How To Use An Underwater Compass
If you're like a lot of us, compass navigation is one of those skills you're too embarrassed to admit has got you beat. If Boy Scouts can do it, why can't you?
You'll be glad to know it's not entirely your fault. Blame the Babylonians, who divided the circle into 360 degrees. And blame the way the compass is taught. You're asked to calculate your direction based on that unfamiliar numeric system instead of the north, south, east and west that we understand intuitively. Blame the fact that compass features aren't universal, then blame the way most of us scuba dive--blindly following a dive guide or relying on natural navigation cues. On top of all that, a compass is harder to use under water because it has to be held level and steady. That's right: Blame the ocean, too.
For the compass-phobic among us (and you know who you are), we've got an easier way to navigate, one with none of the math and, for most scuba purposes, every bit of the precision.
Gear Guide: Anatomy of a Compass
The card: This round piece of magnetic material floats inside the body of the compass and always points north. Look for a simple, uncluttered display with the cardinal points and unadorned tick marks between those points, though the card may have degree numbers printed on it.
The lubber line: A lubber line on the body is just a sight to help you point and aim the compass.
The bezel: This is a ring you can rotate to mark your course, usually by framing the position of north on the card. The bezel may be the biggest source of confusion in numeric compass navigation because there are two styles--direct reading and indirect reading--and they indicate headings in different ways.
Side window: A side-view window allows you to hold the compass between your eye and your destination and read your course, though this often requires an additional (read: confusing) set of numbers. And you can see only numbers, not tick marks, through the tiny window.
Ignore the Numbers
To begin with, forget the numbers and build on what you know, the cardinal points of north, south, east and west. This simplification will be especially helpful in finding your way back to your starting point, the use divers most often make of a compass.
For example: If you've been swimming north, you know immediately and intuitively that your return course is south. If you've been swimming northeast, your return course is southwest.
Your compass will probably have eight bold tick marks between two adjacent cardinal points--between N and E, for example. (Some will have small tick marks between the bold ones. Try to ignore them.) The third and sixth bold marks will probably have numbers (30 and 60 in this case). Ignore the numbers. Think of them only as extra-bold tick marks. They can help you count, because each represents three ticks.
Think of a particular course as a number of tick marks to the left or right of a cardinal point. Four ticks to right of north, for example, instead of 40 degrees. (The bolder tick mark, where the number is, helps you count because you know it is the third tick to right of north, and one more is four.)
What's the return course? Since the card always points north, the opposite of "four ticks to right of north" is "four ticks to right of south." The only change you need to make is to substitute the opposite you already know, south for north. Likewise, if your outbound course is "three ticks to left of west," your return course will be "three ticks to left of east." Just substitute east for west.
To write courses quickly on your slate, use + to indicate "right" and - to indicate "left." That way, "four ticks to right of north" becomes N +4. The return from N +4 is S +4. The return from E -3 is W -3.
Ignore the Bezel
The bezel — the part of the compass you can rotate with your hand — is a Band-Aid on a self-inflicted wound. It's just a memory aid to help you cope with an unnecessarily complicated numeric system. Depending on the style of compass, it may have its own numbers and markings which only add to the confusion.
Our simplified system doesn't use the numbers, so you don't need the bezel. To make it less distracting, rotate it until it coincides with the "lubber line." That's the fixed line that points to where you are going or facing. Or, if you like, you can rotate the bezel to correspond with your course, just as you would in the number system, for a quick visual reference.
Step-by-Step: Finding and Following a Heading
1. Check your buoyancy. Become motionless, or swim forward steadily.
2. Hold the compass where you can read it, and with the lubber line pointed toward your target.
3. Check that the compass is level by rotating it slowly back and forth. The card should remain stationary.
4. Count the tick marks from left or right of the closest cardinal point to the lubber line. Then repeat Step Three to be sure the card is still swinging free and therefore accurate.
And that's it. Your course is, for example, N +4. Finding the return course and making right-angle turns are simple matters of substitution. What's the return course? S +4. What's a 90-degree turn to the left? W +4. To the right? E +4.
