Monday, June 21, 2010

How To Use A Compass

It’s a traditional skill often passed down from father to son. You can get a merit badge for learning it in the Boy Scouts. But how many people really know how to use a compass? Many folks mindlessly include it in their backpacking supplies, and it is frequently seen as a necessary item in a bug-out-bag. Yet, if you found yourself in an emergency situation, needing to navigate to safety, would you know how?

History of the Compass

The earliest evidence of any sort of compass being used dates back to around 1,400 – 1,000 B.C. with the discovery of an Olmec hematite artifact. If this recent discovery bears out to be accurate, it would predate the Chinese discovery of lodestone (the mineral initially used to make magnetic compass needles.)

Military Lensatic Compass
Military Lensatic Compass

The earliest Chinese compasses likely had very little to do with navigation. More than likely they were used for feng shui. Feng shui would have made use of the compasses to harmonize the environment of a building to be in certain principles of flow and design.

References to an iron “south-pointing fish” that floated in a bowl of water are first recorded in a book from the Song Dynasty that dates to around 1041 A.D. Later in 1088 A.D. there is reference in a book written by Chinese scientist Shen Kuo of magnetized needles that are hung from a single silk strand. Apparently not having knowledge of magnetic polarity yet, Kuo noted that the needles always pointed towards the north or the south. A mere 35 years later there is the first documentation of what we would consider the modern compass, where it is described by author Zhu Yu as being used by sailors to navigate waters in the dark or when cloudy.

In their earliest incarnations, compasses were marked simply with only markings showing magnetic north, or later the cardinal directions (north, south, east, west.) Later the Chinese divided these into 24 points on the compass card.

The compass did not come to Europe until sometime shortly before 1269 A.D. when Petrus Peregrinus wrote of a floating needle compass to be used for astronomical purposes. By 1290 A.D. the compass was found on sailing ships, allowing for a longer shipping season through the normally cloudy Mediterranean winter, although it was not until around 1410 that the compass was normally used as a steering reference.

In Europe, the divisions on the compass card were increased into 32 equally spaced points. Later, the 360-degree system became popular. Then, in the 1800s some European nations decided to use the gradian system. In this system, the compass card is divided into 400 points, with each right angle being composed of 100 gradians or “grads.” These gradians could then be divided into 10ths to give a total of 4,000 decigrades around the compass.

Magnetic Declination and Deviation
The Earth’s magnetic axis is slightly offset from its rotational axis. This means that a compass usually does not point to true north. Magnetic north is approximately 1,000 miles away from the geographic North Pole. In addition, the magnetic field of the earth is not the same everywhere. It changes depending on latitude, altitude, and the composition of nearby rock formations. There are two corrections that must be made to compensate for this difference: declination, and deviation. Deviation is the effect that local magnetic fields from ferrous objects and electrical circuits have on the magnetic needle of a compass. The declination (also known as the variation) is the angle between true north magnetic north. Most modern maps have the declination angle for the area they cover printed on them. Areas with no correction necessary make lines known as agonic lines. These areas exist in three places in both the eastern and western hemispheres.

Magnetic Deviation
Magnetic deviation is the response of the magnetic needle to local magnetic fields or the presence of ferrous materials.

The Earth’s magnetic field is not uniformly consistent. It varies greatly in intensity and inclination depending on latitude and altitude. Compass needles are specially balanced during manufacturing to compensate for differences in the magnetic field. Each balance design is intended for use in one of five zones on the planet, from Zone 1 which covers the majority of the Northern Hemisphere to Zone 5 which includes Australia and most of the Southern Pacific and Indian Oceans. Balancing the needles in this fashion prevents the needle from dipping too much and sticking.

Compass needles are also affected by nearby metals such as steel and iron. This problem is most common on ships where there is a large amount of metal. To correct for the magnetic deviation compasses on ships are housed in a binnacle and surrounded by large chunks of iron and magnets that are positioned in specific locations as compensation. These measures alone do not completely eliminate deviation however. Natural formations within the earth cause slight magnetic deviations which, while not enough to significantly alter the course of a hiker, can throw off ships by dozens of miles over a the course of a thousand mile journey.

