Adjusting for variation?

[scotty123]

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?

 

[Pye_End]

I've never found that way of doing variation easy.

I use: Grid to Mag Add; Mag to Grid get rid.

Works here, at the moment. Mind you variation is so small now round here you can forget about it a lot of the time.

 

[jimi]

I always remember CADET

Compass to True Add East ... works for me

 

[doug748]

This will show Magnetic North (353deg) & True North (360deg).

I think your error is in the above line. The direction of true north will read 7deg.


Cor, now you have got me going.
These things often maggot away at the unreliable interface between semantics and geometry.
This one could run and run so I have got my fingers in my ears and am not going to listen any more - La La Lar La De Da La.....

 

[johnalison]

I've never been able to remember any of these mnemonics or short-cuts as I tend to remember things entirely visually. I can only cope by visualising a chart with a compass rose on a meridian and rotating the parameters in my mind. Thus if the rose has north pointing say 5 degrees to the left, the meridian will cross the 5 degree marking and I will need to steer 45 degrees to maintain 40 degrees true.

 

[john_morris_uk]

I've never found that way of doing variation easy.

I use: Grid to Mag Add; Mag to Grid get rid.

Works here, at the moment. Mind you variation is so small now round here you can forget about it a lot of the time.

The key phrase here is 'Works here at the moment.' The rule doesn't work for Westerly variation. CADET is just as easy and can be applied to all circumstances, so long as you write it out in a linear format and remember to make everything opposite as necessary.

ie

Compass to True = Add Easterly variation
therefore subtract Westerly variation.

Inverse of everything to go the other way.

For what order to do Variation and Deviation etc I use "Cadbury's Dairy Milk Very Tasty." ie Compass - apply Deviation - gives Magnetic - apply Variation gives True. Reverse everything to got from True to Compass.

 

[Sgeir]

Scotty, with respect, you seem to be over complicating matters. Better to forget these handy "rules" memos which seem to confuse rether than clarify. It's simple enough; to steer true north in your example, you steer 007 degrees magnetic, because your magnetic compass rose north is actually pointing at 353 degrees true.

Similarly if you want to steer due east, steer 97 degrees magnetic, etc, etc... Keep it simple.

 

[andygc]

Scotty, with respect, you seem to be over complicating matters.

Not over complicating, either being perverse to start a thread or just being daft.

Why talk about applying a correction to the compass rose printed on the chart? The 353 degrees on the chart is the direction in which the 000 degrees on the compass points. The compass card against its lubber line indicates true plus westerly variation. That's all.

The mnemonic never was "West is best" anyway, it is variation/deviation West, compass best, variation/deviation East, compass least. If you are going to use a mnemonic, it's best to learn it properly.

 

[alahol2]

I was always taught "Error West Compass Best" ie if the 'error' (Variation/Deviation) is west then the Compass shows the higher figure. Also remembering True-Magnetic-Compass. Easterly 'errors' are obviously the opposite. (Error East Compass Least).
This is a purely mathematical method to me. If I try to visualise the results on a compass rose it makes my head hurt.

 

[Danny Jo]

I always remember CADET

Compass to True Add East ... works for me

I'm another CADET. I've tried other methods; I once even convinced myself that I was smart enough to do it intuitively only to make a complete idiot of myself in front of my incredulous crew.

When you do it often enough, of course, you don't need to keep reciting the pneumonic (er sorry, that's the medicine kicking in) I mean, mnemonic. But after little more than a week ashore it's back to "Compass to True, ADd East" etc.

 

[colingr]

I was always taught "Error West Compass Best" ie if the 'error' (Variation/Deviation) is west then the Compass shows the higher figure. Also remembering True-Magnetic-Compass. Easterly 'errors' are obviously the opposite. (Error East Compass Least).
This is a purely mathematical method to me. If I try to visualise the results on a compass rose it makes my head hurt.

That one works for me too

 

[jordanbasset]

That one works for me too

+1

 

[Summer Sailor]

I always remember CADET

Compass to True Add East ... works for me

Me too!

 

[alant]

The key phrase here is 'Works here at the moment.' The rule doesn't work for Westerly variation. CADET is just as easy and can be applied to all circumstances, so long as you write it out in a linear format and remember to make everything opposite as necessary.

ie

 

[alahol2]

What's going on here? Posts keep appearing then disappearing? Is someone realising their error?

 

[CFarr]

Hi Alant
This can get confusing when it's in your head. The best way is to draw a circle, mark true north at 000deg, then mark magnetic north at say, 355 deg or in other words - error west 005deg.
You can see that then if you want to steer true north you would have to steer 005deg MORE than what the compass (magnetic north) is showing you i.e. 005deg magnetic.
Hope you can visualize that!

 

[scotty123]

Hi Alant
This can get confusing when it's in your head. The best way is to draw a circle, mark true north at 000deg, then mark magnetic north at say, 355 deg or in other words - error west 005deg.
You can see that then if you want to steer true north you would have to steer 005deg MORE than what the compass (magnetic north) is showing you i.e. 005deg magnetic.
Hope you can visualize that!

?

 

[CFarr]

Scotty123
Sorry if it didn't make sense.
I'll try to attach a diagram.
If that's worked you will see I've got true north straight up, magnetic north at error west 005deg and a desired bearing of 045 deg true.
If we aimed directly 045 deg on a compass it would be 005 deg less than 045 true because we're starting from 355 deg and not from true north.

Error west - Compass best. i.e. if the error is west ADD the variation to the True bearing desired to get your heading.
In this case 050 deg M

 

[Dab]

Mag To Grid Does Work

The key phrase here is 'Works here at the moment.' The rule doesn't work for Westerly variation. CADET is just as easy and can be applied to all circumstances, so long as you write it out in a linear format and remember to make everything opposite as necessary.

ie

Compass to True = Add Easterly variation
therefore subtract Westerly variation.

Inverse of everything to go the other way.

For what order to do Variation and Deviation etc I use "Cadbury's Dairy Milk Very Tasty." ie Compass - apply Deviation - gives Magnetic - apply Variation gives True. Reverse everything to got from True to Compass.

I was taught Mag to grid get rid and grid to mag add when I first learnt to map read. The prevailing variation then was West and it still is now in the UK.

As a latecomer to sailing, I still use it, on the basis that the formula is ingrained on my brain and that it works for westerly variation. If i ever sailed where the variation was East i would reverse the sums.

So for westerly variation: Compass (Mag) to True (Grid) Add East (get rid (subtract) West.)

 

[DanTribe]

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.