Bottom Line
It's that simple. But is it too simple? Can you really navigate accurately just by using tick marks? The short answer is "yes."
If you were able to stay within one or two degrees of your course, and if you were going a long distance, the greater precision of the number system would be valuable. But think of how hard it is to hold that small dive compass level and steady and how hard it is to swim a straight line at the same time.
In practice, most of us can't read a bearing or swim a course more accurately than within one tick mark. Given the short distances we need to cover under water, that's plenty.
In fact, the tick-mark system is the basis of the numeric one and the way mariners navigated for centuries. You've probably seen a picture of a traditional compass with a "compass rose" sunburst pattern of tick marks, but no numbers. The numbered compass only became common in the 20th century when bigger and steadier ships became capable of steering to and holding a course to within one or two degrees.
By the Numbers, Recruit!
So what happens if your crusty ex-Navy instructor is barking at you to use only numbers? The rules are fairly simple even if following them is not.
1. Note your course in degrees. Most compasses have numbers marked every 30 degrees; for instance, 30, 60, 90, 120, etc. But note that many compasses substitute E for 90, S for 180, W for 270 and N for 0/360. This 0/360 mark is usually also marked by an arrow or the company's logo. Let's say your course is 40 degrees.
2. To find your return course, either add or subtract 180. Use the number that gives you a result between 0 and 360. In this case, you'd add 180, because 40 + 180 = 220. 220 degrees is your return course.
3. To make a right-angle turn, add 90 to your present course for a turn to the right, or subtract 90 for a turn to the left. That gives 40 + 90 = 130 for a right turn and 40 - 90 = -50 for a left turn. Now add 360 (which is the same as adding 0) to get a number between 0 and 360. -50 + 360 = 310.
4. Check your addition and subtraction--better write it on a slate. Rounding off to the nearest 10 degrees will simplify addition and subtraction. Over the short distances typical in recreational diving, an error of five degrees or so won't matter.
Got that? It may help to remember this general rule: To calculate any course or bearing, add or subtract the desired course from your present course (add for right turns, subtract for left turns), then add or subtract 360 as necessary to get a result between 0 and 360.
OUR ADVICE: KEEP IT SIMPLE. With our simplified navigation system, all you really need is a compass body with a clear lubber line on it, a floating card with N, S, E and W plus tick marks between--and nothing else. Most scuba compasses, unfortunately, are loaded down with features designed for use with the number system, and retain only vestiges of the cardinal points system. Sometimes you'll have to look hard for the N, S, E and W and for the tick marks but they are usually there. If you can train your eye to see the traditional system which is hidden among the numbers you can quickly learn to navigate with confidence. Then, when you're comfortable with your compass, you can return to the number system, the bezel and the rest of it if you find a need for them.
One feature that is vital in an underwater compass is tolerance for tilting. If the compass body is not held level, the card can't swing freely and it will not point to north--you'll get a false reading.
To compare two compasses for tilt tolerance, hold both compasses flat against a slate, then tilt the slate until it is nearly vertical. Now rotate the slate 90 degrees. Neither card will be able to swing freely. Slowly tilt the book or slate back toward horizontal, watching to see which card swings freely first.
All compass navigation, regardless of whether you use degree numbers or tick marks, is based on the card's magnetic attraction to north. Though the compass card appears to move as you change course, in reality, the diver is only moving the body of the compass around the free-floating card. It's this consistent alignment to north that makes the compass a useful tool for determining and monitoring a path of travel.
Where To Mount the Compass
It's common to see a compass incorporated into the instrument console. It's convenient, but the fairly short high-pressure hose makes the console awkward to hold so that the compass is both level and easy to see.
Mounting the compass on your wrist or on a retractor clipped to your BC makes it easier to manipulate. Even better is to mount it in the corner of a small slate. The edge of the slate works as an extension of the lubber line, making it easier to point the compass accurately. The surface of the slate is also a reference plane, making it easier to hold the compass level. The slate itself is useful for noting courses. And if you're making a sketch of the dive site, you can align north on the sketch to north on the compass. Now your sketch aligns with reality: What's to your left on the slate will be to your left as you swim.
