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 SEMIJim 07-17-2007 08:59 PM

Magnetic Variation

For some reason, this one alway confuses me :confused:

Given a chart compass rose like this one NOAA 200th Historical Nautical Charts Collection: New York Harbor; Compass Rose

I believe this compass rose is indicating a magnetic variation of -13 degrees? So I want to steer a magnetic course of -13 degrees to get a true (geographic) course, right? E.g.: If I want to steer a course of 45 degrees (true), I want to steer a compass course of 32 degrees?

TIA,
Jim

 sailingdog 07-17-2007 09:11 PM

Umm... saying you want to steer a course of -13˚ doesn't make sense...you've actually got it backwards. The magnetic variation is 13˚ WEST.

For example, if you want to sail a course of 0˚ true, then you would steer a course of 13˚ magnetic. If you wanted to sail a course of 45˚ true, then you would steer a course of 58˚ magnetic.

Basically, the easiest way to find the variation is to look at the compass rose and line up on True North, and what ever is lined up with it is the variation—in this case 13˚ West, since the 0˚M is 13˚ west of 0˚ True. Variation and deviation have both a magnitude and a direction.

Now the discussion above assumes that the compass doesn't have any deviation, which is not usually the case.

To go from Compass to Magnetic to True, you can remember the mnemonic "Can Dead Men Vote Twice". It stands for Compass, Deviation, Magnetic, Variation, True.

This means to go from Compass to Magnetic, you add the Deviation, to go from Magnetic to True, you add the variation.

BTW, when you are correcting a heading or course—ie going from Compass to True— you add East... When uncorrecting—going from True to Compass—you subtract East.

ed: corrected variation from 13˚ East to 13˚ West. :D My bad, thanks Sailaway...

 SEMIJim 07-17-2007 09:30 PM

Quote:
 Originally Posted by sailingdog Umm... saying you want to steer a course of -13˚ doesn't make sense...
I meant -13 deg. magnetic from the true course. But...

Quote:
 Originally Posted by sailingdog you've actually got it backwards. The magnetic variation is 13˚ east.
I thought magnetic variation was expressed in terms of what the variation was, not the correction that needed to be applied to obtain the true bearing. So if, at a particular point, the compass pointed west of true north, the magnetic variation is said to be negative.

But I think I still had it backwards. To correct for magnetic variation, you have to subtract if the variation is positive, add if it's negative.

Quote:
 Originally Posted by sailingdog For example, if you want to sail a course of 0˚ true, then you would sail a course of 13˚ magnetic. If you wanted to sail a course of 45˚ true, then you would steer a course of 58˚ magnetic.
Yes. And how I figured that out was noticing two things when playing with my brand-spankin'-new Weems Protractor: 1. If I picked a course, and lined-up one edge of it with a chart compass rose, and read the course from the inner rose, it read plus N degrees. 2. The Weems Protractor has magnetic variation "compensation" built-in, in the form of "east" and "west" degree marks either side of 0. (It's a writeable area, so you can put a pencil mark at the current correction.) The "west" label is to the clock-wise side the the "east" label to the anti-clockwise side.

Told you this subject always confused me :p

Jim

 Sapperwhite 07-17-2007 09:37 PM

Jim,
Try this, Draw a line all the way through the compass rose on any given point. Make sure the line passes through the X in the center of the rose. This line represents the relationship that the compass rose is express to tell you. If the line passes through 0 true, then it also passes through 13 magnetic. So to sail to a position on the chart that is 0 true, you would sail 13 deg. by the ships compass.
Try it on different points, 35 true, 276 true, etc etc.
Also, when you are out sailing, take a look at that tower off to port. Now look at you compass and it says the tower is at 13 deg. magnetic. Do the inverse math and you will know that the tower is at 0 true from your current position.

Just draw a line through the rose.

 sailaway21 07-17-2007 09:44 PM

Jim,
Good, you've got a compass rose in front of you. That's a big help. If you look you'll see the two rings; one being true and the other being magnetic. If you look at true North on the outer ring, marked with a star over it, and then within at the magnetic ring, you'll see that the direction to magnetic north is 13 degrees to the east of true North. You could for purposes of argument consider this -13 degrees, since if you subtract it from the magnetic course, you'll have your true course. Less confusing to look at or imagine the compass rose and the Variation being 13 degrees east.

