Map Reading: Understanding Magnetic Declination

Map Reading: Understanding Magnetic DeclinationSince we have already discussed the art of map reading in a general way, today let’s discuss one very specific and at times daunting portion: Magnetic Declination. There is much to do in the cold months of winter that may otherwise be overlooked and pushed to the back burner due to our bear-like hibernation tendencies. Of the many great things to do during winter, breaking out your old map and compass may be the best.

Those months that Mother Nature strips down to her bare minimum and shows off all herself, practicing the lost art of map reading can be made not only easy but a ton of fun. Your visibility becomes higher due to the nudity of the woods/landscape.

Once the trees have removed their cloaks of leaves and the underbrush lays down, you can see clean across scapes that were once too thick. During this time, you can really compare the lay of the land to the lines and dots and shapes that make up your topographical map.

Since we have already discussed the art of map reading in a general way, today let’s discuss one very specific and at times daunting portion: Magnetic Declination.

Earth’s Magnetism: How and Why

Earth’s magnetic power is exhibited through an invisible force. However, its magic can be demonstrated to our naked human eye by way of a home-rigged science project. Simply sprinkle some iron filings over a piece of paper and hold the sheet of paper above any old bar magnet.

The iron shavings will automatically align themselves in “lines of force”. These lines, flowing from pole to pole, duplicate, in principle, the larger system that envelops the earth.

Think of the earth as the large bar magnet. The needle of a compass aligns itself with the earth’s “lines of force” exactly as the iron shavings over the bar magnet.

A compass needle has a more permanent magnetism along with a pivot used to reduce friction; this allows the needle to maintain a magnetic orientation. The compass card (or circle) then gives the needle something to actually be oriented to.

Deviating Influences

Earth’s magnetic “lines of force” are not exactly as well defined as those about the bar magnet. There are many deviating influences functioning here causing the lines to whirl in peculiar patterns over the earth.

The lines of force are subject to multiple changes in those patterns. However, most of these changes do not really affect compass work. It is still good to be aware of and understand them.

When entering the wilderness, one should never go forth with the notion that just because a map claims a specific declination of, for example, “X” degrees east, it is not always accurate.

The lines of force are subject to fairly predictable changes that occur due to solar activity called magnetic storms or sunspots. They are also subject to less predictable deviations due to local weather conditions, specifically during thunderstorms.

Other constant variations are due to a number of things; one possibly being due to the “moderations” mankind has made in and on the surface of the earth (go figure…).

There is another change in declination. And this one is of some importance; very important, in fact. This change is significant because it is one direction only, unchanged level, and of substantial proportions over only a few years time. This is known as annual westward change.

Related article: The Lost Art of Map Reading

“Slippage Theory”

Imagine the entire magnetic field of the earth, as an intact covering of sorts. This cover changes position with respect to the surface of the earth. There is one theory that is most commonly accepted, and it is a bit of a strange one.

This entire magnetic field is thought to be produced by the flow of liquid of earth’s center. This is to say that the earth’s lines of force and magnetic poles are associated with its center rather than its crust.

As you know, the earth spins eastward on its axis. During this rotation, it is believed that some slight slippage westward of the liquid center takes place.

Think of it like this: imagine rotating a pail of water in a constant circular motion. Physical law states that bodies that are at rest tend to stay at rest. In the case of the pail of water (or the interior liquid and earth’s crust), both the pail and the water will tend to defy the rotational force. The water will be more successful than the pail.

You can do this test yourself to get a decent idea of the theory of “slippage”. Get a bucket filled with water; it is pretty easy to spin the bucket without greatly disturbing the water.

Begin to spin the bucket in one direction. The friction between the water and pail will cause the water’s circular speed to almost equal that of the bucket.

Now, drop a small wood chip or leaf onto the water’s surface. The chip will slip backward over a period of time, giving the example of (sort of) what happens with the earth’s rotation and its liquid center.

By accepting this theory, it is pretty easy to visualize what actually is taking place in declination change. While the earth always rotates eastward, its entire magnetic field slips westward by some amount that is equal to the “slippage” of earth’s liquid center.

Dealing With Annual Westward Change

Now, whether or not the “slippage theory” is or isn’t correct, the beheld annual change is, in fact, a fact. And this fact does affect topographic maps. In order to keep them current without absolutely having to reprint them so regularly, mapping agencies typically will date the declination diagram on each map.

Along with that date, there should be a declination change factor on the map.

Do not get the direction of declination and the direction of its annual change confused.

Declination- refers to the angle between True North and Magnetic North from where you stand with your compass.

Annual Change – is always westward and it involves the entire magnetic field (not only your position…).

Magnetic Declination and Deviation

Now, back to the compass needle. First, let’s explain magnetic deviation:

Deviation – any magnetic effect on the compass needle which isn’t caused by earth’s “normal” magnetic field. Its other names may be local attraction or variation. However, all three names impart the same meaning, an extra or abnormal source.