Two Ways To Find Your Way
A Return Course
The Hard Way
Let's see, the heading reads 290°. So the return course is 290 plus 180, which is--no, wait, 290 minus 180, which is, uh, 110 degrees?
It's confusing because the number system requires you to add 180 to your present course and, if the total is more than 360, subtract 360. Or, alternatively, subtract 180 from the present course if, as here, it is more than 180.
The Easy Way
My course is E +2. Substitute W for E to find my return course: W +2.
A Right Turn
The Hard Way My course is 290°. To make a right turn, I add 90 to 290 and get, uh, wait a minute, 380? Then I subtract 360 to get 20°. Is that right?
Again, it's confusing because it often involves both addition and subtraction of three-digit numbers. You add or subtract 90 to your present course, depending on which way you're turning. Then you subtract or add 360 if necessary to get a result between 0 and 360.
The Easy Way My heading is E +2. A right turn is two ticks to the right of the cardinal point to my right, or S +2. A left turn would be two ticks to the right of the cardinal point to my left, N +2.
Three Handy Compass Tricks
CROSSING A CURRENT: You know you need to aim upcurrent in order to compensate for the flow and arrive at your target. But how much upcurrent? As a rule of thumb, aim two tick marks upcurrent for every half-mph of current.
Here's why: the best speed most divers can maintain over a distance--not a sprint but faster than cruising--is about 11/2 mph. If you swim at 11/2 mph at a right angle to a current of one-half mph, it will sweep you off course by about 20 degrees, or two tick marks. Obviously, if the current is more than one knot--four tick marks--you'd better think carefully before you try it. If the current is not at right angles to your course, or you are prepared to swim harder, you can figure on less than two ticks per half knot.
TRIANGULATING A DIVE SITE: Dive site triangulation is the classic method for fixing a dive site location on the surface of the water. It depends on taking compass bearings to fixed and obvious landmarks on shore. Where the compass courses from those landmarks cross is the spot over the wreck.
Courses from two landmarks are enough if you are careful, but a third is better because it is a check on the first two. Try to pick landmarks so that at least two courses cross as near to a right angle as possible. The chance of error is much greater when there are fewer than 4 ticks or more than 14 between them. (To find a right angle from your course, substitute the adjacent cardinal point. A right angle from N +4 is E +4 or W+4.)
SWIMMING A BOX COURSE: This requirement appears sometimes in underwater navigation courses, underwater ballet routines and probably nowhere else. (OK, in a search and recovery operation.) Still, it's a useful training exercise for practicing course changes. It's a succession of right-angle turns, so you substitute adjacent cardinal points.
If you're like a lot of us, compass navigation is one of those skills you're too embarrassed to admit has got you beat. If Boy Scouts can do it, why can't you?
You'll be glad to know it's not entirely your fault. Blame the Babylonians, who divided the circle into 360 degrees. And blame the way the compass is taught. You're asked to calculate your direction based on that unfamiliar numeric system instead of the north, south, east and west that we understand intuitively. Blame the fact that compass features aren't universal, then blame the way most of us scuba dive--blindly following a dive guide or relying on natural navigation cues. On top of all that, a compass is harder to use under water because it has to be held level and steady. That's right: Blame the ocean, too.
For the compass-phobic among us (and you know who you are), we've got an easier way to navigate, one with none of the math and, for most scuba purposes, every bit of the precision.
Gear Guide: Anatomy of a Compass
The card: This round piece of magnetic material floats inside the body of the compass and always points north. Look for a simple, uncluttered display with the cardinal points and unadorned tick marks between those points, though the card may have degree numbers printed on it.
The lubber line: A lubber line on the body is just a sight to help you point and aim the compass.
The bezel: This is a ring you can rotate to mark your course, usually by framing the position of north on the card. The bezel may be the biggest source of confusion in numeric compass navigation because there are two styles--direct reading and indirect reading--and they indicate headings in different ways.
