The Compass

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The Compass
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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.

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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 earths
  magnetic field.
• Electronic compasses, with no moving parts, are
  made by measuring small currents in coils of
  wire, induced by the earths magnetic field.

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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

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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.

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Other Features Abound

• Distance Scales
• Magnifying Lens
• Slope Measurement
• etc

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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 its magnetic
  direction capabilities.

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A Look at Some Common Compass Types
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Zipper Pull Compass

• Good for generaldirection

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Lensatic

• 5 markings
• So-so for sighting
• Useless for plotting

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Baseplate

• 2 markings
• Good for map plotting
• Hard to sight ona distant object

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Mirrored

• 2 markings
• Good for sighting
• Good for plotting
• May have adjustable declination

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Sighting Baseplate

• 1 markings
• Best for sighting
• Good for plotting
• No declinationadjustment
• My personal favorite!

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Parts of a compass
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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.

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Needle Parallax
View from Above
View from Behind
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Parallax Side View
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Needle Parallax
Good
Bad
Keep the needle parallel to the meridian lines.
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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.

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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