Compass Navigation When Scuba Diving
I didn't look into the truth of the statement, but my dive instructor said that it was mandatory to own a compass to become open-water certified. I was annoyed by this for two reasons: 1) compasses are expensive and 2) I knew that long before I did a dive where navigation by compass was necessary, I'd have a dive computer. I guess it's the cost of playing the scuba game.
I do, however, believe that underwater navigation skills are important and so I'm going to cover the basics here. First off, all compasses have three basic features:
- A north arrow that rotates freely on a plane with degree marks.
- A lubber line (more on this later).
- A rotating bezel with north and south reference marks. Like item 1, there are degree marks along the perimeter.
Compass navigation involves determining a heading and then following it. With enough practice, you can combine measured distances with compass navigation to explore an area using a square, rectangular, or triangular pattern rather than having to backtrack.
On the off chance that you don't know, a compass has a moving part that always points north. When navigating underwater, the basic process is to face the heading you wish to follow and then align the lubber line so that it is parallel to your body. Once in position, rotate the outer bezel so that the north marker lines up with the north marker on the free-floating plane. Swim in the direction that keeps the two north markers in alignment.
To return, rotate the bezel so that the north marker lines up with the south marker on the free floating plane. Turn around until the two north markers once again line up. Swimming in that direction will take you back to where you started (assuming the current has pushed you to either side).
Advanced Compass Techniques
The real power of a compass comes from being able to navigate in shapes. This allows you to explore more of your area since you're not forced to backtrack along the path you initially took.
To navigate a square, make 90 degree turns every time you've traveled a specific distance. Of course, a square has equal sides so you'll need to be pretty good at swimming at a steady pace and watching your time or counting kick cycles. Use a similar technique to navigate a rectangle except that sides 1 and 3 need to be the same length as do sides 2 and 4.
With either shape and assuming you're turning counterclockwise, every time you turn, use your compass to position the north marker on the bezel at the 90 degree mark for the first turn, then 180 degree mark for the second turn, then the 270 mark on the third turn, and finally back to the point where both north markers are aligned.
Triangular patterns are also common, but the degree marks are 120, 240, and then back to having the north markers aligned.
As at the surface, there's no question that a compass is a useful tool for navigating. So when your dive instructor says you have to buy one, do as I did and suck it up. You can get your instructor back by asking him a bunch of questions about how to use it and making sure he gives you an actual opportunity to try it out while under his watchful eye.
One last tip: get a compass that is compatible with a large number of consoles and maybe it won't be a wasted purchase after all.