How To Use A Compass For Scuba Diving

How To Use A Compass For Scuba Diving

Getting comfortable with compass navigation underwater might seem intimidating, especially if your initial dives mostly involved following the Divemaster without much self-navigation. Compass navigation is pretty simple in theory, but the mechanics can get tricky. But fear not! Getting the hang of it is easier than you think, even if you're feeling a bit rusty after your Open Water Diver course. Once you do, your compass becomes an indispensable tool for underwater exploration.


Parts of Your Compass

Underwater compass


• The core of a compass is a freely rotating needle magnetized to point north. This needle, or card, floats in liquid to offset the effects of water pressure. The card is painted with the north arrow and the cardinal points (N, S, E, W) and degrees. 

• Surrounding the card is the rotating bezel with index mark.

• Compasses also feature a “lubber line” often in red. It is important for accurate navigation that the compass is held so the lubber line is always a forward extension of your body.

• The side-view window allows you to hold the compass between your eye and your destination and read your course.


Side-Window and Bezel Methods

There are two simple methods for using your compass; both complement each other but can also be used individually.

  1. Let's start with side-window navigation. This method involves aligning the lubber line on your compass with your desired direction, waiting for the card to settle, and then turning the bezel until the double triangle on the edge (index mark, adjacent to the number zero) aligns with the north arrow on the card. As long as you hold your compass flat with the lubber line indicating your direction of travel, you should see the north arrow within the notch. If not, adjust until it aligns. However, you may lose your heading if you accidentally move the bezel underwater! 
  2. To mitigate this risk, it's best to combine both methods. Align the notch with the north arrow on the card, and remember the number displayed in the side window. If you've set the bezel correctly, the number in the side window will match the one at the front end of the lubber line. This means that when the bezel is adjusted accurately, the bezel and side window numbers will correspond. For example, if 160 is aligned with the lubber line, the corresponding number in the side window will also be 160. Using this method, you'll always know your heading, even if the bezel is accidentally bumped.


Koh Tao Underwater Navigation


Tips and tricks for using a compass underwater

  • The first step in using a compass is to know how to hold it so the lubber line is pointed in the direction you want to go and your body is in line with it.
  • Always hold the compass level. Tilting the compass will most likely make the compass card stick, so it doesn’t rotate. Meaning you could deviate from your course without knowing it.
  • Keep it as far away as possible from metal. Metal messes with the magnetism in the compass, potentially giving you a false reading.
  • Check it often. Again, don’t rely on your sense of direction. Just because you’ve read your compass once doesn’t mean you can ignore for as long as you “just have to go straight”. Check frequently to make sure you’re on course and make corrections if you’re not.
  • A Compass Never Lies!


Compass Navigation Techniques

You can use reciprocal courses to navigate back to your starting point after exploring the reef.

Method 1
• Point the compass's lubber line towards your destination.
• Turn the bezel until the double index mark aligns with the magnetic north needle.
• You are swimming in the correct direction if the needle aligns with the two index marks.
• When you reach your turn-around point, turn around until the needle aligns with the single index mark, and keep them aligned as you swim back to your origin.

Method 2
• Note the number directly across from your initial heading using the side window. For instance, if you started at 150, the opposite number would be 330. Keep 330 in your side window on the return trip to guide you back.

Both methods ensure you head in the correct direction.


Calculating Courses Using Numbers

If you're checking out compass info elsewhere, you might come across a trick that says, "Add 180, and if it's over 360, subtract 360" for your return course!

Say What?

  • Check your compass for the degrees. Most have markings every 30 degrees, like 30, 60, 90, etc. But remember, E is 90, S is 180, W is 270, and N is 0/360.
  • For the return course, add or subtract 180 to your current course. Keep the result between 0 and 360. So, if your course is 40 degrees, adding 180 gives 220, your return course.
  • For a right-angle turn, add 90 for a right turn and subtract 90 for a left turn. Then, adjust to keep the result between 0 and 360. For example, from 40 degrees, a right turn is 40 + 90 = 130, and a left turn is 40 - 90 = -50. Add 360 to negative numbers to keep it between 0 and 360, making -50 + 360 = 310.
  • Round off to the nearest 10 degrees and jot down your calculations on a slate. A small error will make little difference in recreational diving.

