Compass-HowTo.com

How to Use a Compass

Where is Magnetic Declination Highest and Lowest?

It is easiest to answer this question with a picture. Check out the map below from the National Geophysical Data Center. Declination is the same at any point along one of the lines (called isogonic lines).  Notice the vertical blue line with a 5 on it in the middle of the map.  This is the 5 degree east magnetic declination line.  If you walked from North Dakota all the way to Mexico you would be following the same path as this line and you could leave the magnetic declination on your compass set to 5 degrees east.

On the other hand, if you were to walk from Boston to San Francisco there is a 30 degree difference in magnetic declination (from about 15 degrees west at Boston to about 15 degrees east in San Francisco). So, you would have to adjust your declination frequently as you crossed the country if you wanted to be able to use a map and compass together.

More information on magnetic declination maps can be found at the NOAA NGDC site.

Making a Declination Correction on Your Non-adjustable Compass

If your compass does not have a magnetic declination adjustment, you may need to add or subtract degrees from your direction of travel to compensate for the difference between true north and magnetic north in your area of travel. I say may, because if you are using a compass to navigate to and from a destination without a map, there is no pressing need to account for the difference between true  north (which your map is oriented to) and magnetic north.  The exception is when you are also using sun angles and other techniques to determine direction and you are in an area with a huge difference between magnetic and true north.  Here, making a correction would help you switch back and forth between compass and sun/celestial navigation.

If you are using a map (which you should be) you will first need to determine the magnetic declination for your area (see Finding Declination for Your Area).  If it is an east declination you will subtract the appropriate number of degrees when taking bearings from a map (east is least) and add degrees when you sight an object with your compass and want to transfer that bearing to the map.

For example, If you are going for a cross-country hike near Portland, Oregon where you have an approximate magnetic declination of 20 degrees east (this means your compass is pointing toward the magnetic north pole which is 20 degrees east of north on the map).   Let’s say you place your compass on the map, lining up the edge of the compass with your starting point and your destination (or landmark of interest). To determine your bearing, you twist the dial until the orienting lines on the bottom of the compass line up with north on the map, then read the compass direction.  Let’s say this direction, which is true north, is 85 degrees. You will now need to twist the dial to set your compass to 65 degrees before using your compass to navigate toward that destination.

Let’s do another example.  If you are hiking in upstate New York, the magnetic declination is closer to 15 degrees west.  If you were to plot a course of 50 degrees on a map, you would add 15 degrees and set your compass to 65 degrees for navigation.

There are several mnemonic phrases and rhymes to help you remember how to make these corrections.  I created my own to reflect my forestry profession:

“Every Day Tree Tops Make Shadows”

The translation is, “East Declination True to Magnetic Subtract.”  In other words, if I am in an east declination area and plotting a course on a map (true north) and need to correct it to magnetic north (compass) I subtract the appropriate number of degrees when I set the compass. Conversely, I add degrees if I am converting between a compass bearing (magnetic) and map bearing (true) in this same east declination setting.  Adding and subtracting is reversed in a west declination area.

 

Understanding Magnetic Declination

Magnetic declination is the difference between the direction your compass is pointing, called magnetic north, and the direction that your map is oriented, which is true north.  The Earth’s magnetic field lines up roughly, but not exactly with the true poles—those points on the surface that correspond with the Earth’s axis. Check out the “grapefruit example” in the Magnetic Declination video.

Magnetic and True North

Magnetic and True North

In some parts of the world the two norths appear to line up perfectly, so declination is 0 degrees, but in other places it is 25 degrees or more. The difference between declination from location to location can add up quickly.  For example, the magnetic north pole lies 15.5 degrees west of the true north pole from a position in Portland, Maine.  If you are standing in Portland, Oregon, your compass points 16 degrees east of True North.  If you travel between these two locations and forget to set your compass declination, your compass bearing could be off by over 30 degrees!

The magnetic declination for a given area is printed right on some maps (see the USGS topographic map image below), and there is a website that will determine your declination based on your website or rough latitude and longitude coordinates (see Finding Declination for Your Area (coming soon))

Setting the declination on hand held compasses with an adjustable declination option makes it easy to “set and forget” about declination while you are navigating in a single general location.  When you move more than a hundred miles or so, and every few years (the magnetic pole wanders a bit) you’ll need to reset the magnetic declination adjustment. See  Setting Declination on Your Compass (coming soon) for more ils.

