are other boaters that dont just have their paper charts there as their 'backup' and take navigation seriously.
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Old 14-10-2010, 10:10   #34
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Quote:
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Always plot in true,,, then convert,,, your deviation can change on every trip depending on what type of vessel you are in,,, steel hull for instance,,,,,,,
plot true, then convert,,,,,to ships compass
INDEED !!!
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Old 14-10-2010, 10:25   #35
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INDEED !!!
If you start out at magnetic instead of true, wouldn't you be one step ahead? Just account for deviation? mdc instead of tvmdc ? (accounting for changed in variation if it changes in the course)
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Old 14-10-2010, 10:45   #36
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If you start out at magnetic instead of true, wouldn't you be one step ahead? Just account for deviation? mdc instead of tvmdc ? (accounting for changed in variation if it changes in the course)

there will be VERY little change no change in variation in the course,, variation is listed in the compass rose adn changes yearly,,
take this situation,,,, you are sailing on a compass course of 75 degrees,,,, your true course might be around 65 degrees,,,, your ship has trouble and you call the coast guard,,, NO GPS(boat got hit by lightning),,, you tell the coast guard that you left such and such place on a course of 75 degrees 4 days ago and have been traveling at 6 knots per hour. that is over 500 miles. The coast guard will take it that you course was in true and be looking for you in the wrong spot.

there is a thing called compass error,, easy to figure out,,, professional mariners always plot in true,,, why do you think that is,,,,every marine school, weather 1 week or 4 years teaches to plot in true,,,

maybe everybody who plots in magnetic should call them and tell them they are doing it wrong
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Old 14-10-2010, 10:48   #37
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Just account for deviation? mdc instead of tvmdc ? (accounting for changed in variation if it changes in the course)

to account for deviation you need a deviation card specific to your boat done by a professional,,, also the deviation can and will change on your boat,,, add a radio and it changes,, add a chartplotter and it changes,, magnetic field from electronics affect the compass,,add a radar arch and radar and it changes,,,, get the picture
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Old 14-10-2010, 11:08   #38
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The Canadian Power Squadron Seamanship course requires one to plot both course and fixes with BOTH true and magnetic as per example below.
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Old 14-10-2010, 11:09   #39
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To the original poster (I didnt read all the replies as I know what form they will take)

For me I use True as the magnetic variation where I have been for almost a year has been very small.

I am also from the other hemisphere so the vairation is now opposite (and small).

Also using electronic is really allowing peole to not really care.

I only use the magnetic compass when in a squall and everything is disoriented. In those instances it doesn't really matter if its Ture or Magnetic. 20 deg variation is fine

Of course when I was young training people to navigate on paper it was magnetic... but all that was before plotters. My world has moved...



I will probably now always use True unless the variation is more than about 10 dgerees. Even then its more likely I'd use T and just note the Variation.

By the way, when the weather forcast says 'North' in an area of 20 deg Variation what do they mean? T or M?

I know the answer... be interested in what others think

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Old 14-10-2010, 11:17   #40
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it should be true north, but you might have thrown in wrench when you said 20 degree variation
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Old 14-10-2010, 11:25   #41
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Quote:
Originally Posted by GordMay View Post
INDEED !!!
Quote:
Originally Posted by captainKJ View Post
Just account for deviation? mdc instead of tvmdc ? (accounting for changed in variation if it changes in the course)

to account for deviation you need a deviation card specific to your boat done by a professional,,, also the deviation can and will change on your boat,,, add a radio and it changes,, add a chartplotter and it changes,, magnetic field from electronics affect the compass,,add a radar arch and radar and it changes,,,, get the picture
I get it. Been boating for 25 years. I understand it well. Rarely get to practice it though. My comment implied using a dev card, how else would you account for deviation without one?

My question still is, rephrased, since variation doesn't change (in your local area for the most part), wouldn't you be a step ahead using your deviation card to account for deviation on your boat (assuming it was created accurately) doing mdc instead of tvmdc? Just seems like one extra step. Just trying to understand why you would plot in true and adjust for a constant to get another constant.

Again, im talking about the planning stage of a trip, not doing dr or anything while on the water. If I am looking at my chart with a course that I previosly plotted, if in true, I would have to adjust for variation, then look at my dev card for the heading and adjust again to find the course to steer. 2 calculations

I could plot in magnetic, then look at my dev card and adjust to find the course to steer. 1 calculation. Wouldn't the final answer be the same in either case?
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Old 14-10-2010, 12:19   #42
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I get it. Been boating for 25 years. I understand it well...
My question still is, rephrased, since variation doesn't change (in your local area for the most part) ...
Apparently not.
Variation does change, within a local area, over time. Your chart's rose will show this change as "total degrees at the time of publication, and degrees per year. This gradual change is called the secular variation of the magnetic field.
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Old 14-10-2010, 12:31   #43
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Most small boat skippers use Magnetic and would best indicate it as such. I think the big ship Navy guys use True.
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Old 14-10-2010, 12:39   #44
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Apparently not.
Variation does change, within a local area, over time. Your chart's rose will show this change as "total degrees at the time of publication, and degrees per year. This gradual change is called the secular variation of the magnetic field.
wow this is a very literal forum..

I am talking about planning a trip... so it should be implied that the variation will unlikely change during that particular journey!

let me rephrase again.... during your trip (for example for day or so), the variation will unlikey change during that timeframe, although there is, generally, a change of variation over a greater period of time, as indicated on your compass rose on yoru chart...
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Old 14-10-2010, 12:43   #45
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When plotting a route, I use true north. While at the helm I use the compass to tell me which way the bow is pointed, relative to any variation and/or deviation the compass is currently experiencing. Constant verification as to which way the boat is actually going is maintained via a handheld GPS (set to true north) or when I mark my position on the chart and run my DR line.
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