Compass Accuracy

Just how accurate is your magnetic compass?

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.

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