Side window: A side-view window allows you to hold the compass between your eye and your destination and read your course, though this often requires an additional (read: confusing) set of numbers. And you can see only numbers, not tick marks, through the tiny window.
Ignore the Numbers
To begin with, forget the numbers and build on what you know, the cardinal points of north, south, east and west. This simplification will be especially helpful in finding your way back to your starting point, the use divers most often make of a compass.
For example: If you've been swimming north, you know immediately and intuitively that your return course is south. If you've been swimming northeast, your return course is southwest.
Your compass will probably have eight bold tick marks between two adjacent cardinal points--between N and E, for example. (Some will have small tick marks between the bold ones. Try to ignore them.) The third and sixth bold marks will probably have numbers (30 and 60 in this case). Ignore the numbers. Think of them only as extra-bold tick marks. They can help you count, because each represents three ticks.
Think of a particular course as a number of tick marks to the left or right of a cardinal point. Four ticks to right of north, for example, instead of 40 degrees. (The bolder tick mark, where the number is, helps you count because you know it is the third tick to right of north, and one more is four.)
What's the return course? Since the card always points north, the opposite of "four ticks to right of north" is "four ticks to right of south." The only change you need to make is to substitute the opposite you already know, south for north. Likewise, if your outbound course is "three ticks to left of west," your return course will be "three ticks to left of east." Just substitute east for west.
To write courses quickly on your slate, use + to indicate "right" and - to indicate "left." That way, "four ticks to right of north" becomes N +4. The return from N +4 is S +4. The return from E -3 is W -3.
Ignore the Bezel
The bezel — the part of the compass you can rotate with your hand — is a Band-Aid on a self-inflicted wound. It's just a memory aid to help you cope with an unnecessarily complicated numeric system. Depending on the style of compass, it may have its own numbers and markings which only add to the confusion.
Our simplified system doesn't use the numbers, so you don't need the bezel. To make it less distracting, rotate it until it coincides with the "lubber line." That's the fixed line that points to where you are going or facing. Or, if you like, you can rotate the bezel to correspond with your course, just as you would in the number system, for a quick visual reference.
Step-by-Step: Finding and Following a Heading
1. Check your buoyancy. Become motionless, or swim forward steadily.
2. Hold the compass where you can read it, and with the lubber line pointed toward your target.
3. Check that the compass is level by rotating it slowly back and forth. The card should remain stationary.
4. Count the tick marks from left or right of the closest cardinal point to the lubber line. Then repeat Step Three to be sure the card is still swinging free and therefore accurate.
And that's it. Your course is, for example, N +4. Finding the return course and making right-angle turns are simple matters of substitution. What's the return course? S +4. What's a 90-degree turn to the left? W +4. To the right? E +4.
Bottom Line
It's that simple. But is it too simple? Can you really navigate accurately just by using tick marks? The short answer is "yes."
If you were able to stay within one or two degrees of your course, and if you were going a long distance, the greater precision of the number system would be valuable. But think of how hard it is to hold that small dive compass level and steady and how hard it is to swim a straight line at the same time.
In practice, most of us can't read a bearing or swim a course more accurately than within one tick mark. Given the short distances we need to cover under water, that's plenty.
In fact, the tick-mark system is the basis of the numeric one and the way mariners navigated for centuries. You've probably seen a picture of a traditional compass with a "compass rose" sunburst pattern of tick marks, but no numbers. The numbered compass only became common in the 20th century when bigger and steadier ships became capable of steering to and holding a course to within one or two degrees.
By the Numbers, Recruit!
So what happens if your crusty ex-Navy instructor is barking at you to use only numbers? The rules are fairly simple even if following them is not.
1. Note your course in degrees. Most compasses have numbers marked every 30 degrees; for instance, 30, 60, 90, 120, etc. But note that many compasses substitute E for 90, S for 180, W for 270 and N for 0/360. This 0/360 mark is usually also marked by an arrow or the company's logo. Let's say your course is 40 degrees.
