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Blake Miller: Compass Accuracy and Why You Should Care

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Accuracy is critical when it comes to hunting rifles, writing news stories and court testimony, but what about that old reliable (?) compass? How accurate is it, and how important can that be?

Navigation expert Blake Miller explains what accuracy  standards to look for in magnetic compasses, and why the backcountry traveler should care.

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by Blake Miller

After speaking at a land navigation lecture at a recent sportsman’s show, an attendee asked my opinion about the accuracy of his compass. His compass was a new model and about the size of a nickel. The rotating body was a circular plate rather than a more traditional red magnetic needle. The four cardinal points of a compass (e.g., north, east) was the only bearing information displayed. His compass was very limited in the information it could provide. It could only be counted on to provide a general trend of direction. I told him the accuracy of a compass depends on many different factors.

Do not assume all compasses give the same information; they do not. The type of compass purchased impacts the dependability of its information. Compass selection is critical to accuracy.

Grandfather’s compass from decades ago may no longer be the best selection for the hiker. The image at left is of a compass made between 1910-1920. Though the dial is fairly detailed, the accuracy may be reduced due to the polarity of the magnetic needle.

In general terms, polarity is how the magnetic needle will react to the earth’s magnetic field. Over a period of almost 100 years, the compass’ magnetic needle may not move in relation to magnetic north as it did when new; that could mean the differences of several degrees.

To prove this point take a new baseplate compass and compare the two (do not hold them near each other.) A navigator can also compare it a new compass or a location where a street or trail is known to run true north.

My recommendation then is to leave the old antique compass in a place of honor at home. It is time to consider buying a new compass.

When buying that new compass what compass should the navigator consider using?

Unique to backcountry travel, the selection of the compass determines what tolerance for error is acceptable. Are 5° degrees of error acceptable? Remember 5° degrees of compass error over a one-mile hike will put the navigator over 400 feet off the desired course.

Most backcountry hikers don’t think that way in terms of compass capability but it certainly makes an impact. If an inexpensive compass is purchased (generally less than $5) then the navigator should assume the tolerance for error is high. Rarely do people consider the accuracy of their compass during a purchase. Having an accurate compass could mean the difference of making it to a critical rendezvous or trail junction.

So, let’s take a general look at the compasses on the market and discuss what options the navigator may expect. For simplicity, the categories fall into four general groups (listed in most accurate to least accurate):

•Mirrored sighting compasses

•Declination adjustable compasses

•Non-adjustable compasses

•Trend of direction

Silva Ranger compass

 

The most accurate and best choice would be a mirrored sighting compass such as the Silva Ranger model (left). It is declination adjustable. The sighting system is unique and the analogy is that it would be like using the sights of a rifle.

Such sighting inherently offers more accuracy. Expect to pay approximately $50. Please note that there are sighting compasses that offer a declination diagram etched in the compass housing but these models can not truly be mechanically adjusted.

 

Baseplate compasses that can be adjusted for declination is another solid option. Expect to pay approximately $15 – $30.  The owner’s manual provides instructions to adjust the compass.

Remember that the magnetic needle still points to magnetic north but the compass housing’s dial has been adjusted for declination. The accuracy is acceptable but not as good as the mirrored sighting compass. I carry a Brunton 8010 G in my hunting pack and a Suunto
M2 in my SAR pack (back up compass.)

Inexpensive base plate compasses ($5 to $10) that cannot be adjusted for declination are just obsolete; I acknowledge my prejudice. It is just not the best option.

Though more adequate measurements can be made, the problem is that one must calculate for declination (add or subtract) which may lead to math errors. Such calculation errors can be significant in the backcountry. Finally, in my map and compass classes, I have observed that students have a degree of difficulty dealing with declination calculations. Navigation should be kept simple and very clear.

The small ball compasses and key chain compasses will provide only a trend of direction. An example would be that the hiker is walking in a northerly direction but not in a specific direction such as 045° (degrees true) or North East. Fine accuracy is just not achievable. Such a trend of direction will get you back to a baseline such as a long road or river. This could be a suitable back up.

