Nav Work 101 - Cardinal Directions and Declination

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Koda
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Koda » January 26th, 2013, 12:53 pm

Lurch wrote:
10°M = 26°T

Declination East, Magnetic Least
I see Potatoes point and agree, this is difficult to remember and understand. If my declination is 16* east, then I add 16* to the needle: So if I want to face true (map) north, then 0*T = 16*M (0 + 16)... I rotate until the needle is at zero, then rotate again (adding) until it points at the current declination value (16*). But likewise if my BEARING is true north (0*T) then I must rotate the compass dial subtracting the declination value. Not understanding these two methods leads to confusion. Are you adjusting the needle, or the compass dial?

another way I see it is to know where you are in relation to the agonic line. Then its easy to visualize the direction the needle needs to point when adding your declination to your bearing.
retired jerry wrote:
Doesn't matter what the declination is

The main reason for compass is to keep going in a straight line when you can't see the surroundings.
I disagree, the main reason for a compass is to find your bearing. Using a compass to walk in a straight line is actually finding your bearing many times over in greater frequency along your route to your destination/bearing. Or maybe I am saying the same thing...?
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raven
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by raven » January 26th, 2013, 1:12 pm

Question of how to correct for declination are reminders of the occasional importance of getting it right -- like when you are trying to locate a point in the woods or to determine, now that a line being targeted has been reached, which way to turn.

The possibility that one might correct for declination in the wrong direction at a critical time has always been on my mind when choosing a compass. I never have made the error, but night travel in an emergency with an injured member of the party dependent on good judgment... Its an old story: 1 problem at a time, simple; two, manageable; three, overwhelming. So any compass I depend on has settable declination -- set at the beginning of the trip or before. Also a mirror, but that's another conversation.

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Re: Nav Work 101 - Cardinal Directions and Declination

Post by vibramhead » January 26th, 2013, 1:28 pm

Great intro! But what do we do about the fact that the next magnetic pole reversal is apparently underway? "After some 400 years of relative stability, Earth's North Magnetic Pole has moved nearly 1,100 kilometers out into the Arctic Ocean during the last century and at its present rate could move from northern Canada to Siberia within the next half-century."

Maybe we can just take some Wite-Out to the N and S on our compasses, and write in the new magnetic world order.
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Koda
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Koda » January 26th, 2013, 1:40 pm

raven wrote:So any compass I depend on has settable declination -- set at the beginning of the trip or before.
agree with Raven here. I also will not buy a compass that does not have a built in declination adjustment.

Using a compass with a built in adjustment for declination is so much easier and eliminates any possibility for a simple math error in the field, especially if under distress. Its also very difficult to read the exact bearing mark on any compass adding to the problem.
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Lurch » January 26th, 2013, 1:40 pm

Koda wrote:
Lurch wrote:
10°M = 26°T

Declination East, Magnetic Least
I see Potatoes point and agree, this is difficult to remember and understand. If my declination is 16* east, then I add 16* to the needle: So if I want to face true (map) north, then 0*T = 16*M (0 + 16)... I rotate until the needle is at zero, then rotate again (adding) until it points at the current declination value (16*). But likewise if my BEARING is true north (0*T) then I must rotate the compass dial subtracting the declination value. Not understanding these two methods leads to confusion. Are you adjusting the needle, or the compass dial?

another way I see it is to know where you are in relation to the agonic line. Then its easy to visualize the direction the needle needs to point when adding your declination to your bearing.
I understand the confusion, that's why I said not to think about it too much! When I get into compasses I'll talk about adjustable declination where this doesn't really become an issue, but for the most part you need to remember two things. First off, the rhyme Declination East, Magnetic Least and then you need to know whether whatever number you're looking at is in Magnetic or True. If you're doing mapwork and pulling a bearing from the map, then it's going to be in true. If you have an undeclinated compass and you're taking a bearing in the real world it will be in magnetic. Once you know which you're working with you can do the math, and just remember that Magnetic should always be LESS than true.

