Magnetic Declination

Compass Declination

Man, you thought latitude and longitude was rough, just check out this declination stuff!

I'm sorry to be the one to tell you that your compass doesn't work quite right. It doesn't really point to the north pole or the south pole.

Not only that, next year it will point someplace different than it does this year! And, someday it may be completely backwards!

Two North Poles


There are actually two north poles - the Geographic north pole which is the axis around which the earth spins, and the Magnetic north pole which is where compass needles point.

Why are there two different poles? Good question!
The magnetic north and south poles are the ends of the magnetic field around the earth. The magnetic field is created by magnetic elements in the earth's fluid outer core and this molten rock does not align perfectly with the axis around which the earth spins.

There are actually many different sources of magnetic activity around and in the world. All those influencing factors combine to create the north and south attractions at each spot on the globe. The actual strength and direction of 'north' is slightly different everywhere, but it is generally towards the 'top' of the planet.

Magnetic Declination


The difference between the north geographic pole and the north magnetic pole is called magnetic declination or usually just declination.

Depending on where you are on the earth, the angle of declination will be different - from some locations, the geographic and magnetic poles are aligned so declination is minimal, but from other spots, the angle between the two poles is pretty big.

Here is a map of the angles of declination for the U.S. as of 2010:

U.S. magnetic declination map
(click for large image)

Notice the black line with on it running down the Mississippi river? Along this particular line, both the geographic and magnetic north poles are in alignment so there is no declination. If you move East of this line, the magnetic north pole will pull your compass needle further and further to the West of geographic north - the angle of compass declination is West Declination. Moving west of the Mississippi river will pull your compass needle further and further to the East.

compass declination in U.S.
This makes it a bit more challenging to find your way because your compass tells you North is a different direction, depending on where you are. Topographic maps used for navigating should have the declination printed on them like this example. This declination is for an area in the Bighorn Mountains in Wyoming where the declination is 12 degrees East in 2001, when the map was made.
(Click the image for a bigger version)
 
It also says that the declination is changing 6.9 minutes West each year. Using this old map, the declination would be about 11 degrees East in 2010 - about 1 degree less than the map has printed.
Unfortunately, the rate of declination change changes over time. When the map was printed, the rate of change was 6.9 minutes West each year. But, it has now sped up to 9 minutes West each year. This has been changing every year since the map was printed so the current declination for this area is 10 degrees 41 minutes East. That is already a larger change than what we would expect for 2010, based on the map's information.

Does it make good sense to use a map's declination information if that map is more than a couple years old? NO, you should use recently published maps or know the current declination for the area and use that information for your navigation.

There are a few ways to solve this extra challenge of finding true north - either changing your compass or using math in your head.

Adjust Your Compass


declination adjustment On many compasses, you are able to adjust the declination by twisting a ring, using a screw, or some other method of changing where the orienting arrow sits in relation to the ring.

If you used a compass set with 0 degrees declination in Wyoming where the declination is 12 degrees East, the compass would tell you that you're heading North when you're actually heading 12 degrees East of North.
You'd quickly wind up off course and lost.
 
 
 

magnetic declination By adjusting the compass to match the declination on our map, the orienting arrow now appears to be offcenter from North, which is how it should be.
Now, when you put RED in the SHED (needle inside orienting arrow), the North indicated at the index pointer is true north and matches your map.
You can continue to check your location and chart your course correctly.
Whenever you stop and check your heading or take a bearing on a distant object, the degrees read on the dial will be the actual true degrees. The only thing that looks a bit odd is that the north end of the compass needle does not point directly at the N when you are heading due North.

Using Your Head


compass declination calculation If you have a compass with no declination adjustment or you just like math, then you can do the declination calculations in your head.
If you are in Wyoming with a 12 degree East declination just subtract 12 degrees from the heading you read on your compass dial.
 
In this example, the reading is 250 degrees.
But, you are actually facing 262 degrees.
So, to actually head 250 degrees, you need to get a reading of 238 degrees and then you're heading the right way.
 

Inclination


declination magnetic compass Inclination is the angle of pull down toward the earth that the magnetic field exerts on a compass needle. At the north magnetic pole, the north end of the needle is pulled straight down toward earth.
Using your brilliant mind, you probably figured that a compass doesn't do much good if it is pointing straight down. And, actually, for hundreds of miles around the magnetic poles, compasses are worthless. For even more hundreds of miles around that, compasses can be erratic.
So, if you are planning an arctic or antarctic expedition, don't count on your Silva or Brunton for getting you there and back.

Not only that, but there are southern hemisphere and northern hemisphere compasses.
I have an old Forest Service compass that you can see here. It has a weight on the southern end to offset the inclination pull in the northern hemisphere. If this compass is taken to the southern hemisphere, the weight would cause it to drag and not balance properly.
You might also notice the two declination adjustment screws and the fact that it is currently set for about 20 degrees East declination. And, you should note that this is not a good compass for mountaineering for many reasons.
 
 
 

The Drifting Poles


north declination Back to the magnetic north and south poles - remember I said they were caused by the earth's molten outer core? Well, things are always shifting and churning down there. So, as the liquid rock moves, so does the magnetic field.

This map shows how the north magnetic pole has shifted over the past century. Notice that it has picked up speed and has moved more in the last 15 years than in the 50 years before that, now moving about 25 miles per year on its way out of Canada and heading to Russia.

So, how does this effect compass use and navigation? Well, if you follow your compass north this year, you will be drawn towards a different place than 15 years ago. Declinations are constantly changing and that means what is written on a map you buy this year will be incorrect next year. The difference depends on where the north pole moves to and what your current declination is.

magnetic declination change For example, if you pointed north from Fairbanks, Alaska in 2006, you would follow Line #1.
100 years ago, you would have followed Line #2 - huge difference!
From other places, such as Winnipeg, Manitoba, Line #3 in 2006 and Line #4 100 years ago are nearly the same.

It's interesting to compute the declination for your location.
This will pop up a new window.
In the new window, enter your Zip Code and click the "Get Location" button. Then, click the [Calculate] button to find the declination for today, or enter any date and find out how the declination will change over time.

These moving magnetic poles continually effect navigation. It's more serious for airplanes and ships, but even for hikers and backpackers, it causes problems. Using a 10 or 15 year old map is a bad idea since the declinations are no longer accurate.
If the north pole movement continues at its present speed, it moves 250 miles every 10 years, but it has been speeding up so no one knows what the next 10 or 20 years hold in store. Historically, the magnetic field of the earth has churned over and reversed and some scientists say we are well overdue for such a reversal.

See big PDF map of US declinations
the whole world - notice the south magnetic pole

Next: Read a Heading

declination of compasses Why didnít True North marry Magnetic North?
 
She didnít like his bearing and he was just a drifter.


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