This is the second part of a two-part series and you should read both parts for a more complete understanding of issues to do with choosing/buying a compass.
In the first part of this Buyer’s Guide to Compasses we discuss the two or three basic design styles/types of compasses, and issues associated with the accuracy you can realistically expect from a compass.
In addition to these different types of compasses, there are a number of features that may be present in various forms or absent entirely on each particular model of compass.
We discuss the most obvious of these below.
The larger the circumference of the compass card, the greater the distance between the degree markers and the more accurately you can use the compass. There’s not a lot of variation in compass size, but where there is a difference, the bigger it is, the better it is.
Compass Build Quality
This is a slightly subjective feature, and hard to evaluate on a brand new compass. But a better made compass will stay more accurate for longer, and won’t have parts come loose and start to wobble and wiggle. Any movements in any of the parts that are used to align the compass will of course detract from the compass’ ongoing accuracy.
A better built compass will also be more robust and resilient such that you can occasionally drop it or otherwise treat it in a less than perfect manner, and still function.
A better built compass will also be operable over a broader range of temperatures. Particularly if the compass is liquid filled, there will be a temperature at which the liquid will freeze, and somewhere above that temperature is the lowest temperature the compass will work happily at.
Compasses point (sort of) to the magnetic north pole. Unfortunately, this is not the same location as the north pole used by map makers and which most grid reference systems are based upon.
We’ll spare you the geometry of it, but the net result is that to switch from your compass’ reading of magnetic north to true north, you need to add or subtract an adjustment to compensate for the difference between the magnetic and true pole. This adjustment is termed the declination.
Compensating for declination is, on the face of it easy, but few things in life are truly easy, and there are three things to consider. The first thing, which confuses many inexperienced navigators, is that sometimes you have to add and sometimes you have to subtract the declination from the magnetic (or true) degrees indicated. Knowing when to add or subtract can be quite taxing of your brain, particularly in a high stress environment (which reminds us of the really big problem some people have with compasses – mistaking the north and south ends of the needle!).
The second challenge with declination is that it is not the same everywhere. It changes as you move around. For example, at present, in Coeur d’Alene, ID, the declination is +15°1′. Go east to Billings, MT, and the declination is +11°11′. Go southeast to Cheyenne, WY and the declination now is +8°44′.
That’s a lot of change in not much area. You can ignore declination changes over short distances (for example, the 140 miles, as the crow flies, between Coeur d’Alene and Missoula, MT has the declination change from +15°1′ to +13°52′ – a change of a relatively minor 1°9′. And the further south you get, the less the rate of declination change.
The third challenge is that the magnetic north pole is moving. It isn’t fixed. It is moving – and at an accelerating rate, currently of about 25 miles a year. In the American Redoubt region, that translates to a reduction of declination by about 9 minutes every year at present.
Anyway, now that you know more than you ever want to know about declination, back to the simple point. Some compasses allow you to automatically build in a correcting factor for declination. This is a very valuable feature, and saves you needing to struggle to remember if you should be adding or subtracting at any given time and situation.
Scales – Degrees and/or Mils
You probably already know that there are 360 degrees in a circle. Most compasses are calibrated in degrees accordingly, with 0° (or 360°) being at the ‘top’ for North, and so on.
There are other scales for angular measurement as well as degrees, however. There is a metric measurement, the ‘grad’ which is happily almost never used (there are 400 grads in a circle – about as stupid a number as 360, really). But the one you are more likely to come across is the ‘mil’. This is a subdivision of a radian (there are 2π radians in a circle – about 6.283, a number which seems awkward, but which has some benefits in geometry). There are 1000 mils in a radian, or 6283 mils in a circle.
Just to make this simultaneously simpler and more complicated, the 6283 mils are often rounded up to 6400 (why not 6300?) in this country, but in some other countries, may be rounded down to 6000.
There are 17.8 mils in one degree.
