Saturday, April 23, 2011

Which Way North - Evaluating the Compass

It's been a while since we looked at the merits and shortcomings of specific compass designs.  Over the past six months or so I've been testing and evaluating a number of handheld compasses from various manufacturers.  It has been something of a grail quest, searching for the perfect compass.  Along the way I've learned a lot about compass design, accuracy and usability.  In the next series of postings we'll discuss specific compasses and perhaps help you select a compass for your needs.  Not all of what I evaluated were modern compasses; I've included a few examples in this evaluation that were manufactured over 60 years ago and represent designs that go back over 100 years.

Before we get started let me state that there is no such thing as the perfect compass.  No one design can meet all needs.  The best approach in compass selection is to pick the design that best meets the needs of the job at hand.  If you are running an orienteering course you'll probably want to pick a compass specifically designed for that sport.  However, if you are doing serious backwoods land navigation you'll want a direct reading compass that can give accurate azimuths to within 1/2 of a degree.  On the other hand, if you are a geologist doing stratigraphic mapping you'll need a compass that will also allow you to measure the strike and dip of rock formations.  Always pick the tool that best suits the task at hand.

Over the next series of postings in the Which Way North series I'll evaluate specific compasses on the following criteria:

1. Basic Design - How good is the basic design of the compass, particularly if the compass was designed for a specific function?  How well does the design meet the stated goal?

2. Features - Are any added features (luminous markings, baseplate scale markings, built-in clinometers, etc.) useful and do they add to the functionality of the compass?

3. Execution - Did the manufacturer do a good job building the compass?

4. Usability - How easy is it to use the particular compass in the real world?

6. Accuracy - How accurate is the compass (in degrees) when used as it was designed and intended to be used?

Much of this evaluation will be subjective.  After all, it is my blog so I set the rules.  However, my subjective evaluation is based on over 40 years of compass use as a geologist, topographer and Army engineer officer.  I may be a bit subjective in my evaluation, but it is a well honed subjectivity.

Other things you should understand about my evaluation criteria are:

1. I value accuracy above all else.  If a compass is not accurate (within it's design parameters) then it is a piece of junk.  I don't care who made it or how expensive it was, if it ain't accurate it's junk.  I think it is important to understand how I evaluate accuracy.  I have access to an azimuth station where I test all my compasses for accuracy.  An azimuth station is a site where survey points have been established and the precise azimuths between points (in relation to true north) is established.  Each compass is checked against three points each roughly 90 degrees apart.  All are checked for handheld accuracy and if possible, supported accuracy when mounted on a jacob staff.  Each compass is tested three times in each mode and the magnetic azimuth results averaged.  The averaged result is then adjusted for local declination (as determined by NOAA) and checked against the known azimuths between each of the survey points.  The difference between the surveyed azimuth and the adjusted magnetic azimuth is the compass accuracy.

2. Ruggedness of design is next on my list of criteria.  A compass that is marketed for outdoor use should be able to withstand that use and deliver reliable and accurate service.  I'm not saying that we should expect to use a compass as a substitute hammer, but it is not unreasonable to expect a land navigation compass to easily withstand the normal bumps and shakes that come with being used out of doors.

3. Declination.  A lot of compasses have adjustable declination scales.  I do not consider the presence or absence of an adjustable declination scale of any particular importance.  In my opinion adjusting for declination on the compass gives a false sense of security.  There are far too many variables in the declination equation to allow it to be handled by a coarse declination scale built into a handheld compass.  However, an adjustable declination scale can be useful for another purpose - adjusting out any error built into a particular compass or compass design.

4. You rarely use the compass all by itself so consider it part of a land navigation kit.  This kit should also include maps of the area you are working in, a plotting scale and a notebook and pencil.  All my evaluations are done in consideration of the compass as part of this kit.

Also keep in mind that I didn't test every compass available.  My schedule (and wallet) won't allow that.  However, I did obtain the most common examples available from the major manufacturers like Silva, Suunto, Brunton, K&R and a few others.  If you have interest in a particular model or style let me know.  I may already have it on my list for evaluation or may be able to get an example to test.

So stay tuned for the plain truth on compasses!


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