How to navigate with a compass

My first open water dive was in Littleton Lake somewhere near Staines on 4 December 1983. I felt prepared, excited, even confident. After half an hour in a 5mm wetsuit in 3ºC and two-metre vis, we surfaced right back where we started. I was now an expert in disorientation and hypothermia. I was also convinced that I would never have what it takes to become an instructor – sonar. “I’m glad you knew where you were going, ‘cos I didn’t have a clue,” I said to my instructor as we reviewed the dive.

Learning to navigate underwater is a fundamental step that enables your confident progression from novice to experienced diver. We navigate every day of our lives without compasses and even occasionally without sat nav. We remember land marks, memorable places and orientate ourselves with the straight lines of roads and pavements that don’t change appreciably in orientation because gravity keeps us on the ground. But when traffic bulletins report visibility of 50 metres or less on motorways, you’d think the end of the world is nigh! It’s not surprising then that some people are put off trying UK diving when they assume that it’s all about simultaneously achieving neutral buoyancy and orientation in poor visibility while in an environment where there are few, if any, naturally occurring straight lines to follow.

My approach to underwater navigation is to spend the minimum amount of time consulting my compass and the maximum amount of time taking stock of the environment. Navigating by the natural features of a site is known as ‘pilotage’ and is an important part of navigation because your compass is not always reliable – for example in among wreckage or other magnetic influences. In this article we’ll review the use of your compass.

Using a compass – step by step

• The ‘direction of travel line’ (some people call it a ‘lubber line’) is the centre line of the compass. This must always be in line with your line of sight and line of travel. The restriction in suits and BCD may make this difficult to achieve with a wrist-worn compass. A similar problem can occur using console-mounted compasses where the high-pressure hose is too short (hoses longer than the usual 90cm are available).

• With the exception of the Suunto SK7, most compasses must be held perfectly level in order for the compass to rotate. The restriction in your field of vision, particularly at the bottom of your mask, may force you to tip the compass so that you can see it better. In my experience this factor is the most common error when using a compass.

• Use the sight glass on land to get an accurate start bearing and then set the bearing on the leading edge of the compass using the bezel. To stay on course you must keep the lubber line in line with your body and the north pointer lined up with the north marker on the bezel.

• The simplest type of course is the ‘out and back’, or reciprocal. In other words, if your bearing out was 100°, the reciprocal course will be always be + or – 180°, in this case 280°. Now I hate maths at the best of times and underwater it’s even worse. When you want to return, simply leave the bezel where it is and turn around until the south pointer is lined up with the north marker.

• The drawback with the reciprocal course is that the return leg often involves swimming through the silt you kicked up on the way out. A square or triangular course can keep you swimming through clearer water and still bring you back to where you started. For a square course, choose your initial bearing and set it on the leading edge of the compass with the bezel. If you want to turn right simply line up the east pointer with the north marker on the bezel. Your next turn will line up the south and the third and final turn will line up the west pointer with the north on the bezel.
 
• Leaving the bezel set on the original heading reduces the chances of errors underwater. If you get confused at any point during the dive, stop swimming, turn until the north marker is once again lined up with the north on the compass and you will be facing in the same direction that you were when you set the compass bearing on land. This will help you get your sense of orientation back as you realise where you are in relation to the landmarks that you saw before you submerged.

• A sense of direction is half the battle, so is estimating distance underwater. There are two approaches, you can either count fin kicks or use elapsed time. Both have their drawbacks. Counting fin kicks is a bit like counting perforations in a tea bag; tedious, uses too much brain power, and you’ll probably lose count. Also, if there is a current you’ll travel further with each fin kick when you go with the current than when you kick against it.

• Using elapsed time also works well but time marches on if you stop to look at something. Unless your finning rate along the bottom is constant time can be difficult to judge.

• If you are a bit out of practice with compass work then spend a bit of time practising the bearings on dry land first.