Anatomy of a baseplate compass.
Take a direct bearing:
Point compass, then box the needle.
Read the bearing.
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Getting bearings off a map:
1. Line up compass between starting point and destination. 2. Turn dial to align North with top of map, then read the bearing. 3. Find the bearing in the field .
Find your current position on a map using a compass.
(Three diagrams above from Suunto.com)
Red stick pin indicates Magnetic North. Green stick pin is Geographical, or True North.
The magnetic force runs in many directions but all lines point to the north and south magnetic poles.
An isogonic chart tells you how far from true north a compass will point anywhere along an isogonic line. Eastern declinations like California must be compensated for by subtracting the number from your readings. For western declinations like New York, add the declination value to your readings to get an accurate bearing. To set your compass ahead of time, change its declination adjustment by moving it clockwise or counterclockwise to get True North. See photo below.
Declination adjustment screw highlighted in yellow. Loosen it, move the ring until "N" lines up with the declination value, then retighten the screw. If "N" is lined up with 360 or 0 (as shown above), then no declination adjustment has been made.
A military, lensatic compass.
Mounted magnetic card compass.
North is universally accepted as the starting point on the outer part (or circumference) of the compass. Somewhat similar to a clock face, the device is divided into units - degrees, minutes and seconds. There are 60 seconds in one minute, and 60 minutes in one degree. A bearing (or direction) is identified as a number falling somewhere inside a 360- degree spread.
To get a concrete idea of a bearing, try holding a compass in the palm of your hand. Picture yourself following an imaginary line that extends out from the center of a circle represented by the compass. The starting point is where you stand, and there are 360 distinct directions to choose from. Each is represented by one degree of angle around the circle.
Looking at the numbered dial, called an azimuth, you'll see that it counts off the degrees from 1 to 359, moving clockwise. North resides at Zero. Turning right, you find the designation East, at 90 degrees. Move right again and you find South at 180 degrees, then right again to find West at 270 degrees. All of this holds true in the field as well. To the right of North is always East. South is always to the right of East. And West is always to the right of South.
If 360 possible directions seem like overkill, consider that the farther you venture out from your starting point, the greater the distance becomes between each of those lines. If you walked ten miles, for instance, but veered ten degrees off-course, you’ll end up nearly two miles to one side of your destination. That’s why long-haul trekkers never leave home without a compass.
The instrument has no electronics and doesn’t rely on batteries or any other power source. Instead, a sliver of metal is secured to a central pivot and allowed to spin freely. One side of the metal is magnetized and marked in red. When you hold the compass level with the ground, this sliver, called the compass needle, will drift for a few moments and then stop. The Earth’s magnetic field exerts a force on its magnetic side, so that the needle's red side points north.
Once that happens, you can turn the azimuth dial until a fat orienting arrow lines up with or encases the needle. This process is called boxing the needle. For a compass to be of any use, you must normally orient it in this manner. Then you can start navigating.
The most common type of compass on the market today is the baseplate model shown on the right. It's designed for use both in the field and on top of a topographic map. Its transparent baseplate is marked with a thin, forward-pointing arrow in the middle that indicates the direction of travel. This prevents you from ever holding the compass backwards Rulers on each side of the baseplate also make it easy to measure distances on a map.
Getting Your Bearings
The principle calculation taken with a compass is called a bearing. Like a heading used in flight and marine navigation, a bearing is a direction derived with cardinal points and the 360-degree circle.
If you’re standing on a hill where you can see your destination, for example, but have to drop down and navigate through a thick forest in order to get there, you’ll take a bearing from the hill with your compass and note it. Then, as you descend into the woods, the bearing will guide you until the destination becomes visible again. The bearing is expressed as a number - e.g. 271 degrees, 30 minutes.
To take a bearing of a physical destination or landmark with a baseplate compass:
Perhaps you’ve been wandering down in the Grand Canyon and can’t find a trail that leads back up and out of it. If you’re carrying a map, you can use your compass in two ways. First you can find north and then turn the map so its north matches up with north in the field. Then, by matching up any relevant landmarks, like the hotel that sits on the north rim of the canyon, you can estimate your current position. Now you should be able to determine which is the nearest trail to your location. Take a bearing of where the trail starts. by placing your compass on the map.