Another factor associated with the very old models or cheap compasses is that they lack a liquid-filled compass housing. The liquid dampens the movement of the magnetic needle when motion stops while sighting. That dampening capability is a very desirable characteristic because it allows the needle to settle and become steady quickly.

Finally, the correct technique of how the navigator holds and aligns the compass while sighting is very important. First, while following the owner’s manual instructions, adjust the compass for declination. Second, ensure that no magnetic objects are near the compass (such as a flash light, belt buckle or a knife). Third, while holding the compass at waist level, turn squarely toward a distant object. Hold the compass so that the direction of travel arrow points directly at the object. (Point the direction of travel arrow away from you; perpendicular to your body.)

Use a mirrored sighting compass the same way. It has many of the same features as the baseplate compass. Held at arm’s length when used, the sighting compass provides better directional control and bearing accuracy.

Education and knowledge of equipment function is a critical baseline that the hiker must attain. Having a solid foundation of Global Positioning System (GPS) operation, map and compass skills is absolutely essential for land navigation. Practice is important.

GPS receivers are accurate to +/- 15 meters or better. I don’t know anyone who would buy a used GPS unit that has an error of 100 yards; that level of accuracy isn’t acceptable.

Why then, would any navigator put his or his loved ones’ life at risk by not having a reliable, accurate compass?

Blake Miller has made a career out of staying found and knowing where he is at all times. His formal navigation training began when he joined the U.S. Navy in 1973. He served as an officer aboard several Navy ships over his

Blake Miller

twenty-year career; many of those tours included the duty of Navigator. Blake began working with satellite navigation systems at sea in 1976, culminating with the then-new satellite positioning systems aboard the Battleship WISCONSIN in early 1990.

In 1998, Blake started Outdoor Quest, a business dedicated to backcountry navigation and wilderness survival. Blake has taught classes to wild land firefighters, state agency staffs, Search and Rescue team members, hunters, hikers, skiers, fishermen and equestrians. He regularly teaches classes through the Community Education programs at Central Oregon (Bend) and Chemeketa (Salem, OR) Community Colleges.

As a volunteer, Blake teaches navigation and survival classes to students in the local school districts, and conservation groups. He is a member of a Search and Rescue team.

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5 comments
  • Alan

    Blake, you missed one of the most important issues relating to land nav.
    You must have a good USGS Topo map, or equivalent to the locale.

    Once you have your map (unless you are in the desert, with zero landmarks in sight) you can orient yourself to the map and then triangulate handily from visible landmarks. Only a doofus, or a 2lt on a night movement to contact, marches while watching his compass.

    How and why you shoot a back azimuth is more than we have time for here. Google is useful.

    Just remember, You don’t need a pocket theodolite, the bells and whistles can be helpful, especially in the mountains, but any compass that isn’t actually broken is better than none. A simple oil filled Silva $9.00 starter compass on a thong around my neck, far superior to the USGI lensatic, has taken me all over the world. Don’t loan it out, and don’t tell the other guys relying on the issue compass, why you get there faster.

    The Mohave around Fort Irwin doesnt count, But if you’re in the real desert, familiarize yourself with celestial navigation, because without GPS, you will get lost sure as Moses.

    Have fun, be safe, and carry two knives and two compasses.

  • Leon

    I like old stuff. And you’re correct about the compass in civilization. But we’re talking about the backcountry here, where accuracy can be critical.

  • vocalpatriot

    “Grandfather’s compass from decades ago may no longer be the best selection for the hiker.”
    I find this to be a counter-intuitive statement.
    In the old days, if a traveler was aiming for, say, a town, his old day compass and his experience got him there or
    he’d spend time wondering when he would arrive…after he passed it by..
    Today on any similar journey a traveler has a miriad of data points, roadways and signs, other towns and landmarks
    to inform him. A basic compass would likely suffice as long as one had a general idea which way his target is.

  • fifth_disciple

    Magnetic lines of declination account for significant variance from true north/south. Know what the variance is for the part of the country/world you are in.

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