Compasses and having a "Blonde moment"

[Biker]

Having just looked at Eric's cartoons (see "other chatter") and in particular the cartoon about compasses it occured to me, in my usual clueless way: Why can't a compass be made to compensate for the deviation of true north and magnetic north?

okenest:

I mean if you're in the UK and true north is say 5 degrees (I'm not sure of the exact figure) to the right of where the needle is pointing couldn't a compass needle be offset by those degrees so it's pointing in the right direction to start with?

The more I think about it the more worms are spilling out of the can here. :theyareon

Maybe the answer is: If you're at zero longtitude i.e. the UK this side of the globe and true north would 5 degrees to the right of where the needle's pointing, am I correct in thinking that if you're 180 degrees the otherside of the world (mid Pacific) true north would it then 5 degrees to the left of where the needle's pointing. Yes?

Have I just answered my own question?

It helps if you have an orange slightly tilted in your hands to figure this one out.

Then if that's the case compasses could only be manufactured and used in specific countries.

Ahaaa! Would a compass for Antarctica point downwards?

Feel free to start hurling the stones at me for my blissful, childlike ignorance. I'm wearing my best Kevlar codpiece in anticipation.

 

[wicca]

I'm off for a lie down in a darkened room while I think about that one Biker, 'ang on where's me orange?
Right found it..hopefully by the time I get back someone will have given you an answer.

 

[Tor helge]

So for travelling in Norway alone you would need 15 (NB.Not an exact number) compasses instead of just one as we use today.
Brilliant.

Tor

 

[phill_ue]

I believe there has been one made that will offset the magnetic variation.

 

[Bravo4]

Biker, I do believe that this can become one of those things that the explanation of, becomes more complicated than the thing itself.
Yes, there is such a compass. It's base rotates within it's rotating base, it's not in the needle in other words.
Yes, your mental picture of global magnetic variation is off, a bit.
Yes, a compass can point downwards.
If you have the time/high speed connect, look at these two maps (charts) for a minute.
http://www.nga.mil/portal/site/mari...i_baseMenuID=e106a3b5e50edce1fec24fd73927a759
To properly explain, in words, what is contained in these charts would take a book, so long and tedious to read; hence the magic of a map, the beauty of a diagram and the genius of a cartoon.

Now, forget all about magnetic variation for awhile. Work out your nav "problem'' with wanton disregard to variation (also sometimes called deviation but let's go with variation because deviation is something else as is declination. Three different words that can be used to refer to the same thing but are really three different things. See how the confusion starts?) Get comfy with the absolute basics and then come back to variation. It'll always be there, if we're lucky.

Most basic nav, then variation; then maybe deviation for awhile, followed by some declination discerning and then we can get into various uses of a solar compass which has zero variation to account for. Of course a solar compass doesn't work at night, has a couple of hick-ups and confusions, but it beats studying magnetic variation, unless that's what you're into.

 

[johnboy]

I believe there has been one made that will offset the magnetic variation.

Yep the Silva Ranger 515 allows you to adjust the declination on the compass card.

 

[wicca]

Biker, you notice I chickened out straight away. After Bravo4's explanation you can see why.
I'm probably wrong but...in general, certainly in the UK ' variation' refers to magnetic variation, ie: the effect of the Earths magnetic field on the compass, the Magnetic North 'Varies' from True North.

'Deviation' is a separate error caused by another influence. Leave your Maglite alongside your compass and the batteries will effect the compass reading.
causing it to 'Deviate' from it's correct heading.

On my boat I have to watch for deviation caused by metal objects close to the
compass binacle. So deviation can be countered, by removing the cause. Variation has to be allowed for, and the amount of correction is shown on maps and charts and alters yearly.

I understand, and Bravo4 will no doubt confirm, that 'deviation' is used in a different sense in the United States. (and probably other places to)

So far on my voyages I haven't collided with any islands so I think I'm not too far off course...whoops sorry about that..

 

[torc]

http://www.silva.se/en/Products/Com...ductId={7BAC2DDA-E172-477B-82C4-B152023529A8}
The declination can be preset in this compass to do away with the whole mag. to grid thing.
The magnetic poles move about, so for good accuracy you need to be able to recalculate declination from time to time. By doing the addition and subtraction yourself or setting the compass card allows you to move around your general latitude on the planet.
Changing your latitude is a problem though as your European compass will need a differently balanced needle. The compass needle does not follow the curvature of the Earth surface but points directly to the magnetic pole itself so in the southern hemisphere the needle will not be level, drag on its housing base and cannot swing freely when held level.
I think compasses are sold for 5 different zones from Northern Europe to Oz.
Global compasses are also available.
http://www.thecompassstore.com/globneedcom.html
As you move towards the geographic North or South poles the variation calculation becomes more difficult and early polar explorers in addition to compasses used solar navigation and dead reckoning.
Happy trails...torc.

