Wilderness Navigation: Magnetic Declination

[Wood Trekker]

Magnetic declination; the thing which we never want to talk about because it gives us headaches! It is unfortunately, a very important part of wilderness navigation, as it can considerably alter our course of travel over a significant distance. I will try to offer a very simple way to look at it.

What is magnetic declination:

Magnetic declination is the difference between the place where the needle on your compass points (magnetic North), and the actual or true North direction in a particular area. That?s right, the compass does not always point directly North. Depending on the area of the globe where you are located, the difference can be quite significant. Failing to compensate for that difference can have important implications to navigation.

How do I find out the magnetic declination in my area:

Fortunately, the nice people at the National Geophysical Data Center of the National Oceanic and Atmospheric Administrations (NOAA) have been kind enough to create a magnetic declination calculator for us. You can go to the website here: http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp, punch in either your longitude and latitude, or just your zip code, and you will get the magnetic declination in your area.

What do I do with this number:

Here is the part that throws most people off, just because of the terminology used. Before we start, remember this:

Positive declination is the same as Easterly declination. (10 degrees is the same as 10 E)
Negative declination is the same as Westerly declination. (-10 degrees is the same as 10 W)



Positive or Easterly declination means that the needle of the compass will point to the East (or clockwise) of true North by however many degrees you got from the above calculation. The first diagram shows 10 E or 10 degrees on a compass. Wherever the needle of the compass points, it is 10 degrees East (or clockwise) of true North.

Negative or Westerly declination means that the needle of the compass will point West (or counterclockwise) of true North by however many degrees you got from the above calculation. The second diagram shows 10 W or -10 degrees on a compass. Wherever the needle of the compass points, it is 10 degrees West (or counterclockwise) of true North.



Now that you know this, you can put a mark on your compass, so you know where true North is depending on where the needle of the compass points. Some compasses have a wheel that you can turn, so all the markings are readjusted.

Why is this important:

It may seem like an insignificant technicality, which is not worth the effort, but it can make a huge difference to your navigation. Remember, when you are taking a bearing from a map, you are using a very small surface to calculate a vector along which you will then travel for a long distance. Small deviations can have a big impact. Let's say your bearing is 110 degrees. In NY the magnetic declination is 13W. This would make my bearing 123 degrees, a significant difference. Over a 10 mile trip, failing to adjust properly, can take you quite off course.

[Wilderbeast]

Magnetic declination; the thing which we never want to talk about because it gives us headaches! It is unfortunately, a very important part of wilderness navigation, as it can considerably alter our course of travel over a significant distance. I will try to offer a very simple way to look at it.

I underlined because this is something that should not be a factor in land navigation.  Over significant distances one should be taking numerous ranges and using the features of the landscape to continually correct course.   

This is not to say that you should not be accounting for declination, you certainly should, especially in areas with significant declination.
Only that you should be taking frequent ranges and double checking to prevent an accumulation of error.

I hope that doesn't sound like I'm being picky.  It just seemed worth noting.

[greyhound352]

Great post Woodtreckker, I just wanted to add one thing. One thing I look for in a compass is a way to adjust for magnetic declination built into the compass. This is a great feature because you can set it and forget as long as you stay in the same area.

[Gurthy]

Great post Ross 

[Half Axe]

A link to some more information on this topic:

http://education.usgs.gov/lessons/compass.html

Hope it's useful.  If you are on the Agonic Line, lucky you! 

[PetrifiedWood]

Another great informative post, thanks!

Also worth noting:

USGS Topographic maps will usually have the local magnetic declination printed on them at the bottom. They also will give you the rate of change per year + or -. You can look at the date on the map, then extrapolate the current local declination by adding or subtracting the rate X the number of years since the map was published, up to a certain point. I wouldn't rely on this method for maps over ten years old tops.

[Wood Trekker]

Magnetic declination; the thing which we never want to talk about because it gives us headaches! It is unfortunately, a very important part of wilderness navigation, as it can considerably alter our course of travel over a significant distance. I will try to offer a very simple way to look at it.

I underlined because this is something that should not be a factor in land navigation.  Over significant distances one should be taking numerous ranges and using the features of the landscape to continually correct course.   

This is not to say that you should not be accounting for declination, you certainly should, especially in areas with significant declination.
Only that you should be taking frequent ranges and double checking to prevent an accumulation of error.

I hope that doesn't sound like I'm being picky.  It just seemed worth noting.

It's not picky. It's a good point. If you have clear reference points in the terrain, you should certainly take a new bearing and adjust your prior one if necessary. In my experience however, that is not always possible. I've had cases where I have not seen any distinctive feature for miles, and then I've had other cases where I have mistakenly identified a feature in the landscape and then incorrectly navigated from there.

[Bibbs]

Now what about using a sun compas? That gives ypu a good general direction, though it is a little bulkier than a standard compas

[MnSportsman]

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[Wood Trekker]

Now what about using a sun compas? That gives ypu a good general direction, though it is a little bulkier than a standard compas

Since it doesn't use a magnetic field, I don't imagine it will be effected by magnetic declination. There are actually fairly small compasses that use the sun for direction. From what I know, they usually require a good amount of calculation in order to get a fairly accurate number. I don't know much about it through. 

[Bibbs]

Now what about using a sun compas? That gives ypu a good general direction, though it is a little bulkier than a standard compas

Since it doesn't use a magnetic field, I don't imagine it will be effected by magnetic declination. There are actually fairly small compasses that use the sun for direction. From what I know, they usually require a good amount of calculation in order to get a fairly accurate number. I don't know much about it through.

I wasnt meaning that as a question of would it be effected, but rather a question of, why not use that as a verification of how far off magnetic north and true north really are?

[Wood Trekker]

I'm not sure it is accurate enough to verify the results. For example, there I am, magnetic declination is 13W. I'm sure I would be off by at least that much if I tried to use the sun.

[Exploriment]

For those of us up in Canada, a magnetic declination calculator
http://geomag.nrcan.gc.ca/calc/mdcal-eng.php

[Wood Trekker]

Cool. Thanks.