Mar 122013
 
A typical medium quality sighting compass.

A typical medium quality sighting compass.

This is the first part of a two-part article on choosing/buying a compass, the second part talks about compass features, and we’d recommend you read it after this first part so as to have a more complete understanding of what to look for when selecting a compass.

These days compasses seem very old-fashioned.  If people wish to know which way is north, they can look at their GPS; and if they have a compass at all, it is as likely to be a digital compass as a traditional magnetic compass.

Indeed, the prevalence of GPS units has made the underlying entire skillset of map reading and direction finding, and the need to even know where north is, apparently obsolete.  You just enter a location into your GPS then follow its arrow to get there.  You no longer need to know where you are, or even where your objective is, or what heading to travel along.  All that matters is to follow the arrow.  Even better still, if you have to make a detour due to some obstacle along the way, the arrow is continually updating and recalculating, always pointing you the most direct way to your objective.

The Problems with Modern Digital Navigation Devices

But, as wonderful as they are, it is not prudent to rely on modern high-tech devices when planning for a future Level 2 or 3 scenario.  It is very likely, in such cases, that the GPS service will be degraded or fail entirely.  In addition, sooner or later, the electronics inside your high-tech devices will fail, either a ‘natural/normal’ failure or one perhaps induced by an EMP event; and whatever the cause, you probably don’t have the knowledge – or the spare parts – to repair them when they fail.  Maybe it will be something else equally inevitable – at some point, you’ll simply run out of batteries.

While modern high-tech devices are subject to all these possible failure conditions, a regular ‘old fashioned’ analog compass is a gloriously low-tech device that is very hard to break, and moderately easy to repair.  By all means have a selection of high-tech navigation aids, but ensure you have some compasses too to get you as close as possible to guaranteed availability of navigational aids in the future.

Note also that neither GPS units nor digital compasses are very accurate.  Our sense is that digital compasses are generally accurate to about 5 degrees at best, and may be much less accurate than that.  Never mind that it displays a nice exact seeming number of degrees, the underlying sensor is not all that accurate.

GPS units can only show heading data based on the change in location between now and their previous location reading, which, particularly if you are walking or moving at slow speed in a vehicle, embodies errors to do with both the location fixes, and if the distance between the two fixes is small, then the accuracies become a large percentage.  As you hopefully already know/realize, when you are stationary, a GPS has no idea which way is north.

Different Requirements Call for Different Compass Capabilities

There are two main purposes for a compass.  The first is obvious – for finding your way to your objective when traveling somewhere.  The second use you may have for a compass is to assist when locating an object – possibly yourself, or possibly something else.

These two purposes place different demands on a compass, and – perhaps surprisingly – in most cases a navigational compass need not be as accurate as a locator type compass – particularly when it is used in conjunction with a detailed map of the area you are traveling through.  Indeed, compasses and maps are almost inseparable partners, with both being much more useful when used together with the other.

That’s not to say that more accuracy is ever a bad thing, but more accuracy usually is associated with more cost, and an outdoor hiking/backpacking type application for a compass can sometimes result in compass damage or loss (ie by dropping it), so it can make sense, depending on the nature of your travels to sometimes leave the ‘best’ compass at home and stick to using an expendable type compass for outdoor navigation.

Compass Types

There are two main types of compass.  They both have a needle, and both point to magnetic north.

The major difference between them is how you can use them ‘in the field’.

baseplatecompassbBase Plate Compasses

‘Base plate’ type compasses are designed to be best used on a map.  They are flat, with the compass itself usually set into a rectangle with some straight edges and lines which you use to align to the relevant parts of your map.  This rectangle is the ‘base plate’ implied by the name of the compass type.

There might be scales on the edges to help in map reading, and often the rectangle shape is made out of clear plastic so you can see to the map underneath.

This illustration of a K & R Baseplate Compass shows how a typical baseplate compass is designed.

The base plate compass is good for using with a map and working on bearings and headings.  But how do you then take that information and use it to work out where to travel, ‘in the field’?  That’s where the second type of compass comes into its own.

compass_sightingbPrismatic and Lensatic Sighting Compasses

These types of compasses have an arrangement whereby you can simultaneously see a compass reading on the compass and also line it up with an object around you.  You sight the compass on an object – maybe a mountain peak, a tree, whatever; and then, without moving yourself or the compass, read the bearing to it from the compass card.

Alternatively, if you know you should proceed in a certain direction, you can line your compass up to that direction, then look through the external sighting slot and choose a distant object to proceed towards that will mark your appropriate direction of travel.

The illustration here gives an example of a Cammenga Model 27CS Olive Drab Lensatic Compass and shows how it can be used to simultaneously display the compass reading and align it to an external object.

These compasses are excellent for fieldwork, but not so good to use on a map.

