Leroy R. Grumman Cadet Squadron

Civil Air Patrol - The official auxiliary of the United States Air Force

Compass Terminology and Use

The compass consists of a magnetized metal needle that floats on a pivot point. The needle orients to the magnetic field lines of the earth.


Orienteering Compass

The basic orienteering compass is composed of the following parts:

 Orienteering Compass

 

 

Types - Orienteering Compass

 


 

Lensatic Sighting Compass

 

Lensatic Sighting Compass

Lensatic Sighting Compass - Animated

 

 


What is North?

 

No, this is not a silly question; there are two types of north:

 

  • True North: (also known as Geographic North or Map North - marked as H on a topographic map - see Figure 6.8) is the geographic north pole where all longitude lines meet. All maps are laid out with true north directly at the top. Unfortunately for the wilderness traveler, true north is not at the same point on the earth as the magnetic north Pole which is where your compass points.

  • Magnetic North: Think of the earth as a giant magnet (it is actually). The shape of the earth's magnetic field is roughly the same shape as the field of a bar magnet. However, the earth's magnetic field is inclined at about 11° from the axis of rotation of the earth, so this means that the earth's magnetic pole doesn't correspond to the Geographic North Pole and because the earth's core is molten, the magnetic field is always shifting slightly. The red end of your compass needle is magnetized and wherever you are, the earth's magnetic field causes the needle to rotate until it lies in the same direction as the earth's magnetic field. This is magnetic north (marked as MN on a topographic map). If you locate yourself at any point in the U.S., your compass will orient itself parallel to the lines of magnetic force in that area.

Declination:

You can see that location makes a great deal of difference in where the compass points. The angular difference between true north and magnetic north is known as the declination and is marked in degrees on your map. Depending on where you are, the angle between true north and magnetic north is different. In the U.S., the angle of declination varies from about 20 degrees west in Maine to about 21 degrees east in Washington. The magnetic field lines of the earth are constantly changing, moving slowly westward (½ to 1 degree every five years).

 

 

This is why it is important to have a recent map. An old map will show a declination that is no longer accurate, and all your calculations using that declination angle will be incorrect. As you will see, understanding this distinction becomes important when navigating with a map and a compass.

 

Which North to Use:

 

So we have two types of north to contend with. When you look at your map, it is drawn in relation to true north; when you look at your compass, it points to magnetic north. To make the map and compass work together you must decide on one North as your point of reference and base all your calculations on that. As you can see the following chart, failure to take declination into account can put you way off target.

 

Declination or Degrees Off Course

Error Off Target after Walking 10 Miles

920 feet (280meters)

4,600 feet (1,402 meters)

10°

9,170 feet (2,795 meters)

 

What's your Map Declination?

 

The first thing you need to know is where you are in relation to magnetic north. You can find this information by looking on your map legend. If you look at the map of North America (above) you will see the line roughly marking 0° declination. If you are on the line where the declination is 0 degrees, then you don't have to worry about any of this, since magnetic north and map north are equivalent. (Wouldn't it be nice if all your trips were on the 0 degree of declination line?) If you are to the right of that line, your compass will point toward the line (to the left) and hence the declination is to the west. If you are to the left of the line, your compass will point toward the line (to the right) and hence the declination is to the east.


Bearings:

The compass is used primarily to take bearings. A bearing is a horizontal angle measured clockwise from north (either magnetic north or true north) to some point (either a point on a map or a point in the real world). Bearings are used to accurately travel to a destination or to locate your position. If you are working from your map, it is called a map bearing and the angle you are measuring is the angle measured clockwise from true north on your map to this other point on the map. If you are taking a bearing off a real point on the landscape with a compass, you are using your compass to measure the angle clockwise from magnetic north to this point on the landscape. This is called a magnetic bearing. Remember that the bearing is measured clockwise. If you think of true north as 12 o'clock then a bearing to the right of that (1 o'clock) is greater than true north and a bearing to the left of True north (11 o'clock) is less than true north.

 

 

 

Map Bearings & Magnetic Bearings:

 

Think about your map as an artist's rendition of the world. It displays true north, but it doesn't include magnetic fields as the real world does, so you need to make accommodations when going from your map to the real world. The real world doesn't have a true north—it's merely a construct of the map—so you have to make accommodations when going from the real world to your map. The basic principle is this: to correct for declination, you want the map bearing and the magnetic bearing to be equivalent. If you are lucky enough to be on the line where the declination is 0°, both are already equivalent, or if you orient your map with your compass (see page 00) then you have made the two equivalent. Otherwise, you will need to make your own bearing corrections by adding or subtracting the declination amount. That gives us 4 possible permutations to work with:

 

  1. West Declination - Going from a Map Bearing to a Magnetic Bearing
  2. West Declination - Going from a Magnetic Bearing to a Map Bearing
  3. East Declination - Going from a Map Bearing to a Magnetic Bearing
  4. East Declination - Going from a Magnetic Bearing to a Map Bearing


West Declination:

 

If your declination is west, then magnetic north is less than true north and the map bearing is less than (<) the magnetic bearing. You need to make the two bearings equivalent by adding or subtracting the declination.

