Magnetic declination

[AJB]

EDIT-"I’ve just realised I’ve made a terminology gaff! You can all line up and beat me with your intellect. I used declination instead of deviation; shoot me for the son of a dog! Sorry!"

Hi,
I’ve started this as a new thread as I thought it might be useful, but it follows on from my Tru-Nord thread. I received two Tru-Nord compasses this weekend, which point in different directions!

Whist trying to check what the local magnetic DEVIATION should be I found this link, which seems really useful.

http://www.threelittlemaids.co.uk/magdec/index1.html


Thanks for reading

Andy

 

[tomtom]

I received two Tru-Nord compasses this weekend, which point in different directions!

is that what you ordered

 

[Lithril]

good link, thanks for that.

 

[AJB]

Hi tomtom,

No, I ordered two compasses set of the UK. Stupidly, I didn’t ask before hand what “set for the UK” was, but what ever it is (I guess it should be about -4deg as a rough approximation), I assume it should be the same for both, sorry for the sarcasm, but I’m a bit irritated!

Anyone else had problems or even checked theirs?

 

[Hoodoo]

Andy, I've never had this problem but I would not be a happy camper if my compasses showed up that way. I would contact Tru Nord as soon as possible.

 

[AJB]

Perhaps you are supposed to buy 360 of them, one would be right!

Joking aside, I assume it’s just a cock-up. But it’s worth checking your own to be on the safe side.

Also I would be interested to know, from the boffins out there, if the information in the link that this thread is about is accurate and reliable.

It seems to be difficult information to get hold of.

 

[Goose]

I am pretty certain that Magnetic Variation is always changing, by small amounts but noticible from year to year, so setting variation into a compass wouldnt really work.
Grid North is also different from true north and can vary over the map by a measurable amount, the GMA(Grid Magnetic Angle) is usually explained in the legend of OS maps.
I think the compasses that are for specific areas vary in the balance of the needle, to account for the way the magnetic field pulls the needle out of balance depending on how far from the pole you are, and shouldnt really point in a different direction.

At the risk of teaching you to suck eggs, how close are the compasses when you compare them?

http://www.silva.se/outdoor/products/comp_general.htm

 

[AJB]

Hi Goose,

From the Tru-Nord web site:-
“The Tru-Nord Compass is compensated for different regions of the country in order to provide a corrected "Grid North" for map navigation. Should you want it changed to a different map region just mail your Tru-Nord Compass to the address below. Be sure to specify the map region or location where your Tru-Nord Compass will be used and please enclose $3.00 for postage and handling”

The compasses are designed to read Grid North. Yes the magnetic north moves around and on occasions (on a geological time scale, if I can remember my o level geology) swaps polarity!

But the whole point of these is that you don’t need to take into account magnetic variation in the area they are set for, with in reason, as all compasses can be affected by local variations. After time they will become wrong reading, if the declenation changes, then you get them adjusted.

 

[raskusdrotti]

Hi,

All OS maps have the variations for that map printed on the legend - the variation is different everywhere - and is always changing.

 

[Goose]

I can't see how it would be possible to compensate for grid north, that can vary from map to map, the zones, 5 I think, are to do with the compass getting pulled down rather than compensating for grid to mag.
http://www.thecompassstore.com/whatisglobne.html
This explains the zones better than I can.

 

[bp1974]

I'm having some trouble understanding what that link's about. I think that that link is about working out magnetic deviation, not declination. The difference between mag and grid is the deviation - I think it's about 4.5 degrees in the UK at the moment. Declination is the angle the magnetic field makes with the ground in any location, for which a compass must be adjusted in order that the needle doesn't follow the declination and sit on a downwards/upwards slope inside the compass housing. Seems to be an error in terminology on the website.

 

[AJB]

Hi all,

Yes, I must admit the responsibility for incorrect terminology; I was using declination instead of deviation. And hence I started to doubt myself, but have just had clarification from Tru-Nord. Their compasses (are for quick reference only, US disclaimer!) are built to read grid north not magnetic north in the area that they are set up for. Admittedly, due to their size they are not terribly accurate especially here where the variation is not substantial so their adjustment has limited effect, but I can imagine in other areas where it is much more pronounced they could be a useful tool. Anyway, that’s not the point, I wanted one as they are cute (lets be honest).

