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The Compass

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  • pg 1
									The Compass
             Some History
• As far back as 2500 BCE the Chinese knew
  that a loadstone on a piece of floating wood,
  would always point itself in the same
  direction.
• Later with the move from bone to iron
  needles, it was noticed that an iron needle
  placed near a loadstone would also take on
  these directional properties.
        The Modern Compass
• Modern compasses typically have a
  magnetic needle or capsule that floats freely
  on a central pivot, so that it can align itself
  with the earth’s magnetic field.
• Electronic compasses, with no moving
  parts, are made by measuring small currents
  in coils of wire, induced by the earth’s
  magnetic field.
           Angular Measurement
• A compass will have a angular measurement ring to
  observe the angle between the compass needle and the
  compass body.
• The most common angular measurement system is a 360°
  circle.
• Other systems include
   – Four 90° quadrants
   – Mills, a system that divides a circle into 6400 parts
   – Avoid these for wilderness navigation purposes
         Sighting Mechanism
• Many compasses include some mechanism
  for sighting to a distant point to measure the
  angle of the line to that object relative to the
  compass needle.
        Other Features Abound
•   Distance Scales
•   Magnifying Lens
•   Slope Measurement
•   etc…
                 Compass Uses
• In wilderness navigation a compass is used
  to do the following.
  –   General orientation to the 4 cardinal directions
  –   Taking a bearing to a distant object
  –   Traveling along a heading
  –   Plotting or measuring a bearing on a map
       • This is using the compass as an expensive
         protractor, and does not use it’s magnetic direction
         capabilities.
A Look at Some Common
    Compass Types
        Zipper Pull Compass
• Good for general
  direction
                 Lensatic
• 5° markings
• So-so for sighting
• Useless for plotting
               Baseplate
• 2° markings
• Good for map plotting
• Hard to sight on
  a distant object
                   Mirrored
•   2° markings
•   Good for sighting
•   Good for plotting
•   May have
    adjustable declination
            Sighting Baseplate
•   1° markings
•   Best for sighting
•   Good for plotting
•   No declination
    adjustment

• My personal favorite!
Parts of a compass
   Taking a bearing to an object
• Sight to the object with the compass.
• Turn the ring to align the orienting arrow
  with the red end of the magnetic needle.
• Read the bearing from the ring
  at the index line.
      Needle Parallax
View from Above   View from Behind
Parallax Side View
       Needle Parallax
      Good                         Bad




Keep the needle parallel to the meridian lines.
        Taking a back bearing
• A back bearing is taken looking back to
  where you took the original bearing.
• A back bearing is 180° different from a
  forward bearing.
• An easy technique is to align the south end
  of the needle rather than the north end.
     Classroom compass exercise
•   Pair up with another student
•   Take a bearing and a back bearing on each other.
•   Bearings should agree within +/- 2°
•   Try other positions in the classroom
•   Try using other types of compasses

								
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