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39
The Compass
Instrument navigation is the most accurate type if done
properly. The primary navigational instrument of divers is the
magnetic compass: a device consisting of a fluid-filled housing
containing a rotating disk called the compass card, which is
visible under a clear lens. The card displays numeric markings
that reflect the 360 degrees of a circle, and it contains a small
magnet that seeks magnetic north — the northern extreme
of the earth’s magnetic field. This magnet is positioned at an
arrow, so the arrow always points north. Magnetic north is
different from true north, and although this must be accounted
for in long-distance navigation, it is not a concern for divers
who operate within the small geographic areas of recreational
dives. The lens of a compass contains a red or black line called
the lubber line, which allows the diver to select and maintain a
direction of travel. On most compasses the lens is surrounded
by a rotating bezel that is also marked with the degrees of a
circle and has two, large index marks at 0° and 180°.
To use a compass most effectively, the diver should point
the lubber line in the direction he intends to travel and read
the number at the bottom of the compass. This number is the
magnetic heading in which the diver will travel. The compass
must be held level, since tilting it will cause the card to hit
the compass’ sides, preventing it from turning freely. The
compass must also be pointed directly forward, in a straight
line between the diver and his destination. Make sure no
metal objects are near the compass as these can attract the
north-seeking arrow, skewing the compass’ reading.
Using The Bezel
While watching the small lens at the base of the compass
and ensuring the desired numerical heading stays in
the window, the diver should swim straight in the
direction the compass is pointing. This is fairly hard;
another, potentially easier, method is to use the index
marks on the bezel to establish a quick reference. This
is done by pointing the compass in the direction of
travel, then rotating the bezel until its 0° index mark is
lined up directly over the north-seeking arrow on the
compass card. Now the diver can swim along in the
proper direction and periodically check to ensure the
north-seeking arrow is still aligned with the index mark.
This quick reference method is generally much easier
than trying to hold the compass at eye level to sight the
heading while keeping the compass level and swimming
in the direction it is pointing.
The bezel can also be used to quickly and easily plot a
return course. If the diver simply turns his body until the
north-seeking arrow lines up with the bezel’s other index
mark (the 180° mark), he will turn 180 degrees, facing in
exactly the opposite direction in which he was swimming.
It is important that the diver rotates his whole body and
not the bezel or the compass. Of course, it’s also possible
to set a course in the opposite direction by calculating a
new heading: Add 180° to the original heading if it is less
than 180°, or subtract 180° if the original heading is equal
to or greater than 180°.
Compass position is important; wrist-mounted models
are designed to be used with the arm bent 90 degrees.
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