How to Adjust for Magnetic Declination

What is Magnetic Declination?

Technically speaking, the earth has two Norths: the magnetic north and true north.

The true north, also known as the Geographic North, is the northern end of the axis in which the earth rotates. It is also the basis of your map’s orientation and the point where all the latitude lines meet. On the other hand, Magnetic North is the point where your magnetic compass needle is attracted to. 

Think of the Earth as one giant magnet. Beneath the earth’s crust are a bajillion tons of molten materials that make up the outer core – many of these materials are dense metals whose movements influence the earth’s magnetic field. Since the outer core is liquid, it moves below the surface and changes the earth’s magnetic field and the magnetic pole along with them. 

This means that the Magnetic North Pole is continuously changing year by year. It is currently located somewhere near Ellesmere Island in Canada and is now drifting towards Siberia in Russia.

It also means that the True North and Magnetic North don’t line up. The difference in angle between these two points is called the magnetic declination, or what sailors and pilots call magnetic variation.

How to determine Magnetic Declination?

The declination between the magnetic and true north poles varies widely depending on where on the earth’s surface you’re currently located.

Another thing to remember is the earth’s magnetic field is also shifting gradually over time. Meaning, the magnetic north that your compass indicates this year might be different in the next few years. This change is referred to as secular variation.

Warning!: Because of secular variation, old maps with outdated magnetic variations are practically unusable without updated declination values. Keep in mind that the annual change corrections included will not be reliable enough since the secular variation also changes unpredictably over the years. If you are unsure, you can always check local declinations online using a trusted source such as this one.

How do I calculate magnetic declination for my area?

To calculate the magnetic variation for the area you are in, you must first grab an isogonic chart. An Isogonic Chart is a graphical representation of the isogonic lines, which depict constant values of the earth’s magnetic declination.

When reading the chart, you should notice that one of the lines is labeled with a zero. This is called the agonic line or the point where the magnetic north and true north is perfectly aligned that there is no declination to speak about.

The agonic line is important as it tells you whether to add or subtract the declination value depending whether you are east or west of this line. 

If you are standing on the east side of this line, you have to adjust for a west declination (the needle points to a westerly direction). If you are on the west side, you have to adjust for an east declination (the needle points to an easterly direction).

To manually adjust for declination, remember that you have to ADD for West Declination and SUBTRACT for East Declination.

ProTip: You might hear more experienced navigators refer to the magnetic variation as either a Positive or a Negative. This follows the same concept as the one we’ve discussed. The magnetic declination is “negative” when the compass needle is leaning towards the west to indicate the magnetic north while it is “positive” when the needle is leaning towards the east. Just remember that when it is negative, you need to add, and when it is positive, you need to subtract to find the actual true bearing.

How Do You Find Magnetic Declination on a Map?

Commonly, topographic maps place the declination under the legend. Sometimes, a compass rose can be found in the bottom left-hand corner of the map. However, these may vary depending on the map’s publisher.

The compass rose usually consists of three lines: the centerline, the grid north line, and the magnetic north line.

The centerline is usually labeled with a star symbol. This line points to the direction of the true north.

The grid north line is used as a reference to the north according to the map’s projection.

Finally, the Magnetic north line represents the point where the compass needle aligns itself.

Between these arrow points, the difference in magnetic variation will be given as a value in degrees.

Occasionally, some maps will show the magnetic declination as a sentence somewhere in the map legend.

Lastly, as we mentioned before, many online resources can help identify a specific location’s current magnetic declination if your map doesn’t show the magnetic declination or is years out of date.

Adjusting for magnetic declination to get a True Bearing

Adjusting for magnetic declination varies depending on the type of compass you are using.

Adjustable Compass

An adjustable compass means that you can rotate the orienting arrow without moving the compass dial itself.

There are plenty of types of compasses that can be adjusted for magnetic declination: there are the tool-free compasses and those which use an adjustment key. Either way, the principle is the same, and the only thing that separates them is that one can be adjusted by hand, and one uses a tool to turn the rotating bezel.

Here are the steps to adjust for magnetic declination:

1. Identify if your compass is a tool-free compass or one that uses an adjustment key
2. Double-check, the declination value indicated on your map or use a trusted online source to get the declination value by filling in the coordinates of your destination.
3. Either by hand or by using the attached screw tool, rotate the screw until the required amount has offset the orienting arrow/lines from the 0 degrees indicated on the bezel.

With this method, as the compass has been adjusted before you take a bearing, the bearing you’ve taken can be said to be a true bearing i.e. a bearing relative to True North.

Non-Adjustable Compass

On the other hand, some compasses cannot adjust according to the declination. These are called non-adjustable compasses. While the bezel cannot be turned, it still contains a declination scale inside the bezel.

Here’s how you can adjust for declination on a non-adjustable compass:

1. Procure the declination value as you would when following the steps above.
2. Compensate for the declination by adding or subtracting the declination every time you take another magnetic bearing. Note: an east declination=Positive, west declination=Negative
3. This MUST be done every time; otherwise, you risk ending up potentially miles off your route.

With a non-adjustable compass, you must remember that you are taking a magnetic bearing i.e. a bearing relative to the magnetic north pole. Only by correcting the magnetic bearing by adding/subtracting the declination will you get the bearing relative to the True North.

Never Miss an Angle Again!

Now that you have the necessary knowledge about magnetic declination, we hope that you are now ready to take the field without the fear of losing your way!

Are there still questions lurking in your mind or methods which we have failed to include? Don’t hesitate to share them with us by leaving a comment below.