Finding Your Way Aided by a Compass

Divers who frequent dive sites with lousy visibility would shake their heads at those who enjoy good visibility and rarely have a use for a compass, so don't know how to use one. If you can see where you're going, what's difficult about keeping the reef on your right during the way out and on your left when you return? But, if you dive in low visibility, featureless sandy plateaus, or a wreck you are not moored over, you need a compass, and you need to know how to use it. If you do know how, read no further.

But pity the poor diver who leaves his dive boat in Truk Lagoon, only to miss the wreck and spend his dive swimming over acres of seabed. It happens!

Traditional compasses have a card marked with the 360° bearings that floats within the instrument on either a pin or oil and a needle that is free to rotate in the same way.

The card or needle wants to point to magnetic north. To use a compass correctly, you need to hold it so the fixed arrow marked along the side of the compass points in the direction you wish to travel. This arrow is called the 'lubber line.'

Ensuring that the compass card or needle can swing freely, rotate the moveable bezel so that its position is marked. Adjust your direction as you swim so that the needle remains in this position. When you want to return on a reciprocal course, you simply rotate the bezel through 180° and swim back. Some compasses have a sight glass that allows you to take a bearing, and you merely need to add or subtract 180° for the reciprocal course.

What can go wrong? Well, lots, actually. First, it is important to hold the compass out in front of you with two hands so that the lubber line really points in the direction of travel. If you wear the compass on your wrist, this almost ensures that you'll have the lubber line held at an angle instead. Also, holding the compass ahead of you ensures that a steel tank you might be wearing does not affect the compass magnetism, just as a steel or iron wreck will.

Many computers now offer an electronic compass mode. Take a bearing of where you want to head, and, keeping the electronic compass level, follow the bearing. Some will automatically give you the reciprocal bearing, but none, however high tech they might be, can take currents into consideration.

Unless you are heading exactly and directly into a current, it is inevitable that you will be pushed off course. You will need to vector a course, which is nigh on impossible. Never forget that Christopher Columbus traveled west, but hit the Bahamas rather than what was to become Manhattan. That's why it's important to navigate short distances between specific and noticeable points of interest in the undersea terrain. Slavishly following the compass will lead to mistakes, whereas this point-to-point piloting as you go adds gentle corrections.

Magnetic compasses are cheap to buy and low tech. Before you enter the water, either from the shore or a boat, get someone to point out roughly your intended destination so that you can take a bearing on it. Never forget that there are 360 degrees in a circle, and avoid trying to do anything clever like adding turns and trying to calculate the resulting geometry underwater. That's for armchair divers!

--john@undercurrent.org