Remember: Add or subtract your desired course from your present course, adjust for turns, and ensure the final result stays between 0 and 360.


For the numbers-phobic 


Underwater Navigation


Another easy approach is to forget the numbers on your compass and focus on the cardinal points: North, South, East, and West. This simplification is beneficial when navigating back to your starting point, a common task for divers using a compass.

• If you've been swimming north, it's instinctive that your return course is south. Similarly, if your outbound course is northeast, your return course is southwest.

• Your compass likely has eight bold tick marks between adjacent cardinal points, such as between N and E. Pay attention to the third and sixth bold marks, which often have numbers (like 30 and 60). However, don't dwell on these numbers. Instead, consider them as additional bold tick marks, aiding in counting because each represents three ticks.

• Conceptualize a specific course as several tick marks to the left or right of a cardinal point. For example, "four ticks to the right of north" instead of 40 degrees. To return from this course, reverse the direction, substituting south for north. Likewise, if your outbound course is "three ticks to the left of the west," your return course is "three ticks to the left of the east."

• To quickly record courses on your slate, use "+" for "right" and "-" for "left." Thus, "four ticks to the right of north" becomes N +4. The return from N +4 is S +4, and from E -3, it's W -3.

• Forget about the bezel, the part you can turn on the compass. It's just there to help you remember numbers, which can be confusing. Instead, line it up with the lubber line or the direction you want to go. Easy, you have a simpler way to see where you're headed.


Here's a step-by-step guide:

  1. Check your buoyancy to stay steady in the water.
  2. Hold the compass to read it, with the lubber line pointing in the direction you want to go and your body is in line with it.
  3. Make sure the compass is level by gently moving it back and forth.
  4. Count the tick marks from the nearest cardinal point to the lubber line.
  5. And that's it!

Your course might be something like "N +4". Determining the return course and making turns are easy once you understand this system. What's the return course? S +4. What's a 90-degree turn to the left? W +4. And to the right? E +4.


Comparison of Methods

• The geek Way: If your heading is 290°, calculating the return course involves adding or subtracting 180. The numbers can get confusing... For example, 290 + 180 = 470? No wait, subtract 360 to get 110°!

• The Easy Peasy Way: If your course is E +2, your return course is W +2. Just swap E with W.

A Right Turn?

• The geek Way: For a right turn from a heading of 290°, add 90. So, 290 + 90 is 380..Wait a second, 380? Then I subtract 360 to get 20°. Tricky, indeed!

• The Easy Peasy Way: If your heading is E +2, a right turn is two ticks to the right of the cardinal point to your right, or S +2. No complicated math needed.


Mastering underwater navigation, Koh Tao.Thailand


Blending Compass Skills with Natural Tricks

Navigating underwater doesn't have to be all about numbers and technicalities. Here's a tip to keep your dive on track without constantly checking your compass:
Feature Mapping: Identify specific landmarks like channels between rocks or patch reefs to guide your return.
Sand Ripples: Shore divers can follow ripples, which run parallel to the shore and get closer together near the beach.
Current: If the current is on your right when you head out, it should be on your left on the way back.
Waves: Waves move toward the shore, offering a direction cue.
Depth: If you move from deep to shallow water, head seaward to find your boat.
Shadows: Note the sun’s angle and where shadows fall at the start of the dive for orientation.
Sound: While not always reliable, the sound of a live-aboard boat’s generator can help locate the boat in poor visibility.


Dealing with Compass Inaccuracy

• Remember the potential for deviation, especially around significant metal structures like shipwrecks, which can affect your compass accuracy. If you suspect deviation, move away from the object and ascend a few meters if necessary to recalibrate your compass.

• The card in your compass is balanced on a pivot. If your compass isn't completely flat, the card might brush against the top and get stuck, unable to rotate properly. A quick tip: give your compass a gentle jiggle. If the card moves, it was probably stuck before, meaning your compass was more flat than you thought. 

• Now, about holding your compass: most folks grip it in one hand and end up heading in the opposite direction. Why? Because they think they've got the lubber line pointed straight ahead when, in reality, it's a tad off to one side. Practice makes perfect here. Experiment with different ways of holding your compass to see what clicks!


Koh Tao diving


Feeling unsure about navigation? No worries! Underwater Navigation is a skill that can be learned by anyone. Regain your confidence and take control with a PADI Underwater Navigator course.

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