Using a Compass
A compass is an instrument used for navigation, and consists of a magnetized metal needle that hovers on a pivot point and orients to the magnetic field lines of the earth. The basic orienteering compass includes the following parts:
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*Note - Compasses are affected by magnetic fields. When using your compass, keep it away from metallic objects, as even a metal belt buckle is capable of distorting your magnetic reading. Furthermore, magnetic fields such as those found in cars can temporarily or permanently demagnetize your compass needle. Finally, most compasses these days are made of clear plastic; however, certain chemicals and solvents - like those found in DEET insect repellent - can deteriorate the paint markings or cause the once clear plastic to become cloudy.
Finding North
Believe it or not, there are actually several types of North...
True North/Map North Also referred to as geographic north, this version of "north" is marked as a star on a topographic map. It represents the geographic North Pole, the place where all longitude lines meet. Although all maps are laid out with True North/Map North directly at the top of the map, True North is not at the same point on the earth as the magnetic North Pole, which is where a compass points.
Magnetic North The result of a magnetic attraction in the southern hemisphere near the geographic South Pole that compasses are designed to use to point north. In other words, because your compass needle in magnetized, no matter where you are, the earth's magnetic field causes the needle of your compass to rotate until it lies parallel to the lines of magnetic force in your particular location with the red end of the needle pointing to magnetic north. The map on the left shows an approximation of the lines of magnetic force for the United States. |
Declination
If you examined the lines of magnetic force in the United States illustration above, you probably noticed that your location does in fact make a big difference in where the compass points. This angular distance between true and magnetic north is referred to as declination and is marked in degrees on a map. Long story short, the angle between true and magnetic north is different depending on where you are. The angle of declination varies from about 20 degrees west in Maine to about 21 degrees east in Washington state. With this much variance, it is helpful to think of the Mississippi River roughly as the location of the zero declination line (called the agonic line).
For example, if you are in Maine, your compass "declines" or points toward the agonic line (which is to the left) and therefore your declination is to the west. If you are in California, for example, your compass "declines" or points toward the agonic line (which is to the right) and therefore the declination is to the east. If you happen to be lucky enough to be using your compass on the line where the declination is zero, where true and magnetic north are equivalent, you do not need to worry about declination at all.
Finally, because the magnetic field lines are constantly changing, it's important to have as recent a map as possible. The magnetic lines are slowly moving westward (at approximately 1 to 1.5 degrees every five years). Because older maps possess a declination that is out of date, all of your calculations using that angle will be incorrect.
For example, if you are in Maine, your compass "declines" or points toward the agonic line (which is to the left) and therefore your declination is to the west. If you are in California, for example, your compass "declines" or points toward the agonic line (which is to the right) and therefore the declination is to the east. If you happen to be lucky enough to be using your compass on the line where the declination is zero, where true and magnetic north are equivalent, you do not need to worry about declination at all.
Finally, because the magnetic field lines are constantly changing, it's important to have as recent a map as possible. The magnetic lines are slowly moving westward (at approximately 1 to 1.5 degrees every five years). Because older maps possess a declination that is out of date, all of your calculations using that angle will be incorrect.
Bearings
Now that you are familiar with the differences between True and Magnetic North, you can begin to use your map and compass together (assuming that you read the Map and Map Reading section - if not, get on it!).
The primary purpose of a compass is to take bearings. A bearing is a horizontal angle measured clockwise from north (either magnetic or true north) to some point (either a point in the real world or a point on a map). Bearings are important to understand because they can be used to pinpoint your position or reach a destination. It is important to keep in mind that a bearing is always measured clockwise. If you are taking your bearing using your map (called a map bearing), the angle you are measuring is the angle measured clockwise from True North on your map to another point on the map. If you are taking a bearing from a real point on the terrain with a compass (called a magnetic bearing), you are using your compass to measure the angle clockwise from Magnetic North to this point on the terrain. |
Map Bearings and Magnetic Bearings
The map you are using is a visual representation of the real world. Although it shows True North, it does not take the earth's magnetic fields into account. Furthermore, a True North does not exist in the real world. In order to compensate for declination, you want the map bearing and the magnetic bearing to the equal. This can be accomplished in one of the following three instances:
- Although this does not happen frequently, you may be in position on the agonic line where the declination is 0 degrees - if this is the case, then the map bearings and magnetic bearings are already equal.
- Orienting your map with your compass BEFORE taking a bearing is the easiest way to compensate for declination because then you have already made the two norths equivalent. If you do this, disregard number 3.
- If the map bearing and magnetic bearing are not equal, you will need to make a bearing correction by either adding or subtracting the declination amount as you go either from map to compass or from compass to map. This can be a nuisance, so adhering to number two is the best means to compensate for declination.
East Declination
If you declination is east, then magnetic north is greater than True North and the map bearing is greater than the magnetic bearing. The illustration to the left demonstrates how 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 a map bearing. To determine the magnetic bearing, subtract the declination from 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 a magnetic bearing. To determine your position on the map, add the declination from your magnetic bearing to create the proper map bearing. Magnetic Bearing + Declination = 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. The illustration to the right demonstrates how 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 a map bearing. To determine the magnetic bearing, 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 a magnetic bearing. To determine your position on the map, subtract the declination from your magnetic compass bearing to create the proper map bearing. Magnetic Bearing - Declination = Map Bearing |
Local Declination Compass Adjustment
Some compasses possess an outer degree ring that can be unlocked, effectively allowing the user to reset the compass to account for the local declination. Normally the orienting arrow will point to the N on the compass ring, but the user can adjust the orienting arrow so that it points to magnetic north for the declination of the area. This is handy because once the compass has been corrected to the local declination, the user will not have to add or subtract for declination because the compass is already aligned to True North. This means that when the compass needle is inside the orienting arrow, the compass bearing the user reads off the compass will be in relation to True North instead of magnetic north.
Alternatively, if the compass is a fixed-ring compass, the user can simply mark the declination angle on the compass baseplate with a piece of tape.
Alternatively, if the compass is a fixed-ring compass, the user can simply mark the declination angle on the compass baseplate with a piece of tape.
Comments
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