Magnetic Declination
Magnetic declination is the difference between the magnetic North Pole and the geographic or “true” North Pole. The Earth’s magnetic axis is offset from the geographic rotational axis by a few thousand miles. This means that the indicated or magnetic north on your compass may be off as much as 10-20 degrees from geographic or “true” north.

Most maps have the declination adjustment printed on them. The declination can change rapidly from year to year as the Earth’s magnetic field fluctuates, so it is important to have an up to date map with the most recent declination adjustments. In the absence of a map, you can take a manual measurement of the local declination by measuring the difference between true north as indicated by a sundial or the North Star and the indicated magnetic north of a compass.

If indicated on a map, declination is usually given in the format of “10 degrees west” or alternatively “-10 degrees.” When reading declination adjustments, if given in the latter format, remember that negative numbers indicate west, and positive numbers indicate east. When adjusting the azimuth on a compass reading, it is simple to just add the compass reading with the declination to get the correct heading; e.g. with a 10 degrees west declination, and an indicated reading of 245 degrees, the corrected bearing would be 235 degrees.

When traveling by foot, you rarely have to take multiple declination readings. Unless you are in an area with heavy mineral and metal deposits, using just one declination reading is usually fine for hiking.

The Modern Compass
The most common modern compasses are the hand held lensatic compass, such as the US Military issued lensatic compass with tritium inserts, and the protractor compass. The lensatic compass is a military design initially developed to quickly direct mortar or artillery fire. Most military forces have adopted the “mil” system for measuring the compass dial. The milli-radian system uses 6400 units or “mils” per circle. The mil system was developed as a valuable method for measuring one meter at a distance of one kilometer (one mil = one meter at 1k) allowing for artillery fire to be precisely directed.

Lensatic Compass
The lensatic compass in its modern implementation is comprised of three basic parts; the cover, the base, and the lens. The base and cover are both square on one side ruled. The cover, in addition to protecting the lens and compass when closed, also has a slit, a sighting wire, and tritium dots or phosphorus dots for use in navigation. Lensatic compasses do not have needles. Instead the dial itself is magnetized and floats in a bezel ring in the base.

The base of the compass is outfitted with a sighting lens with a slot in it, a thumb loop, the bezel ring and of course the floating rotating dial. On the dial itself are two glowing letters indicating east and west, the directional arrow indicating north, an outer black scale in mils and another inner scale degrees. The inner scale indicator is in red color. The bezel ring around the dial is able to be rotated around, and has a ratchet that has 120 positions in a full 360 degree rotation, allowing for accuracy of 3 degrees when positioning the index line. The index line is a short luminous line in the glass face of the bezel. Also attached to the base is a thumb loop used for handling and stabilizing the compass.

The defining characteristic of a lensatic compass is, of course, the lens. The lens is set in a piece that folds over the compass face. This lens, in the open position, is used to read the floating dial. The lens also has a sighting slot that is used in conjunction with the sighting wire in the compass cover to sight landmarks. When used in this method, the user lines up the landmark in the sighting wire and the sighting slot, then glances down through the lens to read the bearing off of the compass dial. Opening and closing the lens also serves to lock or unlock the compass dial. In order for the compass dial to spin freely, the lens must be open a minimum of 45 degrees to unlock the clamp that holds the dial when the compass is closed.

Protractor Compass
Protractor compasses are probably the most common commercial compass, as well as the simplest to use. This compass is usually set in a rectangular clear plastic baseplate and is designed primarily for use with a map. One of the first things you will notice when looking at a protractor compass is the direction of travel arrow marked on the clear plastic baseplate. The edges of a protractor compass are generally ruled in various increments to aid in measuring distance on a topographical map.

The bezel of the protractor compass includes a transparent bottom compass housing which turns with the bezel, independently of the needle or the compass baseplate. Marked on the bottom of the bezel is a orienting arrow. This orienting arrow is lined up with the magnetic needle when taking a bearing, and compared to the direction of travel arrow inscribed on the compass baseplate.