 

hybridcompassbHybrid Compasses

Increasingly, there are good compasses now being sold that can work both on a map and in the field, and for most people, this ‘one size fits all’ design approach makes them a good choice.

The illustration shows a high quality Suunto brand hybrid compass.

Compass Accuracy

The concept of accuracy is often misunderstood.

For example, in this compass listing, which is the most popular prismatic compass on Amazon, note one of the first reviews (giving the $10 compass an improbable full five stars) which says

If you are looking for a direct sighting compass with 1 degree (or better) accuracy, that is quick to use and virtually foolproof, this is the bargain of the decade.

Currently, 245 out of 250 people found the review helpful.  We are not among them.

The review’s claim that the compass has a 1° or better accuracy is total nonsense.  The compass scale is only marked in 5° increments, so there’s no way it could be five times (or more!) accurate than its own scale.

There are at least four sources of error in reading a compass, and two of those four are independent of the compass itself.

Error 1 – Mechanical

The first compass source of error is mechanical.  Maybe the needle sticks a bit – chances are you can move the compass very slightly and the needle will follow, it likely needs some force to dislodge it from where it had settled, and maybe it then sticks again before settling at the exact new point.

Maybe the needle is not perfectly balanced (indeed, it is probably deliberately not perfectly balanced) and you’re not holding the compass perfectly level, so that makes the heavier end want to drift downwards and scrapes along the bottom of the compass card, or points upwards and scrapes along the top of the viewing glass.

Maybe the outside bezel has a bit of slop in it relative to its position to the internal markings.  Maybe the sighting posts also have a bit of wobble or have been slightly bent.

All these issues can add to the mechanical lack of precision inherent in a compass.

Error 2 – Precision

The second source of compass error is the precision with which the scale is marked and aligned with the compass needle.  Maybe when the markings on the compass card were printed the printing wasn’t exactly aligned.  Maybe then the needle isn’t exactly centered in the card.  Maybe you’re looking at it on an angle too, introducing further error.

Even with these possible error sources minimized, the best case accuracy is probably no more than half of one marking unit.  If the compass is marked in ten degree units, maybe you can guess when the needle is halfway and fairly say ‘this is probably about 5 degrees because it is sort of in the middle’ but you know that there’s no way, with only ten degree markings, you could claim with any confidence a degree measurement to within 2 or even 3 degrees, based on the scale errors alone.

Error 3 – Magnetic Interference

Now for the two major external sources of errors reading a compass.  The first of these is the possible presence of stray magnetic fields or metal that might bias the compass reading.

What’s that in your pocket – your cell phone?  And on your hip – your pistol?  Both of those will impact on the accuracy of your compass reading, as will other metal objects or magnetic fields nearby.

Error 4 – User Errors

The second source of external error is – let’s be as polite about this as we can – you.  The person using the compass.  Maybe you’re not aligning the compass exactly with whatever bearing point you are taking a bearing from/to.  Maybe you didn’t wait for the compass needle to fully settle.  Maybe you just misread the number on the compass card.  Human error is always potentially present.

So how accurate is your compass?  As you can see from the four sources of errors discussed above, that really depends on how skillfully you are using it, and how careful you are to search out and eliminate some of the possible error sources.

We use a rule of thumb that whatever resolution is shown on an instrument’s scale is probably indicative of what the instrument’s capabilities are.  This rule of thumb is sometimes invalidated with digital devices, where it is inexpensive and simple to add extra digits to the digital display, to imply an accuracy that is completely absent.  But for analog compasses, it is probably an acceptable rule of thumb to say that most of the time, whatever the resolution on the compass card is will be about the same as the best resolution you can hope to achieve in the field.

So the highly rated compass with the claimed 1° of accuracy we started off by citing?  With 5° markings, the best you could hope for in a perfect world is 2.5°, and probably, in real life, after allowing for the other three types of errors above, you should consider it to be accurate to about 5°.  That’s a great deal less accurate than incorrectly claimed in the review.

Summary

Good quality compasses can give high quality results.  We’d think that obvious, but the $10 compass that was given a five star review and claimed to be five times more accurate than it truly is, and which received 245 approving responses out of 250 total, suggests that we should revisit the obvious.  🙂

When buying  a compass to help you locate yourself or other objects (ie ‘surveying’ type applications), you should choose one with the most accuracy.  This is implied by its marking resolution, and further implied by the size of the compass card (ie the diameter of the compass mechanism), and by it being (or not being) a recognized valued brand name and at variously a high or low price.  Yes, you generally get what you pay for, with compasses as with everything else.

But if you are simply seeking to navigate from Point A to Point B, and with a map as well as a compass, a less accurate instrument is fine.

This is the first of a two-part series on buying compasses.  Please visit the second part for a discussion on specific compass features.

  One Response to “Compass Buyer’s Guide 1 : Compass Types”

  1. […]   Compass Buyer’s Guide 1 : Compass Types […]

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