True North (star) and Magnetic North (MN)

Making a declination correction on a non-adjustable compass requires a little arithmetic, but don’t panic, it’s not as hard as it sounds.  See Making a Declination Correction on Your Non-adjustable Compass.

What Causes Magnetic Declination?

Our planet’s magnetic field is created by the movement of liquid iron in the Earth’s outer core. The iron moves because heat causes a convective current and because the Earth is rotating.  This energy creates an electrical current that interacts with smaller magnetic fields to create Earth’s strong magnetic field.

All the movement down below the planet means that the field is constantly changing in intensity and intensity.  In fact, the field even reverses every so often—on average, every 250,000 years, but sometimes a lot more frequently and sometimes there are millions of years between flips.  These reversals, once they get going, probably take hundreds or thousands of years to complete, so don’t worry too much about having to buy a new compass should one start next year.

Position of Magnetic North Pole

Changing Position of Magnetic North Pole

A somewhat shorter-term concern for compass users is the year-to-year change in the location of the magnetic north pole.  Even this is not a huge concern because the movement is slow enough, on a human time-scale, that you could go backpacking for a few years and not have to worry about it.  Observations of magnetism going back to the 16th century show that the declination in London, England has changed from 10 degrees east to 25 degrees west, and now back to about 3 degrees west.  Currently, the magnetic pole is moving at approximately ½ degree per year.

Compass Directions

There are a few different methods for describing the direction you and your compass are facing.  Mariners use a slightly different system than aviators, and the military has its own system.  Here I will focus on the most useful ways of describing directions for land navigation with a handheld compass.

The first rule of compass directions is to remember that no matter what letters and numbers you see at the top of the compass dial, or which way you are facing, the red end of the compass needle always points toward the north magnetic pole.  This sounds easy enough to remember, but when your head is foggy because of lack of sleep, cold temperatures, or panic, you can become easily confused.  Practice just watching the compass needle to see which direction is north.

By default, the other end of the needle points south.  East and west are more easily confused.  You have to remember that east is right of north, and west is left of north.  If you have problems remembering these directions it might be helpful to memorize the directions in the order they would appear if you turned in a circle.  Starting at north, you could make a ¼ turn clockwise to east, another ¼ turn to south, another to west, and another back to north.  Memorizing the phrase “Naughty Elephants Squirt Water” might help you remember the sequence of directions as you turn clockwise in a circle.

These four directions (N, E, S, W) are the cardinal directions and they lie at 90 degree angles to each other.  The intermediate directions of NE, SE, SW, and NW are at 45 degree angles to the cardinal directions.  For modern land navigation purposes the intermediate directions are used only for rough approximations.  For example you might tell someone that you will be following a stream that flows NW until you reach the waterfall shown on your map and then you will climb the ridge to the SW and follow it downhill to the road.  Someone with a compass and/or a map could follow your route very easily with this information.

More precise compass directions are described with numbers or number and letter combinations called a bearing.  Two common methods of describing a bearing are the quadrant method and the azimuth method

The quadrant method breaks the compass dial into two hemispheres, north and south, and then describes how far toward either east or west you are facing. A description of a direction always begins with either N or S, followed by a number of degrees and then the letter E or W. For example, N 40° E is a direction that is 40° east of north.  S 45° W is exactly due southwest.  South 85° East is just 5° south of due east.

The azimuth method is easier to use. The compass circle is broken into 360 increments or degrees (°).  North is 0°, East is 90°, south is 180°, west is 270°, and north is 360° (or zero).  Describing a direction is as simple as saying one number.  For example, a direction of 40° using the azimuth method is the same as N 40° E using the quadrant method.  A direction of 225° is equivalent to a quadrant direction of S 45° W, and an azimuth of 95° is the same as a quadrant bearing of South 85° East.

Some compasses have a dial with quadrant degrees, others have an azimuth scale, and some have both.  Practice converting between these two methods of describing a direction so that you will be able to use either type of compass in an emergency.

How a Compass Works

Humans have learned a lot of tricks for finding their way around this planet—the location of the sun as it sets on the horizon, the pattern of stars in the night sky, even changes in wind direction throughout the seasons.  About a thousand years ago Chinese mariners began employing a new trick that used an invisible force of nature to help chart their course.