These abnormal sources differ widely. From natural mineral deposits to some sort of metal that may be on the person using the compass, an extra source (other than earth’s natural magnetic field)  will the make the compass needle to suggest a reading that is over or under that of the alleged declination for that area.

For practical navigational intents, there is really not much difference in declination and deviation. In the most literal sense, declination is exactly how far the compass needle ebbs away from True North.

It is here when it becomes important to know how much of this decline is due regular sources and how much is due to extra or abnormal sources. Too often, the deviation is unanticipated.

Favorably, all magnetic forces work according to a mathematical decree which explains how these abnormalities have a lessening effect as they gain distance from the compass.

This decree is known as the law of inverse squares (it declares “that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity.” In other words, as the distance becomes larger, the force becomes smaller.

Looking at the Stars for Guidance

The stars, particularly Polaris (in the Northern Hemisphere), afford us a method of verifying the total of declination and deviation.

Polaris is better known as the “North Star”. It is located so perfectly over the earth’s the North Pole that it barely changes position as the earth spins underneath the night sky. Every other star crosses the heavens in curved paths from east to west (just like the sun and the moon).

Two Methods for Locating Polaris:

  1. First, find the Big Dipper. Two stars make up the pan of the front wall of the dipper (the side away from the “handle”). They point from dipper bottom to top. If you consider the distance between these two stars one part or unit, then Polaris is around six of these units away from them. It is also very nearly in line with them. The North Star will be the brightest star in its direct proximity.
  2. As a second way to find the pole star, try to find the Little Dipper. Polaris is the last star that makes up the handle of the smaller dipper. Making the Little Dipper slightly harder to find (therefore placing it in second place for locating Polaris) is the fact that the pan often looks more like a V-shaped due to one of the stars being quite faint.

Using Polaris to Check Deviation:

After adjusting for the local declination, you can sight and read the bearing of Polaris in the same manner. The adjustments will vary given the different types of compasses available.

While checking, if the bearing is 0 degrees, then no deviation exists. It the bearing is other than 0 degrees, it equals the amount of deviation only. Now you simply adjust the outer dial of the compass until a bearing of 0 degrees is obtained.

Magnetic deviation often has an erratic enough nature to change quickly over very short distances. Unstable compass readings are one hint at its existence. Obviously, under certain conditions, it would be pretty impractical to wait for nightfall and clear skies to prove each and every reading.

 However, it is quite reassuring, as well as interesting, to make at least one check on deviation while you are in an area you don’t know very well using the pole star.

You should try to practice checking both forward and back bearings at each and every sighting station as this will help to reveal local deviation. If deviation exists at one station but not at another, the forward and back bearings between the two places will not coincide; in other words, they will not differ by 180 degrees.

D.I.Y. Tool for More Accuracy

More accuracy can be achieved with a simple to make “sighting table”. This is done by gluing an angle-shaped piece of cardboard to a flat board. The cardboard will need to be cut on the angle that approximately equals the latitude of the observer.

Latitude across the earth’s surface is represented by a number of degrees from the equator. The degree distance of Polaris above the true horizon actually equals the latitude of the place from which the sighting is being made. From the equator, for example, latitude zero, Polaris makes an angle of zero degrees with the true horizon. If you live on or simply near, say the 54th parallel, cut an angle of 54 degrees, etc.

This set up allows the compass to rest level on the board, parallel to the sight (angled cardboard), while they are all aimed straight at Polaris. It is key now to remember that other metal around the compass can cause quite the stir.

Polaris has an apparent rotation of about 2 degrees in diameter. This means that twice each night the North Star lines up precisely with true north on any given observers line of sight. One can use the constellations of the Big and Little Dippers for reference to finding approximately when they will line up with true north.

However, given the degree of this declination, and the fact that you are to be traversing with a hand compass, this accuracy is much more theoretical than practical.

Suggested reading: 5 Tips for Successful Natural Navigation

Fun Fact About Polaris

Polaris has actually not always our “pole star”, nor will it be in the distant future. Just over two thousand years ago there was actually no star in close association with True North.

Historians have, at times, seen and expressed an important parallel between the coming about of long-distance navigation and the presence of a dependable marker representing True North.

All things considered, Polaris has been a prominent landmark in the sky for centuries.


All in all, magnetic declination, and map reading in general can seem like a mighty daunting task. But such is not the case. All you require is a decent compass, a decent head on your shoulders, and the high hopes for clear skies at night!

One major suggestion is to get out there while Mother Nature has shed her green glory, and your line of sight is more wide open.

Sure, we, in these modern times, have much more sophisticated navigational tools, the pole star’s role has not changed. Once you have learned to find it and can understand its basic principle (it is an almost permanent compass in the sky), you will be much more prepared to navigate your way “home” at any given time!

This article has been written by Jonathan Blaylock for Prepper’s Will.

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