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The Compass
Although the most important navigational aid used in orienteering is the
human brain, one other navigational device which is allowed and is in
general use is the compass. Compasses are useful for taking bearings
and for orienting the map so that it is aligned with the terrain
- but it is possible, in most areas, to complete a course quite
easily and efficiently without a compass (an exception would be
a flat area poor in prominent features).
Most orienteering compasses have a fluid-filled housing which dampens the
motion of the needle so that you can use the compass without holding
it perfectly still. The compass needle is painted in two colours.
Assuming that the compass is held flat, the red end points to magnetic north
and the white end to magnetic south.
However, some things can go wrong with a compass - the plastic components can break, the
housing can develop a leak, the fluid within the housing may turn opaque over time and,
very rarely, the magnetisation of the compass needle may reverse, so that the
south end now points to north!
An interesting detail is that there are northern-hemisphere and southern-hemisphere
compasses. This is due to the fact that the magnetic field lines to which a compass needle aligns
point into the earth at the north and south magnetic poles. This means that in the
northern hemisphere, the north end of the needle is pulled downwards and so the south end
is counter-weighted to balance the needle.
Therefore, when you use a northern hemisphere compass in, say Australia, the south end
of the magnet is pulled downwards by the magnetic field and is also heavier than the
north end - resulting in a needle that catches and drags on the bottom of the
compass housing when the compass is held horizontal.
Compass Types
There are two main types of orienteering compasses:
- The Baseplate or Protractor Compass
This type of compass was invented by the Kjellstrom brothers during
the World War II era and consists of a rectangular baseplate, which
is marked with a red arrow pointing along the long axis, and a
rotating compass housing marked in degrees (360 degrees for the
full circle in most of the world, but 400 degrees on some European
compasses). Marked on the floor of the rotating compass housing
are an arrow and a set of lines parallel to that arrow. Additional
features may include a lanyard for attaching the compass to the wrist,
scale bars for measuring map distances along one or more edges of the
baseplate, a magnifying glass for reading fine map detail, and
templates of a circle and triangle for marking orienteering courses
on the map.
- The Thumb Compass
In the mid 1980s, a top Swedish orienteer developed an alternative
to the baseplate compass by reshaping the baseplate and adding a
strap for attaching the compass to his thumb. This compass is then
placed on the thumb of the left hand, which holds it on the map.
The advantage of this system is that the map and compass are always
read as a unit, the map is aligned more easily and quickly, plus one
hand is left free; the disadvantage is that running very accurately
on a bearing is more difficult. Personal preference usually
determines the type of compass that is used.
Compass Skills
Using either type of compass, there are two basic skills an orienteer
needs:
- Orienting the map
This is a simple skill and is probably the most important use of
the compass:
- Hold your map horizontally.
- Place the compass flat on the map.
- Rotate the map until the "north lines" on the map (a series of evenly spaced parallel lines drawn across the map, all pointing to magnetic north) are aligned with the compass needle.
The map should now be oriented to the terrain. This makes it much easier to read, just as text is easier to read the right side up than upside down.
- Taking a bearing
Every direction can be expressed as an angle with respect to north.
In the military and the boy scouts, this is called an
"azimuth", and bearings are expressed as a number of
degrees. Orienteers take the easy way out, by just setting the angle
on their compass and keeping the needle aligned, which in turn keeps
them going in the right direction. A simple set of step-by-step
instructions for setting a bearing on a baseplate compass is given below:
- Place the compass on the map so that the direction of the travel arrow
is lined up with the way you want to go.
- Turn the compass housing so that the arrows engraved in its plastic base are parallel to the north arrows drawn on the map (make sure the arrowhead points north and not south).
- Take the compass off the map and hold it in front of you so that the direction of the travel arrow points directly ahead of you.
- Rotate your body until the compass needle is aligned with the arrow on the base of the compass housing.
- Pick out a prominent object ahead of you along the direction of travel, go to it, and repeat the process (this way you can detour around obstructions but still stay on your bearing).
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These pages were last updated during May 2000 by Conor Creedon.
Comments and suggestions to conorcreedon@bigfoot.com.
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