That force, magnetism, had been observed in naturally occurring magnetic minerals for hundreds of years. Then one day, some long forgotten person discovered that a small piece of this magnetic material, if suspended from a string or floated on a liquid surface, always lined up in the same North/South direction.

Try this experiment yourself.  Rub the pointy end of a needle on a magnet to “magnetize” it, then place it gently in a cup of water.  The surface tension of the water should keep the needle from sinking, but if it doesn’t, float the needle on a small piece of cork.  The needle will slowly swing to point north.

The compass needle, like the magnetized sewing needle in our experiment, is also a magnet, balanced so that it can swing freely on a little pedestal. Magnetism is an invisible field associated with certain objects—magnetite and the planets in our solar system, for example. Why magnetism occurs is a question that has received a great deal of scientific attention and there are several good articles available on the web.

More central to our discussion, however, is the question of HOW magnetism works.  We have all had the experience of playing with two magnets and observing how they only align with each other in a certain direction. This phenomenon is called polarity.  Magnets have north poles that attract south poles, and vice-versa.  North poles repel each other, as do south poles.

Magnetic Field Revealed By Metal Filings

courtesy stargazers.gsfc.nasa.gov

When magnets repel and attract in this manner, it is the invisible lines of force that are interacting.  These lines of force, called flux, can be observed in another simple experiment with a magnet, sheet of paper, and some iron filings.  The image below shows the result of the experiment, and what important to note is that the metal filings too far away to be drawn tight to the magnet line up along the curved flux lines between the north and south poles.

Similarly, our tiny floating needle’s magnetic field lines up with the giant magnetic field of planet Earth.  The north-pointing end of a compass needle (usually painted red) is of the opposite polarity as the Earth’s magnetic north pole, so it is attracted rather than repelled, and it aligns parallel to Earth’s strong magnetic lines of flux.

The Earth’s magnetic field is oriented more or less north-south, but there are bumps and wrinkles in its lines of flux associated with metallic mineral deposits.  This knowledge highlights one small inconsistency in my statement earlier that our floating needle points north.  A more accurate way to describe the behavior of our needle would be to say that it lines up with the Earth’s magnetic field (which is lined up approximately north).

Compass Needle Lining Up With Magnetic Field

courtesy www.teachnet-lab.org

A more important distinction related to the term “north” is that Earth’s molten core sloshes around a bit, and causes to Earth’s magnetic poles to slowly wander around.  Currently, the north magnetic pole is several hundred miles away from the “True” north pole that maps line up with.  A magnetic compass points toward magnetic north, so a correction needs to be made if you are using a compass with a map.  See Understanding Magnetic Declination for details.

There are a few more details in the articles to follow that will help you learn how to use a compass, but these concepts are the same for all magnetic compasses:  the north pointing end of a compass needle aligns with the Earth’s magnetic field, pointing toward the magnetic north pole.

How to Use a Compass to Determine Direction

This article explains how to use a handheld compass for land navigation on foot or horseback.  Different types of compasses (see How to Choose a Compass) work in slightly different ways to show you which direction you are facing.  Here I explain how to read a baseplate compass (also called orienteering, or protractor compass), because it is the most commonly used compass for outdoor recreation.

  • Hold the Compass Properly: Keep the compass flat and level, so that you are looking down at it.  Keep the direction of travel indicator at the top of the compass.  A good way to remember how to hold the compass is to dangle it from your neck by the lanyard.  When you lift the compass and hold it flat, it will be facing the correct direction—with the direction of travel indicator facing away from you.

  • Learn What the Magnetic Needle Does: The red end of the needle ALWAYS POINTS NORTH.  It actually points toward the magnetic north pole which is different from the geographic or “true” north pole that maps use, but the distinction is only important if you are using a map with your compass (see Understanding Magnetic Declination for details).
  • Set the Compass: Face toward the way you would like to determine the direction for, whether it is toward a landscape feature like a hill or just a path you plan to hike. Holding the compass still, twist the dial until the hollow orientation arrow lines up beneath the needle.  When the needle is “boxed” in this manner, the compass is set.
  • Read the Compass: With the compass set, read the bearing at the direction of travel indicator located at the top (front) of the compass dial.  This is your direction of travel and it can be expressed in a number of ways including the quadrant or azimuth method (see Compass Directions).
Compass Not Set

Compass Not Set

Compass is set

Compass is set (needle is "boxed")