Map, Compass & GPS

Map, Compass & GPS
Wild flowers along Fall Creek on the way to the Green Lakes - Oregon

Wednesday, March 14, 2012

Compass Accuracy

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 below 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, the 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 the capability of their compass but it certainly makes an impact.   If an inexpensive compass is purchased (generally less than $5) then the navigator should assume that 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

The most accurate and best choice would be a mirrored sighting compass such as the Silva Ranger model (below.)  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.  Two examples of excellent choices are shown below.  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 the map and compass classes that I teach, 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 towards 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?

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