 

  • Map Bearing to Magnetic Bearing: If you are taking a bearing from one point on your map to another point on the map with respect to true north, then you are working with the map bearing. Now you want to figure out where your position is in the magnetic bearing. In order to transfer this information back to your magnetic bearing you need to add the declination to your map bearing to create the proper magnetic bearing. Map bearing + Declination = Magnetic Bearing.

  • Magnetic Bearing to Map Bearing: If you use your compass to take a bearing from your current position to a point on the landscape, then you are working with the magnetic bearing. Now you want to figure out where your position is on the map. In order to transfer this information back to your map you need to subtract the declination from your magnetic bearing compass bearing to create the proper map bearing. Magnetic Bearing - Declination = Map Bearing. 

 

 


East Declination:

 

If your declination is East then magnetic north is greater than true north the map bearing is greater than the magnetic bearing. You need to make the two worlds equivalent by adding or subtracting the declination.

 

  • Map Bearing to Magnetic Bearing: If you are taking a bearing from one point on your map to another point on the map with respect to true north, then you are working with the map bearing. Now you want to figure out where your position is in the magnetic bearing. In order to transfer this information back to your magnetic bearing you need to subtract the declination from your map bearing compass bearing to create the proper magnetic bearing. Map bearing - Declination = Magnetic Bearing.

  • Magnetic Bearing to Map Bearing: If you use your compass to take a bearing from your current position to a point on the landscape, then you are working with the magnetic bearing. Now you want to figure out where your position is on the map. In order to transfer this information back to your map you need to add the declination from your magnetic bearing compass bearing to create the proper map bearing. Magnetic bearing + Declination = Map Bearing. 

 

   

If the declination is...

Then...

Map Bearing to
Magnetic Bearing

Magnetic Bearing to
Map Bearing

West

Magnetic North < True North Map Bearing is < the Magnetic Bearing

Map Bearing + Declination = Magnetic Bearing.

Magnetic Bearing - Declination = Map Bearing.

East

Magnetic North > True North Map Bearing is > the Magnetic Bearing

Map Bearing - Declination = Magnetic Bearing.

Magnetic Bearing + Declination = Map Bearing.

Bearings and Triangulation

Getting A Bearing

As mentioned in the previous section, a bearing is a measurement of direction between two points. Bearings are generally given in one of two formats, an azimuth bearing or a quadrant bearing.

 

Azimuth Bearing Ring on an Orienteering Compass

 

An azimuth bearing uses all 360° of a compass to indicate direction. The compass is numbered clockwise with north as 0°, east 90°, south 180°, and west 270°. So a bearing of 42° would be northeast and a bearing of 200° would be southwest, and so on.

 

Compass Quadrant diagram

 

For quadrant bearings the compass is divided into four sections, each containing 90°. The two quadrants in the northern half of the compass are numbered from 0° to 90° away from north (clockwise in the east, counterclockwise in the west). In the southern half of the compass, the two quadrants are numbered away from south (counterclockwise in the east, clockwise in the west).

 

Quadrant bearings are given in the format of N 40°E (northeast), S 26°W (southwest), etc. Whenever you measure a quadrant bearing, it should always be recorded with north or south listed first, followed by the number of degrees away from north or south, and the direction (east or west) away from north or south. In other words, you would never give a quadrant bearing as E 40°N or W 24°S.

 

Your compass may be an azimuth compass or it may be divided into quadrants. If you have an azimuth compass and are given a quadrant bearing, you’ll have to divide it into quadrants in your head, and the same goes for quadrant compasses if you are given an azimuth bearing.

'Sighting' with a Lensatic Compass to get a Bearing

 Lensatic Compass - 'Sighting' View 1

Lensatic Compass - 'Sighting' View 2


Measuring A Bearing

 

So, you’re in the field with your map at point A and want to get to point B…how do you accomplish this? The first thing you need to do is determine the bearing from point A to point B. There are two ways to go about this:

 

Protractor Use

 

1)     The easiest way is to carry a protractor with you when you’re in the field. If you have a protractor with you, place it on the map so it is oriented parallel to a north-south gridline, with the center of the protractor on point A (or on a line drawn between points A and B). Once you have done this, you can simply read the bearing you need to go off of the protractor.

 

 

Compass Use

 

2)  If you don’t happen to have a protractor with you, you can determine the bearing you need using your compass. To do this, place your compass on the map so that the edge of your compass is oriented parallel to a north-south gridline and the center of your compass is on the line between points A and B.

