Basics of using a Dive Compass
If you always dive with a guide, you might not care about using a dive compass. You follow your guide and end up back where you started. But, at some point, especially once you’ve gained experience, you’ll want to explore dive sites on your own. That’s when you’ll need a compass and know how to use it.
You’ll use your compass to find your way from one point to another, to navigate multi-leg routes of dive sites, and to find your way to your exit point at the end of a dive without surfacing first. A compass is an invaluable dive tool. They come in several different styles; the most common one is built into the gauge console. Others attach to the high-pressure hose above the console and a third type is worn on your wrist. The components are always the same.
Four components of a dive compass you should know are:
Lubber line
It marks the direction you are traveling. Always hold the compass so that the lubber line is perpendicular to your shoulders and parallel to your line of sight.
North arrow 
It points in the direction of magnetic north. If you rotate your compass, the arrow will always point to the north.
Rotating bezel with reference marks 
It contains two reference marks, primary and reciprocal. You use the reference points to mark the position of the north arrow relative to the lubber line. The two primary marks provide a reference, which allows you to navigate in a straight line on the outward leg of a reciprocal course. The reciprocal mark will guide you back to where you started.
Degree calibration marks 
The marks read from zero to 360 degrees. You use these marks when navigating a multi-leg underwater course and when going from one known point to another. The calibration marks may vary from brand to brand in the way that the marks are displayed, but the function is the same.

Dive compasses are easy to use. No matter what type of course you are navigating, you always hold the compass in front of your body, slightly below eye level. The lubber line is positioned perpendicular to you shoulders and pointed straight ahead in the direction you want to go. Always hold the compass level.
To navigate, you must establish a course. Here is how it’s done:
1. Point the lubber line in the direction you want to travel.
2. Rotate the bezel until the primary reference marks are directly over the north arrow.
3. Your normally establish course from the beach just before entering the water or the dive boat anchor line after descending.
4. To maintain course under water, just swim forward, keeping the north arrow between the primary reference marks.
5. If you stray off course, you’ll see the arrow move to one side or the other of the reference marks.
6. To get back on course, swim to the left or right, depending on which way you strayed, until the north arrow is again between your reference marks. As long as the north mark remains between the reference marks, you’re on course.
7. To return to your starting point, simply reverse your course. Slowly rotate your body until the north arrow is in line with the reciprocal reference marks.
8. Swim forward, keeping the north arrow lined up with the reciprocal marks and you should end up back where you started.

There are other factors you should be aware of when using a compass. One factor is the current. A strong cross current makes it difficult to maintain an accurate compass heading. Be aware of the current and compensate for it when using a compass. Also, be aware that concentrations of metal will hinder the accuracy of the north arrow. Be careful while diving wrecks and keep metal objects (like dive knives) away from the compass.
Sometimes you will have to rely exclusively on compass navigation, but when feasible combine your compass readings with natural navigation techniques. The best way to learn compass basics is to enroll in advanced class or navigation specialty class at your local dive shop.

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