Now you can put the map away and get started on your trek. To know which direction you need to hike in:
A compass can also guide you through the terrain when there’s low visibility. If you know your bearing and keep the magnetic needle boxed, you can follow the instrument all the way home. Of course, you’ll have to move slowly to avoid stumbling. As an added perk on many compasses, the azimuth and needle both glow in the dark.
For all its simplicity, there’s one complicating factor involved in using a compass. It’s a phenomenon known as declination. Maps are all oriented to geographical, or True North. On the other hand, all compass needles point to Magnetic North. Sadly, the two locations are not the same. Magnetic North is located 1,300 miles south of the North Pole, off the coast of Greenland. So that's where your compass points, not Santa Claus's home and workshop.
Even worse, the magnetic north pole is moving six minutes west, or one tenth of one degree, every year. That’s not a lot, but if you find a declination value on a map that's more than 10 years old, it's not going to be accurate.
Declination represents the difference between magnetic and true north (or magnetic and true south), expressed in degrees. And the difference varies depending on where you’re at. In the continental United States, your compass (if not adjusted) will be anywhere from 0 to 21 degrees off when the iron needle points north. Thus, a declination value tells you how far away your compass needle is from geographical (aka true) north. This becomes an especially big deal if you're trying to navigate with a map.
To figure out the adjustment needed for your location, start by checking the NOAA magnetism web page, where you can enter a zip code to get a declination value. If you're planning a journey to multiple spots many miles apart, you should download an isogonic chart (be sure to check the date), then carry it for reference during your trip. (Note: If you're located on a isogonic line that extends between Minnesota's Lake Superior and Florida's western panhandle, there is no difference between magnetic and geographical north, making it an "agonic" line instead. See the U.S. chart to the right of this column.)
Once you know your declination, you'll have to factor it in every time you take a bearing with your compass in the field. For western/negative declinations (east coast), subtract the declination value to your readings in the field to get an accurate bearing. For eastern/positive declinations west of Lake Superior, add the declination value.
Alternatively, you can set your compass for declination BEFORE using it in the field. Then all your bearings will be accurate without the need for addition or subtraction. In order to do this, the compass must have a rotating ring that allows for a declination adjustment. On many baseplate models, you can loosen a tiny screw with a clip provided on the compass lanyard. (See photo right of column.) Turn the ring the clockwise or counterclockwise, depending on your declination value. For example, if the declination for your area is 10 degrees East, then turn the ring clockwise until the "N" symbol is pointing at 10 on the azimuth dial. If you have a western declination, you'll turn the dial counter-clockwise. For example, if your declination is 5 degrees west, turn the ring until "N" lines up with 355 degrees.
Once you’ve adjusted the dial, tighten the screw again. Now you can forget you ever heard the word declination -- that is, until you travel somewhere else and have to change the dial again.
One other word of advice: When communicating your compass bearings to other navigators, specify whether they're derived based on Magnetic North or Geographical North. And if someone gives you a bearing, make him or her do the same.
Fixed Dial, Sighting, Military and Magnetic Card Compasses
Before discussing other compass navigation techniques, let’s look briefly at a few other compass types.
A fixed-dial compass has no baseplate and the azimuth remains stationary. With an inexpensive brand, there’s also no forward pointing arrow or mark to indicate the forward direction of travel, and no vial of liquid in which the needle floats. To take a bearing for a destination or landmark:
If you already know the bearing you want to follow, but aren’t sure what direction that’s in:
A more accurate version of the fixed-dial is the sighting compass. It adds a mirror with a vertical sighting line. This allows you to see the the target in the field and the bearing at the same time. Some sighting compasses also include a baseplate and declination adjustment. All the extra features come at a cost, however, and the increased accuracy may be minimal.
A military-style compass uses a lens for even more accurate sighting, as well as a sighting line.
A fourth type of compass is the magnetic card model, commonly found on sailing vessels but also in a cheap miniature form that you can dangle from a keychain. Instead of a needle, the cardinal points and degrees are printed on a card that spins freely inside a casing. Magnetic material attached to the card causes the N or zero degree mark to line up with magnetic north.
When this type of compass is mounted in a boat, snowmobile or other vehicle, a lubber line indicates the direction of travel. It's aligned to the bow or front end of the vehicle. Typically the navigator maneuvers until his boat or land-based conveyance until the lubber line on the compass lines up with the bearing he wishes to head in (probably taken from a map). As with other compasses, you’ll have to compensate for declination – which mariners refer to as variation. Remember, if you’re traveling for scores of miles east or west, your declination value may change several times during your trip.
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