Using a Compass Without a Map

Taking a Bearing
There are two main types of compasses, protractor and lensatic. Let’s address the protractor compass first. When using a compass without a map, you naturally will already know the general direction you want to travel. For example, if you are lost in the woods and you know there is a river a mile to the east that then leads north to a town five miles upstream, you may want to take a bearing heading due northwest. To take a bearing (also referred to as an azimuth) using your compass, turn the bezel until the northwest is on the direction of travel arrow. Next, holding the compass in your hand with the direction of travel arrow pointing away from you, turn your body until the red part of the needle points to the N on the bezel and is aligned within the lines on the bezel. Once the needle is lined up, you are facing the proper direction of travel. Pick a landmark that lines up with your direction of travel and use that as your guide; don’t keep the looking back at the compass. Once you reach the landmark, then take the compass back out, take another bearing, and repeat the process.

To take a bearing with a lensatic compass, you can use either the compass-to-cheek method shown to the right, or the center-hold method shown below. The method is largely the same as with a protractor compass. First, open the cover of your compass so that it is 90 degrees to the base. Open the eyepiece so that it is at a 45 degree angle to the base. Use the image on the right as a reference. Next, place your thumb through the thumb loop and form a steady base with your third and fourth fingers, while keeping your index finger along the side of the compass base. Use the hand not holding the compass as a support for the hand holding the compass. While keeping the compass level, bring both hands and the compass up to your face and position it so that the thumb going through the thumb loop is against the cheekbone. Again, use the image as a reference. Move the eyepiece until the dial below it is in focus. Align the sighting slot of the eyepiece with the sighting wire in the cover and turn until your target is lined up with the sighting wire. While keeping the wire on your target, look down through the lens and read the bearing under the index line.

The center-hold method, shown to the left, is not the preferred method, but it can be easier and faster to use when accuracy is not extremely important. To use the center-hold method, open the compass so that the cover is completely flat with the base. Move the lens of the compass out of the way. Place your thumb through the thumb loop and use your third and fourth fingers to form a steady base while keeping your index finger stretched along the side of the compass. Place the thumb of your other hand between the eyepiece and the lens with the index finger of that hand placed along the other side of that. Wrap the rest of the fingers of that hand around the fingers of the first hand. Finally, pull the compass in tight by tucking your elbows firmly against your sides.

Taking an azimuth is similar to taking a bearing, and though it is faster, it is not quite as accurate. Using the center-hold method, orient your body toward the target and point the compass cover in that direction. Glance down at your compass to read the azimuth beneath the black index line. On compasses equipped with tritium inserts, this method can also be used in darkness. Azimuth readings are most useful when actively navigating terrain, as the user can quickly pause to check the azimuth and confirm that they are still heading in the right direction. By lining up objects in the desired direction of travel, it is a simple matter to move from waypoint to waypoint by simply taking an azimuth reading to the next landmark and continuing in that direction.

Modern Protractor Compass

Using a Compass With a Map
Using a compass with a map is easiest with the use of a protractor compass, thoughlensatic compasses also have a straight edge ruler incorporated into the body to aid in use with a map. To take a true bearing or map bearing (one taken in reference to true north instead of magnetic north) place the compass on the map (layed out flat and level) with the edge of the compass parallel to the direction-of-travel arrow so that it makes a line between your current location and the desired location. Next, turn the bezel until the orienting lines are aligned with a marked line of longitude on the map. Make sure that the edge of the compass does not move on the map as you turn the bezel, or your bearing will be off. The map bearing is the number that is at the direction-of-travel line. Add or subtract the magnetic declination indicated on the map for the magnetic bearing.

You can use this bearing to navigate to your waypoint without the map by checking your azimuth as you travel in the manner described earlier in the article. Depending on the distance you are traveling, it may behoove you to stop periodically to confirm that your bearing is still accurate. If it is not, make a note of the corrected bearing and use that as you proceed along the azimuth toward your destination.

To navigate with a map using a military style style lensatic compass, open the case and place it on the map. Orient the compass on the map with the case cover to the north, thereby placing the black index line to the north. Orient the map by rotating it and compass until the declination marks on the map match the of the needle on the compass and the black index line. To then find the bearing needed to navigate to your target destination, without moving the map lay the ruled side of your compass along a line drawn from your current location to your destination. The bearing read on your compass ring is your heading.

Never Be Lost
With a compass and a map, you’re never truly lost for very long. The basics of compass navigation aren’t difficult to understand, but to truly become skilled at land navigation, you need to get outside and try it yourself. Pick up a lensatic or protractor compass and some topographical maps and try it out yourself. You’ll find that you’re soon able to navigate with ease in nearly any situation.

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