But just to clarify, they are adjusted for Deviation from Grid North not declination (inclination) of the magnetic field and are very, very pretty.

http://www.trunord.com/

 

[bp1974]

Hi Andy,
Thanks for the clarification. I note that the website you link to has the terminology wrong in their title - that's what confused me the most.

For interest, magnetic deviation in the UK is currently decreasing at a rate of about 1 degree every decade.

As to your original problem, did you get that sorted? Putting the compasses near each other to compare them would screw up the needles. Have them at least a metre apart and then see if they're still pointing in different directions.

I once trailed a group I was training as they wandered around Epping Forest at night in circles. After two hours of this I intercepted them and asked who was carrying the compass. They all pointed at the guy who was also carrying the six foot long iron scaffolding pole...

 

[Goose]

I was teaching scouts to use a compass, I told them to make sure their rifles were not near the compass when taking a bearing, took me a couple of minutes before I realised why they were looking at me funny!

I dont usually bother taking into account magnetic variation when walking as even if the compass can deal with the accuracy 90% of people will be out by more than a few degrees after even a couple of paces, although it is best to be aware of it at times when real accuracy is required.
Looking at the type of compass I would be surprised if you could tell if it was accurate enough to notice variation anyway. Although it does look good!

 

[Schwert]

It just all depends...where I live magnetic north is 21 degrees east of True North, so the small compasses adjusted for this can be especially useful. With only a 4 degree compensation required for the UK and given the size of these compasses I think compensation would have less importance.

However, both compasses when reasonably apart for each other should all point to the same direction.

BTW I learned that:

Declination (aka Variation) was the local error for a compass because it points to magnetic north

Deviation was local error due to very close magnetic fields (boat hull for instance)

Inclination was the compass needle tendency to point downward requiring a weight on the needle

 

[AJB]

Thank you all for your help and interest.

To answer questions:-

Yes the compasses were tested well apart and yes they are wrong.

I’ve contacted Tru-Nord, who have apologised, and it seems they indented to send them both as magnetic north compasses. This seems to go against what their compass is for, however, they feel the one which is completely wrong has had the wrong disc placed in it as its error is one of their standard adjustments. I am to send them back and they will fix them.

However, this thread seems to have highlighted a confusion in terminology. When I started it I knew what I meant and I thought I knew the right words. I thought declination was the variance between magnetic north and/or true/grid north. And, I thought, inclination was the angle at which the magnetic field subtends the earths surface. I have been told these are wrong and now right. Can anyone give me definitive definitions as I doubt myself and there are enough people in my life who do that for me.

 

[Ogri the trog]

I'm not sure I could navigate without calculating the magnetic variation now.
I learnt the skill in Scouts many years ago and it has served me faultlessly since then.
By the way, if you want to get good at following a bearing, practice as we did:-
Find yourself a large flat area with no specific ground markers.
Take a large denomination coin (£2) and tread it edge on into the turf.
Then start by walking on a bearing for a counted number of paces, reverse your bearing and recount the paces - coin should be between your feet.
When you can do that go to the next stage - walk an equillateral triangle (ie 60 Deg) then a square (90 deg).
You tend to get a lot better at navigation when you realise that you're leaving a lot of money on the ground!

All the best

Ogri the trog

 

[Stuart]

However, this thread seems to have highlighted a confusion in terminology. When I started it I knew what I meant and I thought I knew the right words. I thought declination was the variance between magnetic north and/or true/grid north. And, I thought, inclination was the angle at which the magnetic field subtends the earths surface. I have been told these are wrong and now right. Can anyone give me definitive definitions as I doubt myself and there are enough people in my life who do that for me.

probably best to start a new thread to ask that

 

[AJB]

I don’t know Stuart; I feel I’ve beaten this one to death and got nowhere!

 

[bp1974]

Yes, confusion. I've always understood deviation to be the local variation, but then I'm not a sailor, and it seems to mean something different in nautical terms.

So long as we all know that there are three different magnetic phenomena that need to be taken into consideration when navigating and what they are, then there's no problem.