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Components of a Compass
By Sgt. Tygh Thompson,
Washington County Sheriff's SAR Coordinator
THE
MAGNETIC COMPASS
WORKING PART
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Magnetized needle
Red end always points to Magnetic North
Magnetic North and True North are not the same!
The difference between the two is called Declination and is
expressed in degrees.
THE ISOGONIC CHART
Declination is determined by calculating the difference
between true north and magnetic north.
True north is of course the north pole, magnetic north however fluctuates
around the
Hudson
’s Bay area. Periodically
Isogonic charts are published which list the current magnetic deviations
.
OTHER PARTS OF A COMPASS
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Base plate
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Azimuth ring or Housing with Degree Marks
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Orienting arrow or Needle Box
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Orienting lines
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Direction of travel arrow
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Sighting Mirror with sighting line and notch
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Scales and Grid readers
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Clinometer
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Declination adjustment screw and tool
UNITS OF MEASURE
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Degrees there are three hundred and sixty degrees in a full
circle
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Minutes every degree has sixty minutes
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Seconds every minute has sixty seconds
WHAT
IS A BEARING
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A bearing is any
direction expressed in degrees, in relation to True North.
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For example each of the four most common directions has a bearing
North
= 0 or 360
South =
180
East
= 90
West = 270
SIGHTING A KNOWN BEARING
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Turn the Azimuth so that the desired Bearing can be read at the
direction of travel arrow
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Without turning the Azimuth any more and using the mirror, turn
the whole compass until the red end of the needle is “Boxed”
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Then using the sight notch you can identify a target on the
Bearing you entered
HOW TO TAKE A BEARING
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Bearings are usually read at the direction of travel arrow
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Now you can turn the Compass so that the direction of travel arrow
is pointed in the direction or at the target of your choice.
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Now you must “Box” the needle.
This means you must turn the azimuth ring on the base plate until the red
end of the needle is centered within the orienting arrow.
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Proper use of the mirror and the sight notch will greatly increase
your accuracy. Remember if you
turn the base plate while trying to center the needle you will then be off
target and your bearing will be no good.
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Once the Needle is “Boxed” and the direction of travel arrow
is on target the bearing can be read at the direction of travel arrow
OTHER TIPS
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Be patient Compass work is a science not an art
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Always use the lanyard, if you lose a compass you are in trouble
 Congratulations!
You are now a wise woods owl.
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