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 generaldirection

Lensatic

• 5° markings

• So-so for sighting

• Useless for plotting

Baseplate

• 2° markings

• Good for map plotting

• Hard to sight ona 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 declinationadjustment

• 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 ParallaxView from Above View from Behind

Parallax Side View

Needle ParallaxGood 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