Now, here's where you're messed up! (vbg) You are not going to steer the magnetic course. You are going to steer a compass course. Let's start from scratch.

TrueVirginsMakeDullCompanions or TVMDC. You lay down a course from Oblivion Island to Tedium Anchorage on the chart. You use your triangles or parrallel rules to walk it over to either the rose or a meridian. The course you read off is True, referenced to True North or Polar North. Let's say it is 045 degrees True. Now you consult the compass rose for the local Variation. Applying the annual increase or decrease found inside the rose to the variation listed there you apply that number to your true course. East is Least. West is Best. As you've already seen, you would subtract the 13 degrees East Variation from the 045 and that would give you a magnetic course of 032 degrees Magnetic. (i believe the dog had it backwards, esy to do)

But you're not done. Now you have to apply Deviation. Deviation is the error that your compass has in it. It is the difference between Compass North and Magnetic North in much the same way that Variation is the difference between Magnetic North and Polar North. (we use "Pn" to describe true North when writing, hence the term Polar North for True North).

Your deviation will vary depending on the heading your boat is on. It is necessary to make up a deviation card for this. We'll discuss that later or you can look up one of the threads/articles on "swinging ship". Unfortunately, swinging ship refers to the process of determining and adjusting for deviation in the ship's magnetic compass and not the Wombat's Austrailian Love Boat. (g)

So with all this in hand, True Virgins Make Dull Companions, should make sense. Starting with the capital letters, make your way through to find your answer. True, with Variation applied, yields Magnetic. Magnetic, with Deviation applied, yields Compass. Remembering East is Least (subtract) and West is Best (add) If you are working the other way, from Compass to True, you must reverse the adding/subtracting.

What the West and East of Variation is telling you is that the Magnetic Pole is to the West or East of the True Polar North.

Quiz in the morning, open chart!

 Sapperwhite 07-17-2007 09:50 PM

Sailaway is absolutely correct, but you have to lay the foundation before you can put on the roof. Slow it down a little.

 sailingdog 07-17-2007 10:18 PM

Basically, Sailaway has done the True -> Magnetic -> Compass, and I did the opposite: Compass -> Magnetic -> True. :D

Sapperwhite-

This is a bit simplistic, since it ignores the compass deviation, and most compasses have some deviation.

Quote:
 Originally Posted by Sapperwhite Jim, Try this, Draw a line all the way through the compass rose on any given point. Make sure the line passes through the X in the center of the rose. This line represents the relationship that the compass rose is express to tell you. If the line passes through 0 true, then it also passes through 13 magnetic. So to sail to a position on the chart that is 0 true, you would sail 13 deg. by the ships compass. Try it on different points, 35 true, 276 true, etc etc. Also, when you are out sailing, take a look at that tower off to port. Now look at you compass and it says the tower is at 13 deg. magnetic. Do the inverse math and you will know that the tower is at 0 true from your current position. Just draw a line through the rose.

Jim...with the protractor, you'll find yourself just plotting the true course then reading the variation off the compass rose and adding it to the true course in your head for a magnetic heading. Don't worry too much about exactness since you can't steer that accurately anyway. Just gives you a quick course that should be pretty close to the heading your GPS gives you. Good early warning system that you've entered the wrong waypoint in the GPS!! Pretty neat device eh?

 sailaway21 07-17-2007 10:31 PM

Ah, Hey Dog? Hello, Dog? The difference being that I have him on the correct course and you have him 26 degrees off? Mea culpa, dog?

(attention, sports fans. this rarely happens-catching the dog out. so i'm taking abrief break and acting like a juvenile. all bark intended-no bite!)

 sailaway21 07-17-2007 10:36 PM

Ah Cam?
The guy's in New Yawk Harbor. And you've got him on the GPS? I thought we were talking about charts and compasses here. As in, I want to steer 045 degrees true, what is my compass course? What, pray tell, does the GPS have to do with that? And yes, the compass is a pretty neat device. Wouldn't cross New York harbor without one. (g)

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