The use of the mil measurement has one useful feature – it helps you when estimating distances or sizes. If you know the size of something, you can tell its distance by the number of mils in size it appears to be. Or if you know the distance to something, you can tell its size the same way.
The way it works is that if an object is x mils wide, and 1,000 units of distance away from you, then that object is actually x units distance wide. For example, a 2 mil wide object that is 1500 yards away would be 2 x (1500/1000) yard wide – in this example, 3 yards wide.
You can also use this for estimating distances. For example, if you say that a typical man is 6′ tall, then if he is filling 12 mils, he would be 500 feet away. If 6 mils, he’d be 1,000 ft, and if 3 mils, he’d be 2,000 ft away. That’s why many rifle scopes have mil markings on them.
Or if you saw two mountain peaks in the distance and wondered how far away they were, and your map showed them to be 1 mile apart, and they registered as 100 mils apart on your compass, that would tell you they were 10 miles away.
A compass with both mil and degree markings is perhaps slightly better than one with only degrees, but unless you have a use for the mil calibration, your first priority should be to get the best possible compass with degree markings.
In addition to working out distance/size from mil angles, some compasses also have quick tables of degree angles subtended vs distance/size that you can use as rules of thumb, conveniently printed on them.
There are plenty of situations where you might be using your compass in low/no light conditions, and if the compass had either a tritium self-illuminated dial or a phosphor coating that would ‘soak up’ some light, eg from a flashlight, then give it off again for some minutes or longer, that might be very helpful.
A phosphor coating of course requires you to have some other light-source to activate it, although it is probably reasonable to assume that you would indeed have a flashlight with you.
The tritium coated compasses are nice, but tritium has ‘only’ an 11 year half-life. In other words, in a decade, it will only be glowing half as bright as when you bought it; in two decades, one quarter as bright; and in three decades, one eighth as bright.
We’re not saying you must get some type of self-illuminated markings on your compass, but if the price isn’t much more, it might be nice to have them.
Better compasses have some type of liquid in the compass housing. This damps the needle’s movement and protects it some from shock as well. If you are looking at a compass that is not liquid filled, you are probably not looking at a good quality compass.
In most cases, the more level your compass, the more accurate your reading will be. Accordingly, some compasses have a bubble level somewhere that allows you to check how level your compass is. This might be in the form of a bubble in the liquid that fills the compass housing (assuming the top of the housing is then slightly curved) or it might be a separate liquid bubble level to one side.
Clearly, this is a good extra feature to have.
The needle in your compass ideally wants to point more or less directly to the magnetic north pole, following the line of force that flows between the earth’s north and south magnetic poles. Near the equator, those force lines are pretty much parallel to the earth’s surface, but as you get closer to the magnetic pole, the force lines are curving inwards and downwards (or outwards and upwards) and so the needle wants to go off-center, off-balance.
This would interfere with its free swinging on its mounting point, and so compass needles are typically made unbalanced, with a compensating weight on one side or the other of the needle’s center, so as to adjust for the magnetic force lines wanting to force the needle up or down.
This means that a compass made for eg North America would not work so well in eg South America, because the balance correction swaps.
Some compasses have a clever mounting mechanism for the magnet separate from the indicating needle which makes it less sensitive to the shift in direction of the lines of magnetic force. This is useful not only for the international traveler, but also for everyone, everywhere, because a related benefit is that the compass doesn’t need to be held quite so exactly level in order for an accurate reading to be obtained.
Of course, these features all have costs associated with them, and remember from the first part of this compass buying guide, compasses don’t always need to be ultra precise.
The more accurate that compasses are, the better they will assist you with tasks such as locating a buried cache on your property or surveying work in general, but even a much less accurate compass can help keep you situationally aware when traveling through unfamiliar territory.
We find that Amazon has a good range of well priced compasses available. There are also specialty compass stores online, and your favorite outdoor retail store probably has a range of compasses that you can actually hold and physically choose from too.
Please note this is the second part of a two-part article about choosing a compass. Please also visit the first part to understand about the two major types of compass design and issues to do with compass accuracy.