 

[phill_ue]

You can set your magnetic variation with a sun/shadow stick and then set it up to sight the north star at night. The difference between your north/south line on the shadow stick and the north star is the variation. It takes a day to do, but it is a very interesting method and worth trying out a few times.

 

[Biker]

This is going to come as some surprise but I actually understood most of what you've replied with. Thanks folks.

Tor, in case you're wondering I have shares in a compass making company. I figure if I can circulate enough doubt in people's minds I can sell a compasses for every degree of longitude.

"This time next year Rodder's we'll be Millyunaires!"

Thanks for taking the time to compose and reply. I'll re-read the specs and sort it out in my head a bit more, gotta be out the door in 5 mins.

Biker

 

[Carbuncle]

Many Suunto, Reacta, Kasper & Richter and even the odd Silva compasses do this by allowing the "shed" to be moved independently from the N/S line.

Like this:

 

[Eric_Methven]

Blimey, it's not rocket science. Magnetic north moves about (slowly)! Grid north is what's printed on the maps. At the moment, magnetic north is in the Northwest Territories of Canada, so that means the compass needle will be pointing west of the north pole. It's about 5 degrees in 2010 (approximately).

So, how do you compensate? Well, you have to either add or subtract depending on whether you are taking a magnetic reading and refering it to the map, or whether you are taking a map bearing and walking off using your compass for direction finding.

The formula is GUMA (Grid unto Magnetic Add) and MUGS (Magnetic unto Gridd Subtract).

The further north you go, the greater the magnetic variation. Bt it will tell you on the map of the area what the variation is, so you just use the same formula and add or subtract the number for that region.

Practice it and it'll come automatically after a while.

All these gizmos for adjusting magnetic variation on your compass housing is just a way for the companies to make money. It's really best to learn it properly, then remember it. There'll come a day when your auto gizmo breaks, or falls into a swamp and you'll have to borrow one, and if it doesn't have the gizmo to automatically set the variation, you'll be wondering "Do I add or subtract?".

Same with GPS users. Brilliant tools, but learn the basics first cause the batteries will fail just when you misplace yourself on the hills.

Anyway, all of the above is academic because in 2012 there's going to be a polar shift and north will become south and south will become north and the zombies will roam the earth. What we need is a hero to save us all. Bushcraft Betty to the rescue!

 

[mikeybear]

I learnt Grid to Mag Add, Mag to Grid Get Rid

 

[dave53]

im totally confused now

 

[Carbuncle]

I learnt Grid to Mag Add, Mag to Grid Get Rid

So did I (along with drawing new grid lines on maps) but . . .

While Eric is right in that it's not rocket science, I find it hard to believe he's used a compass with adjustable declination. The "gizmo" as he calls it is as low-tech as you can get, if it fails, your compass has probably fallen apart. And given that you set it once for your location, unless your walking some serious distances you wont have to touch it once you get going.

As for being expensive, the suunto M2 can be got under 12 quid. Of course, there's nothing you can't do in your head, (or a pin in a cork for that matter) but it's an enabler for "little and often" continuous navigation, and it might come in handy in stressful situations when mistakes could be made.

But what bothers me more is that using the likes of Silva type 4's to teach cadets and scouts can put them off unnecessarily. The only counter intuitive thing about compass navigation is the declination adjustment - take that out of the equation and you get good rewards for little effort. When I think back to the blank faces of us kids when we were first taught it 25 years ago on a Scottish hillside, I doubt half got it then, and less still could do it now. I hate the thought of people growing up relying solely on GPS's for the lack of a simple, low cost, low tech "gizmo".

 

[AJB]

http://www.trunord.com/

I have two, they're lovely!

If you want to see one...

http://www.youtube.com/watch?v=I0lQ0NqTNts&feature=player_embedded

(Skip to 4.04 to avoid a right load of twadle)

 

[Biker]

Thanks for the replies.

At the risk of being burnt at the stake why don't the mysterious they simply make a law that says North is here put a pole in the ground and stick to it, and while they're at it make the circumference of a circle 3 times it's diameter and adopt a single universal currency?

It'd save all sorts of fiddly maths and mnemonics.

I'll go and ge the matches. Purists can bring their flints

Thanks again.


PS Can't help but wonder how a traveller would navigate if he's standing on magnetic North pole. I'm getting dizzy just thinking about it.

 

[Toddy]

Hope it's Summer and he can follow the Sun

cheers,
M

 

[Graham_S]

PS Can't help but wonder how a traveller would navigate if he's standing on magnetic North pole. I'm getting dizzy just thinking about it.

That's easy, no matter which way he goes, he's traveling south

 

[Biker]

That's easy, no matter which way he goes, he's traveling south

Yes but which south? The camp could be in that southly direction or it could be the one just left of South, south south.

You can see why I'm dizzy, and not a blonde hair in sight, not that I have many hairs left .. but that's another story.

Answers like that Graham will be treated with the contempt they deserve ... no matter how funny thay are