It's a hard thing to articulate through type unfortunately :(
Raven wrote:The possibility that one might correct for declination in the wrong direction at a critical time has always been on my mind when choosing a compass. I never have made the error, but night travel in an emergency with an injured member of the party dependent on good judgment... Its an old story: 1 problem at a time, simple; two, manageable; three, overwhelming. So any compass I depend on has settable declination -- set at the beginning of the trip or before. Also a mirror, but that's another conversation.
I'll definitely be talking about different types of compass in my next thread. I agree with you though, adjustable declination and a mirror are key if you want to be fully functional out there. I'm trying to keep this stuff a bit more general than I usually do because I know there are folks on here that travel out of state and do stuff all over, so it's difficult to give hard rules of every scenario.
Last edited by Lurch on January 26th, 2013, 1:58 pm, edited 2 times in total.

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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Lurch » January 26th, 2013, 1:55 pm

vibramhead wrote:Great intro! But what do we do about the fact that the next magnetic pole reversal is apparently underway? "After some 400 years of relative stability, Earth's North Magnetic Pole has moved nearly 1,100 kilometers out into the Arctic Ocean during the last century and at its present rate could move from northern Canada to Siberia within the next half-century."

Maybe we can just take some Wite-Out to the N and S on our compasses, and write in the new magnetic world order.
Hah!

Well first off, it's not really that much more "unstable" than it has been for years..

Image

Secondly, there *are* ways that you can calculate declination if you're sure of your abilities and have known references. For instance you could take a bearing between two known points on the map, and the same in the real world, and figure out the difference for yourself. The only real problem is going to be if Magnetic North is moving rapidly, or in some zones where it's attempting to pull your needle *down*. Suunto has compasses with "global needles" that can function at up to 20° tilt. With the average compass the needle will start to rub on the case if you're not holding it level, or if the magnetic field is tipping the needle, putting more friction on it and compromising the accuracy of your bearing.

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Koda
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Koda » January 26th, 2013, 2:36 pm

Lurch wrote:If you're doing mapwork and pulling a bearing from the map, then it's going to be in true. If you have an undeclinated compass and you're taking a bearing in the real world it will be in magnetic. Once you know which you're working with you can do the math, and just remember that Magnetic should always be LESS than true.

It's a hard thing to articulate through type unfortunately :(
So if I'm taking a bearing in the field, I'm adding. If I'm taking a bearing off a map, I'm subtracting. The reason this is confusing to me is because if I am taking a bearing in the field I dont need to worry about declination...
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Joseph Elfelt » January 27th, 2013, 5:03 am

NOAA, in cooperation with similar agencies from other countries, provides two different ways for software developers to obtain the current declination for a location. Gmap4 uses one of those two ways. These two ways produce slightly different results. That is the reason why you might see small differences if you use different programs to tell you the declination.

Since these differences are small, any program that tells you the current declination will be better than using the declination printed on old topo maps.

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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Lurch » January 27th, 2013, 1:06 pm

Koda wrote:
Lurch wrote:If you're doing mapwork and pulling a bearing from the map, then it's going to be in true. If you have an undeclinated compass and you're taking a bearing in the real world it will be in magnetic. Once you know which you're working with you can do the math, and just remember that Magnetic should always be LESS than true.

It's a hard thing to articulate through type unfortunately :(
So if I'm taking a bearing in the field, I'm adding. If I'm taking a bearing off a map, I'm subtracting. The reason this is confusing to me is because if I am taking a bearing in the field I dont need to worry about declination...
That's partially correct... If you are *only* taking bearings in the field, and using the compass to simply walk in a straight line, than magnetic or true not doesn't really matter. It's when you're converting between the two that the issue becomes apparent.

Since your map is going to be oriented to true, it doesn't care about declination, or that Magnetic North is moving, it's reference will remain accurate enough for a few thousand years. You should be doing all your work in true as well. When it comes to triangulation (intersection/resection for you military types), or any time you want to put a bearing on your map or take it off, you're dealing with the interaction between the two. If you don't have a compass that declinates, you need to do the math. If you're going from compass (magnetic) to map (true) you would need to add, "magnetic least". If you're going from map (true) to compass (magnetic) you would be subtracting.

More often than not, unless you specifically tell it not to a GPS's "goto" bearing is going to be in true. It's easier all around, since it doesn't have to worry about the cluster..fun.. of magnetic field lines to figure out what declination is for where you're standing right this second.

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Martell
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Re: Nav Work 101 - Cardinal Directions and Declination

Post by Martell » August 13th, 2013, 11:37 am

I stickied this post. Maybe you could post a link to Nav Work 102 and Nav Work 103 in the original post?
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