 

Now rotate the map and compass together until the north arrow on the compass points to 0° on the graduated circle. You can then approximate the bearing you need by estimating where the line between A and B crosses the graduated circleGoing From Point ‘A’ To Point ‘B’

 

Once you’ve figured out what direction you want to go, you need to figure out how to use your compass to get you there. In th previous example, you determined that the bearing between A and B is 21° (N 21°E).  All you have to do now is walk a straight line from point A to point B at 21° and, after a little sweat, you’ll be at your destination.

 

Most compasses have some sort of sighting system built into them to allow greater accuracy in determining where you want to go. If your compass has a sight (check your owner’s manual to see if it has one and, if so, learn how to use it), you will orient it the same way as described above, but you can look through the sight at the same time and find an object to walk toward.

 

By finding an object (such as a tree or large rock) that lies along your path you will have more freedom to go around obstacles (such as large gullies, streams, hills, etc.) without losing track of the direction your are traveling.  Once you reach the object you were headed for, sight in on another object along your path, repeating this process until you arrive at point B.


Finding Yourself On A Map

 

Now you know how to get from point A to point B on a map using your compass…but what if you are not sure where exactly point A is (i.e. you are lost)? By far the easiest way to determine where you are on a map is to pull out your pocket GPS (global positioning system receiver) and have it give you your map coordinates. If, however, you are like a lot of people, you don’t want to shell out a hundred bucks for a GPS and, unless you are in an area with very little topographic relief, you don’t need one. You can determine your position quite accurately on a topographic map by using your compass to triangulate between three points.

 

Example - Triangulation

 

The first step in triangulation is to pick three topographic features that you can see and can identify on your map (mountains are ideal). Start with the first feature you have chosen and determine the bearing between you and it, as outlined above. Once you have determined its bearing, pencil in a line with the same bearing on your map that runs through the chosen feature (once again, having a protractor would be useful).

 

Repeat this for the other two features, drawing lines for each. The point where the three lines intersect on the map is where you are. Depending on how accurate your sightings were and how accurately you drew your lines through the features, there will probably be some error in your location. Be sure to double check the map and reconcile it with what you see. If the lines intersect in a valley and you are on a hill, the location is obviously off a bit on the map.

 

It does give a good approximation though and, by looking at your surroundings, you should be able to figure out which hill on which side of the valley you are on. If you have an altimeter with you, you can also use it with the triangulation to help determine your exact location more accurately.

Pace Counting

 Determining Travel Distance by Pace Counting

While pace counting is an old distance determination technique that is seldom used by trail-bound hikers, it is an essential technique used by off-trail navigators who travel cross-country through challenging wilderness.  In certain situations, a map and compass alone just aren't enough.

 

More mistakes are made in land navigation/orienteering by wrongly estimating distance than from any other reason.  While most of us can quickly learn to travel in the right direction, few of us have any idea of how far we have traveled.

 

Think about it for a moment.  Have you ever cut an azimuth through the bush and wondered if you had missed your target, or perhaps not gone far enough, when it did not materialize?  Did you continue on another 10 minutes, then 20 minutes, hoping it would appear?  Or did you backtrack?  You could have eliminated much of the guesswork in this situation by using a technique known as “pace-counting."

 

Pace counting with Pace-Count Beads is well suited for the navigational challenges faced by a wilderness navigator.  For example, pace counting is essential for dead reckoning, where azimuth (or direction of travel) data is combined with pacing (or distance traveled) data.  With this technique, one can establish his or her position in nondescript terrain, foul weather, or even in complete darkness.

 

(Note - The "dead" in dead reckoning is derived from "ded.," an abbreviation of "deduced.“  It's navigation by logical deduction.  It does not necessarily mean it's a deadly form of navigation.)

 

The hardest thing to get a "feel" for is how to adjust your pace-count for weaving back and forth on a route covered with trees, shrubs, and boulders.

 

To be accurate, the navigator must practice pacing over different types of terrain.  First you have to do some calculations.  Measure out exactly 100 meters on three types of ground: 1) flat easy terrain, 2) rougher terrain with some slope and 3) steep hill terrain.  Then on each measured course count your paces (every time your left foot touches the ground or every 2 steps = 1 pace). You will have 3 different pace counts for different types of terrain.   If you wear a pack when in the woods then do your pace testing with the pack on.  Once finished, memorize your pace count of all 3 types.


Pace-Count Beads

 

To use the pace-count beads:  Pull down one of the bottom 9 beads for each 100 meters you travel.  When you reach 1,000 meters, pull all 9 of the bottom beads back up to the starting position and pull down one of the top 4 pace-count beads.  Repeat the process with the lower beads (100 meter increments) until you can pull down the second of the top 4 pace-count beads (2,000 meters).  Repeat as needed.  This set of pace-count beads will help you count up to 4,900 meters.

 

(Note - Instructions for making pace-count beads can be found on the "ES-Downloadable Resources" page.)

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