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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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  #11  
Old 07-17-2007
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Quote:
Originally Posted by sailaway21
... then within at the magnetic ring, you'll see that the direction to magnetic north is 13 degrees to the east of true North.
...
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)
Actually, I had it backwards. SD had it right, but for the wrong reason . (In that the magnetic variation is 13 deg. west in the example.)

When magnetic north is west of true north, it's known as a negative variation (or declination). To correct for magnetic variation, to point to the true bearing from the magnetic bearing, you have to add the inverse of the magnetic variation. Thus, given the compass rose we're using for demo purposes, the magetic variation of which is -13 degrees, to get the true bearing you have to add 13 degrees. E.g.: If you want to point to true north, you have to point to 13 degrees magnetic. (Believe-it-or-not: I figured all this out once, already, when I installed a wind direction indicator for my weather station. My magnetic declination here at home is [well, was] about -7 degrees.)

All of that is confusing. Sapperwhite gave the clearest, most easily understood demonstration of how it works.

Quote:
Originally Posted by sailaway21
Quiz in the morning, open chart!
I think I'm ready!

I'm going to have to look up the procedure for figuring-out compass deviation.

My wife is asked me earlier "Why are you doing all this? We'll have a GPS." "Because I find it interesting and challenging," I told her. "I want to understand how it's done and be able to do it. It's part of the challenge and fun of boating."

Jim

Last edited by SEMIJim; 07-17-2007 at 10:54 PM.
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Old 07-17-2007
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Ah...sailaway...I was simply remarking on the utility of the course protrator I had recommended for quickly checking GPS bearings as well as doing all that arcane map and chart stuff that no one needs to use anymore because of chart plotters. (G)
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Old 07-17-2007
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Actually, Sailaway's right... the variation is 13˚ West.. since the magnetic heading is 13˚ at 0˚ True... however the verbal part of the explanation is correct... My bad... it should be 13˚ WEST. I've corrected the mistake in my original post so as not to confuse anyone...



Quote:
Originally Posted by sailaway21
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!)
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Old 07-17-2007
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Jim, All of the above theory is true(pun), but the compass rose you are referring to is indicating 13 degrees (12.5) WEST variation. Keep it simple to start out. EAST is least(-)WEST is best(+), keep this in mind when working from True to Mag. Use the TVMDC Sailaway was talking about above. Write it vertically with T at the top and fill in what you know, add or subtract as necessary to fill in the blanks. In the compass rose you linked to it shows the variation of 13W. So if you have plotted a True course on your chart in order to figure out your magnetic course(forget Deviation for now) you would ADD 13 degrees from the True course to get your Mag. heading(000T + 13W = 013M. Are you confused yet?

T 000
V 13W
M 013
D 4E Deviation follows same rules as Variation
C 009 Compass course to steer or psc (per steering compass)

If you always write out the TVMDC you can't go wrong if you add all west and subtract all east variation and deviation.
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Jim,
Please do not get caught up in the use of the word declination when describing variation-even though it is technically correct. For instance, Bowditch is known as The American Practical Navigator since we are interested in practical means of navigation. Keep it simple and refer to declination as the difference between Compass North and Magnetic North. Variation is the difference between Magnetic North and True or Polar North.

This is important because, as you've seen, errors tend to be double the number involved, and even Cam can steer within 26 degrees of his course. You'll find many different ways of doing the same thing, one will stick. I'd recommend remembering, for now, the Virgins, thus keeping things in order.

Bowditch explains all of this to whatever level of detail desired and comprehendably.

And while I was joking with Cam, I am entirely serious about the use of charts and navigation. Your instincts are correct. Your wife's attitude, with all due respect, will get you killed. What you are learning now constitutes the basis for all other navigation. Errors in learning or inadequate learning will translate downward to whatever form of navigation you are using. If you intend to sail out sight of land, or in any type of restricted visibility, the concepts you are studying must be had down cold.

Here's a sample question from the USCG exam for Third Mate-Oceans:

While enroute from Montevideo to Walvis Bay a vessel's course is 116 pSC. If the variation for the locality is 25 West and the deviation is 6 west, what is the true course made good if a west wind produces 1 degree leeway?

A 084True
B 086True
C 148True
D 085True note: pSC= per Steering Compass

Here's another. (give you a hint-ignore the Gyro info, it's extraneous)

While proceeding up a channel on course 010 per Gyro Compass you notice a pair of range lights in alignment with the masts of your ship when viewed forward. A check of the chart shows the range to be 009 True and the variation to be 15 West. If the ship's course is 026 pSC, what is the deviation for the present heading?

A. 1 West
B. 2 East
C 2 West
D 1 East


Answers will be found in Off Topic in the AFOC thread.

Enjoy!
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Cam,
What's a GPS bearing? Never heard of such a thing.
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Old 07-18-2007
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I understand that the suggestion I gave was simplistic... that was the point. Once Jim figures out the compass rose then add the next layer. You can't just throw a bunch of jargon out and expect Jim's light bulb to come on.

Now that magnetic variation is understood, we can add deviation.

What is deviation, how do we measure deviation, why is deviation important.

I've seen many posts on Sailnet where someone has a simple question and they get the most drawn out, lengthy, over the top answers you can imagine. Jim had a question about magnetic variation, so you all throw out the whole TVMDC..... just answer the V question and then build on it with the rest....it can be confusing if you are new to it.
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Quote:
Originally Posted by jrd22
Jim, All of the above theory is true(pun), but the compass rose you are referring to is indicating 13 degrees (12.5) WEST variation. Keep it simple to start out. EAST is least(-)WEST is best(+), keep this in mind when working from True to Mag. Use the TVMDC Sailaway was talking about above. Write it vertically with T at the top and fill in what you know, add or subtract as necessary to fill in the blanks. In the compass rose you linked to it shows the variation of 13W. So if you have plotted a True course on your chart in order to figure out your magnetic course(forget Deviation for now) you would ADD 13 degrees from the True course to get your Mag. heading(000T + 13W = 013M. Are you confused yet?

T 000
V 13W
M 013
D 4E Deviation follows same rules as Variation
C 009 Compass course to steer or psc (per steering compass)

If you always write out the TVMDC you can't go wrong if you add all west and subtract all east variation and deviation.
This is good advice But: Going from True to Compass, you add West & Subtract East. Going from Compass to True, You add East & subtract West.
An Always use the Format that JRD has written out.
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Sailaway:
"What's a GPS bearing?"

Well it is either the course to steer that the GPS gives you to your next waypoint OR the little roller ball type things they use to keep the gears inside the GPS turning smoothly! (G)
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Quote:
Originally Posted by camaraderie
Sailaway:
"What's a GPS bearing?"

Well it is either the course to steer that the GPS gives you to your next waypoint OR the little roller ball type things they use to keep the gears inside the GPS turning smoothly! (G)
The GPS only takes you to the Middle of the conversion to Magnetic compass. It only applys the variation to True and not deviation. You have to apply the deviation from your Deviation card and finish the process of converting from True to Boat compass.
You did have your compass swung and now have a Deviation card for your Boat??

Note: this is when the GPS is set on Magnetic and not True. And the GPS will give you the variation of the area that you are in also.
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