Adjusting for variation?

[john_morris_uk]

I also use the CADET method, but have to admit that if I have to do the calculation quickly or under stress, I ignore the maths and work in magnetic. This is on the basis that I'll be about 4 degrees out but if I get the maths wrong I might be 8 degrees adrift. It can be corrected later at leisure.

OK after all the descriptions of how to do the maths, this is what I really do in practice:

I mark the variation on my Breton plotter and just set the bearings to the pencil mark. You don't have to think about it at all. Furthermore I always try and adjust the compass (or have it adjusted) so that deviation is only a degree or two at the most - and then I ignore deviation as I can't steer that accurately.

 

[alan_d]

Another way of remembering which works in UK waters (and anywhere else where the deviation is west), assuming your chart work is done in degrees True:

Bearings taken in the cockpit are handed down to the navigator (i.e. deviation is subtracted). A course calculated by the navigator (in deg.T) is then passed up to the helmsman (i.e. variation added to give deg.M).

 

[Monique]

IMO...

Mine is easier to remember:

True Virgins Make Dull Company = True +- Variation = Magnetic +- Deviation = Compass.

 

[sticky]

Adjusting for variation (and if necessary deviation)

When you reach a certain age, remembering mnemonics becomes a bit of a chore. I have a simple diagram along the following lines:

+W
-E

T
V
M
D
C

+E
-W
When going from top to bottom it is +W -E and when going the other way it is +E -W.

Sticky

 

[chart5011]

Have a quick look at www.cockpitcards.co.uk for description and worked examples of Variation and Deviation.

 

[snowleopard]

The simplest way is to get a Breton plotter and just dial it in.

2nd simplest is to get your instruments to read in True bearings.

 

[Danny]

The simplest way is to get a Breton plotter and just dial it in.

2nd simplest is to get your instruments to read in True bearings.

I think this one would probably do the job.

 

[stephenh]

The simplest way is to get a Breton plotter and just dial it in.

2nd simplest is to get your instruments to read in True bearings.

I'll go for that....simplicity rules......Breton plotter - wouldn't be without one

I was brought up on "Timid Virgins Make Dull Company" - a bit more potential than "True"

 

[Bosun Higgs]

Adjusting for variation, the rules are -

When going from deg(T) to deg(M), 'west is best' so we +West and - East.
When going from deg(M) to deg(T), we - West and + East.


Now, consider a compass rose, with the variation shown as 7deg West.

This will show Magnetic North (353deg) & True North (360deg).
Logically, to adjust from deg(M) to deg(T), the adjustment is from 353 to 360deg, which seems to be +West, not -West as per the rule.

Similarly, if there is 10deg East variation, we are going from (010deg) Magnetic North to back to (000deg) True North.
So, when adjusting from this deg(M) to deg(T), we seem to be -East, not +East as per the rule.

Why?

The 353deg you quote above for magnetic north is degrees T. If it were degrees M it would be zero wouldnt it. So your mnemonic still works. On the example you gave, a course of 90deg T would be 97deg M . Personally I find it much easier to visualise a compass rose rather than use mnemonics. Or if that is too difficult, use your Breton plotter.

I assume you are doing an RYA course which is why you are bothered about magnetic bearings. In the real navigating world, forget the issue. Set your plotter / gps to report in T and do all your navigating that way. Set your autohelm by comparing track with waypoint bearing. Leave the cover on the compass - it's yesterdays technology and in and case if everything goes down and you are daft enough not to carry a spare handheld GPS and batteries, you wont be able to hand steer more accurately that the variation.

 

[alahol2]

Leave the cover on the compass - it's yesterdays technology and in and case if everything goes down and you are daft enough not to carry a spare handheld GPS and batteries, you wont be able to hand steer more accurately that the variation.

Sorry, just can't agree to that. Anyone that takes a boat to sea should be able to do basic navigation without recourse to electronics. That means understanding how to use a handbearing compass to fix a position as well as working out a course to steer using DR. I would agree that most people probably use GPS almost exclusively at the moment but that doesn't mean you can just forget the none electronic methods. Pre-GPS the easiest way to navigate was to work in magnetic, there are very few operations that require True bearings (other than perhaps leading lines/light sectors).

 

[dolphin]

variation & deviation

i use the simple rule : variation & deviation is applied only to the COMPASS!

compass +/- {variation [west (-), east (+)]} = true

we know the variation & either compass either true, easy to calculate the missing one!

NB
please note that on the chart, reading the compass rose, the variation is for the year, when the chart is produced !!! sometimes, one can use a very old chart, lets say edition 1965!
45 years x annual 5'(for example) = about 4 deg !!!
can be a case with variation E and annual W or opposite, not always with the same sign!

 

[Bosun Higgs]

Sorry, just can't agree to that. Anyone that takes a boat to sea should be able to do basic navigation without recourse to electronics. That means understanding how to use a handbearing compass to fix a position as well as working out a course to steer using DR. I would agree that most people probably use GPS almost exclusively at the moment but that doesn't mean you can just forget the none electronic methods. Pre-GPS the easiest way to navigate was to work in magnetic, there are very few operations that require True bearings (other than perhaps leading lines/light sectors).

Didn't suggest he forgot / never learned how to do it the old way - as it happens I teach the RYA courses so I'm all in favour of people learning the old ways. What I was saying was that real life navigation post GPS was much better done in True and forgetting the compass. Though I must admit I do use a handbearing for collision avoidance

 

[Cruiser2B]

IMO...

Mine is easier to remember:

True Virgins Make Dull Company = True +- Variation = Magnetic +- Deviation = Compass.

True Virgins Make Dull Company, so Add Whiskey (+west)

 

[RichardS]

Tele Vision Makes Dull Children Al Ways

True Variation Magnetic Deviation Compass Add West

 

[grumpy_o_g]

Always assuming you can steer a compass course to that accuracy anyway of course. I've found that on every occasion I have to go from true to magnetic or back someone has kindly provided a compass rose, plotter or something similar to cover my embarrassment. Which is fortunate as I used to earn my living maintaining navigation systems.

 

[Monique]

True Virgins Make Dull Company, so Add Whiskey (+west)

Whiskey does make it better...