All About Compasses
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While conceptually simple devices, compasses come in many shapes and sizes. Ranging from a gas station key-chain compass to $700 geological compasses, the sheer variety can be rather daunting to the budding adventurer. Which leads to the inevitable question of, "Which compass do I actually need?" In this article, we explain some of the basic styles and features of modern compasses as well as a couple of basic considerations of compass use.
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Liquid vs. Induction Dampening
Compass needles are almost universally 'dampened'. This slows needle movements and minimizes oscillations, allowing the needle to come to rest faster and preventing erroneous readings. There are two major dampening types. The more common is liquid dampening in which the needle housing is filled with liquid. Liquid dampening benefits from being quick and cheap to manufacture. However, it suffers from two possible malfunctions. First, liquid filled compasses can develop air bubbles in the fluid caused by changes in elevation. This could especially be a problem for those who will be flying in small, unpressurized planes. The second issue is the liquid's freezing point, which limits the minimum operating temperature of the compass. While not an issue if you live in Arizona, in cold northern regions, a compass that maxes out at 20 below zero could pose an issue.
This brings us to the induction dampened compass. Invented in the late 1800's, these use electromagnetic induction to slow the compass needle, removing the need to fill the housing with potentially freezable fluid. Quality induction dampened compasses, such as the Cammenga 3H mentioned below, are rated to operate in temperatures as low as -50 degrees Fahrenheit.
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Mechanical vs. Practical Accuracy
All compasses, even the highest quality models, have some degree of inaccuracy in their measurements. Inherent, or mechanical, accuracy is a measure of how far from magnetic north the compass needle points. Most decent compasses will have a listed specification of their mechanical accuracy. However, the most mechanically accurate compass in the world can't take accurate measurements if you can't point and use it accurately. Lack of sights can make a compass difficult to point accurately and a compass without fine azimuth markings may as well be mechanically inaccurate. For this reason a good compass design attempts to help control the human error, as well as mechanical error.
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So Which Compass do I need?
Although there's an incredible variety of compasses on the market today, they can essentially be broken down into four major categories by their degree of practical accuracy. While many more models are available in each category, we've selected one or two common examples from each category to demonstrate typical features. Beware however of the many cheap knockoffs available that duplicate the appearance of popular, high-end models. Before buying, always do your own research to determine the true quality of a compass.
Ultra-Economy Models
This category accounts for the whole class of very cheap compasses. Often without any aiming devices, poor (or simply no) accuracy specifications and limited azimuth markings, these offer only the most basic directional information and are entirely unsuited for navigation. When manufacturers are unwilling to put their name directly on their product, it's usually a bad sign. These are nice curiosities and great kid toys to teach the basics, but for navigation they have little to no purpose.
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Baseplate Compasses
These compasses almost always instruct the user to point the arrow on the compass baseplate at a given terrain feature to determine it's bearing. Unfortunately, we humans are not physically capable of accurately pointing at a given spot without sights or some other mechanical aid. This leads to errors conceivably up to 5 degrees or more. Add the mechanical error of the compass itself and you'll find that these compasses are really only an option for the price-sensitive user, who doesn't plan to stray far from the beaten path. They are not especially well suited for deep-woods navigation.
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Sighting Compasses
Mechanically, these compasses might not be significantly more accurate than a quality baseplate compass. However, the addition of front and rear sights allows them to be accurately pointed at the target feature, removing most of the human error. The common versions are the lensatic compasses, typified by the Cammenga 3H (shown above on the left) and the mirror compasses, such as the Brunton Truarc 20 (shown on the right). Sighting compasses in this category usually offer practical accuracy of 2-3 degrees, allowing for meaningful accuracy and good navigational capability. This category likely represents the best trade-off between accuracy and cost for the average outdoorsman, with good quality models available for around $100.
Brunton Transits and Similar
While also sighting compasses, these are in a category of their own. With extremely high mechanical accuracy and a long sight radius, the Transit can take azimuth measurements within a mere 0.5 degree margin of error. An integrated bubble level enhances handheld accuracy. It can even be mounted on a tripod for maximum accuracy. More than just a compass, it also has a clinometer for measuring slope angles which is useful to those who spend time in avalanche country. It even has a bottle opener! Although, we would have to advise against drinking and navigating.
These compasses are employed by surveyors, geologists and other professionals who need maximum precision and are so accurate that the army uses them to direct artillery fire. However with the cheapest models costing around $400 and rising sharply from there, unless you spend significant amounts of time navigating the backcounty, or just want to have the absolute best, these are likely more compass than you'll really ever need.
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Use & Storage Considerations
While very reliable and accurate when used properly, there are some things to keep in mind when using a compass.
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Interference
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At the most basic level, all compasses are just a magnet on a swivel. Therefore, compasses can give erroneous readings if a nearby object interferes with the local magnetic field. High tension power lines are a prime example. Their strong electric field can interfere with compass readings at distances of nearly 200 feet. Electronics, such as phones and computers can also affect accuracy, athough from much shorter distances. Ferrous (magnetic) metals and, of course, actual magnets can also cause issues. Vehicles can affect accuracy from up to 60 feet away. Fortunately, we generally don't find a need for a compass next to our car or in a city filled with power lines. The more likely source of interference in practice, is one's personal gear.
Any steel will affect a compass in close proximity but small objects only need to be moved a few feet away to avoid interference. For hunters, setting your rifle on the ground behind you and taking a step forward, or a least slinging it behind your back is usually sufficient. For any measurements where high accuracy is required, magnetic objects and electronics should be removed from your front pockets before taking measurements. Belt buckles made of ferrous metals are another common culprit.
Most mapping and navigational tools are made of plastic. However, you could find yourself using metal ones, especially if you're improvising. If you use metal tools, be sure to keep them away from your compass while taking measurements. The images below demonstrate a case of interference by a tool. In the image on the left, the metal ruler is about three feet away and has no effect. On the right, the ruler was brought within about three inches and created a large error. This ordinary metal ruler affected readings from up to about 18 inches away. So when using a compass, remember, MIND YOUR METAL.
Storage
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For the same reasons that electronics, magnets and ferrous metals ruin the accuracy of your readings, they can also ruin your actual compass itself by changing or even reversing the polarity of the needle if left in close proximity for long periods of time. Therefore, when storing your compass, keep it somewhere away from electronics, magnets and ferrous metals. It doesn't need to be on the other side of the house. But a few feet of distance is desirable. Before starting out on a trip, you should also verify that your compass is reading within it's specified accuracy by checking it against a known azimuth.