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Life is a wild ride!
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Discussion Starter · #1 ·
I've been reading up on charts and how to plot bearings. The books I have all seem to describe converting magnetic bearings to true bearings in the same way and they have me confused.

I understand that the variance given in the compass rose has a stated variance, when it was measured, and the amount and direction of change per year. I understand how to find the current year variation from this information. What's confusing me is the statement "To convert a magnetic compass reading to true to plot on a chart *subtract a westerly variation *Add an easterly variation."

Assuming that my variation after figuring for current year is 5* W. and forget deviation for a moment, let's say I take a compass bearing of 270*. Would my conversion be 270 - 5 = 265*T? If the variance was 5* E then the conversion would be 270 + 5 = 275*T?

I'm not sure why I'm having so much trouble picturing this in my head.
 

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Your variation depends on which side of the country you are on. In the U.S., for me, it will be west. For the west coast (and west of the isogonic line) it will be east.

But yes, you are correct. Magnetic to true you subtract westerly variation and add easterly variation.
 

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Life is a wild ride!
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Discussion Starter · #4 · (Edited)
Your variation depends on which side of the country you are on. In the U.S., for me, it will be west. For the west coast (and west of the isogonic line) it will be east.

But yes, you are correct. Magnetic to true you subtract westerly variation and add easterly variation.
The examples I'm looking at show the compass rose of a chart with the variance noted in the center. I was hoping that following the rule of add east, subtract west will work on any chart.

Another question; When the rose has an inner magnetic rose, it is drawn to the variance noted in the center for that year, right? If the chart is older and the variance over the years was great enough to change the degree variance, I'll still have to allow for that if I use the inner magnetic rose.

Thanks for the response by the way.

Edit: And I guess I reverse the rule when converting from true to magnetic?
 

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Life is a wild ride!
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Discussion Starter · #5 ·
!!!Except in the Bermuda triangle! Or the superstition mountains, but you can't sail there.
I've heard that with all the magnetic disruptions in the Triangle, you can only steer a straight course while drunk on Pusser's rum and you have to create a deviation card for any other rums consumed. :p
 

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The examples I'm looking at show the compass rose of a chart with the variance noted in the center. I was hoping that following the rule of add east, subtract west will work on any chart.
Charts for the west coast will have an easterly variation noted in the middle of the compass rose. Charts for the east coast will have a westerly variation. They aren't interchangeable in the way I think you mean. If you're using a chart for say, the entrance to New York Harbor, you'll use the westerly variation as noted in the compass rose.

They give you both east and west so that the publishers can use the same example on either coast. But you need to use the one that's applicable to your area.

Another question; When the rose has an inner magnetic rose, it is drawn to the variance noted in the center for that year, right?
Yes.

If the chart is older and the variance over the years was great enough to change the degree variance, I'll still have to allow for that if I use the inner magnetic rose.

Thanks for the response by the way.
You'll have to account for the change in variation over the years regardless of which direction you are going (magnetic to true or true to magnetic).

I hope this helps.

A great book that I recommend is How to Read a Nautical Chart by Nigel Calder. Good explanations and it also contains all of Chart No. 1.
 

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Discussion Starter · #7 ·
Charts for the west coast will have an easterly variation noted in the middle of the compass rose. Charts for the east coast will have a westerly variation. They aren't interchangeable in the way I think you mean. If you're using a chart for say, the entrance to New York Harbor, you'll use the westerly variation as noted in the compass rose.

They give you both east and west so that the publishers can use the same example on either coast. But you need to use the one that's applicable to your area.

Yes.

You'll have to account for the change in variation over the years regardless of which direction you are going (magnetic to true or true to magnetic).

I hope this helps.

A great book that I recommend is How to Read a Nautical Chart by Nigel Calder. Good explanations and it also contains all of Chart No. 1.
Thanks, I'll check that book out. I just assumed that if I buy charts for any given area, the variance would be noted on it whether it was E or W. Most of my navigating has been piloting by landmarks or by GPS. I'm trying to self-teach myself on the subject. Using electronics isn't all that difficult but I want to learn how to do it without having to totally rely on them. Would you recommend Calder's book as a good source to learn not just how to read a chart but how to figure course, set, drift, accounting for currents, tides, etc. and all those other things that I should know but don't yet know that I should know?:laugher
 

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Thanks, I'll check that book out. I just assumed that if I buy charts for any given area, the variance would be noted on it whether it was E or W.
It will be. I'm sorry. I thought I was more clear.

Most of my navigating has been piloting by landmarks or by GPS. I'm trying to self-teach myself on the subject. Using electronics isn't all that difficult but I want to learn how to do it without having to totally rely on them. Would you recommend Calder's book as a good source to learn not just how to read a chart but how to figure course, set, drift, accounting for currents, tides, etc. and all those other things that I should know but don't yet know that I should know?:laugher
I don't remember offhand if it teaches you how to figure out your course with current set and drift. I think I left my copy on the boat in my nav bag. Boater's Bowditch has a brief explanation.
 

