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Do you know your compass? Test your knowledge

Proper Compass Photo

The key to using any navigation system is understand how it works on a basic level. Sure, we can shoot azimuths, follow terrain using a map, but do we really understand how it works? It is my experience, that most who venture out, have a poor understanding of the science behind the system they use.

With that in mind, let’s explore the compass.

The compass represents (still today) one of the most popular ways of finding magnetic north. The science behind the compass is truly remarkable. The magnetic compass is inexpensive, durable, dependable, doesn’t need a power source and is very easy to use. But do you really understand how it works? Let’s find out.


First, some background. . . Over 4.5 billion years ago, stuff exploded. Then, the debris of a huge nebula of stars formed earth. The non-burnable nuclear ash from the stars, heavy with particles of iron, nickel and cobalt all sank to the earth’s core. The ferromagnetic elements have electrons in their outer shell that spin, creating magnetic moments that are not cancelled.

Since then, the Earth has created heat from ongoing radioactive decay resulting in a molten core. The motion of these molten elements creates the earth’s magnetic field which extends a few thousand miles into space. This is the magnetic field we use to navigate.

This “Magnetosphere” protects the ozone from solar winds and keeps solar flares and dangerous radiation from extinguishing life.

So, the earth is actually made up of several “dipolar” magnets that are 90% of the Earth’s magnetic field. (Polar Bears are named after magnets!)

So Anyway, how well do you know your compass? Here we go. . . 

The magnetic needle in a compass points to magnetic north.


The magnetic needle of a compass aligns itself with the Earth’s Local Magnetic field and not to any single point.  This field varies a whole bunch across the Earth in intensity and orientation, and the compass actually points to the sum of the effects of many giant magnets at your location.  So, your compass aligns with the magnetic lines of force.  These are called Isogonic lines.

The magnetic needle is “attracted” to the magnetic north pole.

No. . .

If you think about that for a minute, that would mean the magnetic north pole would have to be awesomely strong to attract a little compass magnet in the southern US.   So strong, in fact, ships would get dragged over miles of open ocean, not to mention all the other magnets and metals objects flying north like a flocks of disorganized birds.

The magnetic north pole is stationary.

Not at all.


Well, most of us probably know this, but the earth’s magma is a swirling substratum that is constantly on the move.   Magnetic north is currently heading towards Russia at 65 km per year (a little over 40 miles). 

The magnetic north pole will always be in the north.

I’m sorry. . . that is incorrect (but we have a wonderful consolation prize).

The earth’s magnetic field actually reverses a lot.  Reversals happen at apparently random intervals, ranging less than 100,000 years to as much as 50 million years.  The last time it happened, was 750,000 years ago.

There are three “norths” – Grid, True and Magnetic.

Not exactly, there is a fourth

It is called the “Geomagnetic North”.  Which is the north end of the axis in the magnetosphere which extends into space.  It is the center of the region where the Aurora Borealis (Northern Lights) can be seen.

We correct our compasses for “Magnetic Declination”.

Nope. . . we actually don’t.

We (including me) tend to use the term “magnetic declination” (or magnetic variation) when correcting grid lines taken from a map to a compass, or vice-versa.  Actually, magnetic declination is the difference between true north and the horizontal trace of the local magnetic field and that is the reference we use with most USGS maps.


True north is is usually used on a “Global scale” and does not reflect the grid lines on a regional map.  Global maps project large areas of a curved surface onto a flat surface. However, a level of spatial distortion is caused across a relatively small area of the earth’s surface, such as a USGS quadrangle map.  Because this distortion is unacceptable to navigators, we have developed our own local rectangular grid system (such as UTM) to reduce that distortion.  Consequently, the grid lines of maps do not point to true north.  They, instead, become “grid north”.

So as a navigator, we have to adjust for the difference between grid north and magnetic north when converting between magnetic and grid bearings.  This angle between magnetic and grid meridians is called “Grid Magnetic Angle”, “grid variation”, or “grivation”, NOT “magnetic declination”.

What does it all mean?

Well . . . for practicality’s sake, not a lot, but the more you know. . . 

So yeah – kind of interesting, eh.  The good news, we don’t have to necessarily know all this or do all the calculations – most mapmakers do it for us.   And the information is usually printed on a map.

When the batteries have died, or your GPS has been jammed, spoofed or is miscalculating due to refraction, or when you GPS just can’t catch ephemeris, you can reach into your pack and pull out a very advanced and one of the most reliable pieces of gear you have. . . your compass.

Thanks for taking the time to “Go Farther” with your knowledge.

See also: The Best Compass for Land Navigation