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Well since no one's said it, and at the risk of outdated PCness, I'll repeat the jingles I learned back in Power Squadron and around the docks:

To get from From True to the Compass course to steer, you add Westerly error and subtract Easterly error. So, it's True, + - Variation = Magnetic, which + - Deviation =
Compass (and you add Westerly error and subtract easterly error for both variation and deviation, so in shorthand it's:

"TVMDC--Add W", (meaning True, Variation, Magnetic, Deviation, Compass, and add Westerly errors)

How to remember?? as the old non-PC sailors said,

"True Virgins Make Dull Companions--Add Whiskey"


And to go the opposite way, from Compass to True, it's ("CDMVT, Add E") or the jingle"

"Can Dead Men Vote Twice--At Elections?" (here in Louisiana, of course the answer is yes, ha ha)

There may be a more modern set of jingles, but True Virgins and Dead Men is what I grew up with, and I can remember the jingles when I can't remember the formulas.
 

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Well since no one's said it, and at the risk of outdated PCness, I'll repeat the jingles I learned back in Power Squadron and around the docks:...
:)

He was already thinking so much I could hear his brain cells popping. I was trying not to answer more than he asked.
 

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Life is a wild ride!
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Discussion Starter · #12 ·
Well since no one's said it, and at the risk of outdated PCness, I'll repeat the jingles I learned back in Power Squadron and around the docks:

To get from From True to the Compass course to steer, you add Westerly error and subtract Easterly error. So, it's True, + - Variation = Magnetic, which + - Deviation =
Compass (and you add Westerly error and subtract easterly error for both variation and deviation, so in shorthand it's:

"TVMDC--Add W", (meaning True, Variation, Magnetic, Deviation, Compass, and add Westerly errors)

How to remember?? as the old non-PC sailors said,

"True Virgins Make Dull Companions--Add Whiskey"

And to go the opposite way, from Compass to True, it's ("CDMVT, Add E") or the jingle"

"Can Dead Men Vote Twice--At Elections?" (here in Louisiana, of course the answer is yes, ha ha)

There may be a more modern set of jingles, but True Virgins and Dead Men is what I grew up with, and I can remember the jingles when I can't remember the formulas.
That's a good way to remember it I guess! It would probably help me if I had an actual chart in front of me. The ones I have are TVA charts for Kentucky and Barkley lakes. No Roses!
 

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That's a good way to remember it I guess! It would probably help me if I had an actual chart in front of me. The ones I have are TVA charts for Kentucky and Barkley lakes. No Roses!
You can download free charts from NOAA that contain compass roses:

Nautical Charts & Pubs

The printed charts that I use are full-sized charts divided into 8.5x11 sheets when printed. I end up folding a single-sheet chart anyway so the smaller sizes aren't an issue, although some people don't like them.
 

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That's a good way to remember it I guess! It would probably help me if I had an actual chart in front of me. The ones I have are TVA charts for Kentucky and Barkley lakes. No Roses!
Ah, then you can add Roses to the Whiskey for the True Virgins!! ;-)

okay, to get serious for a nanosecond, you can get the NOAA charts on line, which will get you a look at the Compass Roses:

NOAA's On-Line Chart Viewer

Look at one for, say, Massachusetts Bay, then one for say the Central Gulf of Mexico (such as here in New Orleans) and you'll see that there's considerable variation in the former and almost none in the latter (so down here we seldom use the jingle). It has to do with the location of the iron ore in the northern Canada islands compared to the location of the North Pole, or so I was told. From here, they almost line up, hence almost no variation. But from the Boston area in Mass Bay, those Canada islands are considerably to the left of the North Pole, and your variation (sometimes called Declination) is about 15 degrees West.
 

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Life is a wild ride!
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Discussion Starter · #16 ·
You can download free charts from NOAA that contain compass roses:

Nautical Charts & Pubs

The printed charts that I use are full-sized charts divided into 8.5x11 sheets when printed. I end up folding a single-sheet chart anyway so the smaller sizes aren't an issue, although some people don't like them.
Ah, then you can add Roses to the Whiskey for the True Virgins!! ;-)

okay, to get serious for a nanosecond, you can get the NOAA charts on line, which will get you a look at the Compass Roses:

NOAA's On-Line Chart Viewer
Thanks you guys! I think after supper I will try to print some off and come back to it. I probably need a break.
 

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Not sure if this is to simple and correct. I used hand held GPS compass to correct magnetic one with felt pen. Easier to calculate after that.
I may be incorrect, but I think the point of his learning this is to know how to do it without electronics.
 
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