2. To find your return course, either add or subtract 180. Use the number that gives you a result between 0 and 360. In this case, you'd add 180, because 40 + 180 = 220. 220 degrees is your return course.
3. To make a right-angle turn, add 90 to your present course for a turn to the right, or subtract 90 for a turn to the left. That gives 40 + 90 = 130 for a right turn and 40 - 90 = -50 for a left turn. Now add 360 (which is the same as adding 0) to get a number between 0 and 360. -50 + 360 = 310.
4. Check your addition and subtraction--better write it on a slate. Rounding off to the nearest 10 degrees will simplify addition and subtraction. Over the short distances typical in recreational diving, an error of five degrees or so won't matter.
Got that? It may help to remember this general rule: To calculate any course or bearing, add or subtract the desired course from your present course (add for right turns, subtract for left turns), then add or subtract 360 as necessary to get a result between 0 and 360.
OUR ADVICE: KEEP IT SIMPLE. With our simplified navigation system, all you really need is a compass body with a clear lubber line on it, a floating card with N, S, E and W plus tick marks between--and nothing else. Most scuba compasses, unfortunately, are loaded down with features designed for use with the number system, and retain only vestiges of the cardinal points system. Sometimes you'll have to look hard for the N, S, E and W and for the tick marks but they are usually there. If you can train your eye to see the traditional system which is hidden among the numbers you can quickly learn to navigate with confidence. Then, when you're comfortable with your compass, you can return to the number system, the bezel and the rest of it if you find a need for them.
One feature that is vital in an underwater compass is tolerance for tilting. If the compass body is not held level, the card can't swing freely and it will not point to north--you'll get a false reading.
To compare two compasses for tilt tolerance, hold both compasses flat against a slate, then tilt the slate until it is nearly vertical. Now rotate the slate 90 degrees. Neither card will be able to swing freely. Slowly tilt the book or slate back toward horizontal, watching to see which card swings freely first.
All compass navigation, regardless of whether you use degree numbers or tick marks, is based on the card's magnetic attraction to north. Though the compass card appears to move as you change course, in reality, the diver is only moving the body of the compass around the free-floating card. It's this consistent alignment to north that makes the compass a useful tool for determining and monitoring a path of travel.
Where To Mount the Compass
It's common to see a compass incorporated into the instrument console. It's convenient, but the fairly short high-pressure hose makes the console awkward to hold so that the compass is both level and easy to see.
Mounting the compass on your wrist or on a retractor clipped to your BC makes it easier to manipulate. Even better is to mount it in the corner of a small slate. The edge of the slate works as an extension of the lubber line, making it easier to point the compass accurately. The surface of the slate is also a reference plane, making it easier to hold the compass level. The slate itself is useful for noting courses. And if you're making a sketch of the dive site, you can align north on the sketch to north on the compass. Now your sketch aligns with reality: What's to your left on the slate will be to your left as you swim.
Two Ways To Find Your Way
A Return Course
The Hard Way
Let's see, the heading reads 290°. So the return course is 290 plus 180, which is--no, wait, 290 minus 180, which is, uh, 110 degrees?
It's confusing because the number system requires you to add 180 to your present course and, if the total is more than 360, subtract 360. Or, alternatively, subtract 180 from the present course if, as here, it is more than 180.
The Easy Way
My course is E +2. Substitute W for E to find my return course: W +2.
A Right Turn
The Hard Way My course is 290°. To make a right turn, I add 90 to 290 and get, uh, wait a minute, 380? Then I subtract 360 to get 20°. Is that right?
Again, it's confusing because it often involves both addition and subtraction of three-digit numbers. You add or subtract 90 to your present course, depending on which way you're turning. Then you subtract or add 360 if necessary to get a result between 0 and 360.
The Easy Way My heading is E +2. A right turn is two ticks to the right of the cardinal point to my right, or S +2. A left turn would be two ticks to the right of the cardinal point to my left, N +2.
Three Handy Compass Tricks
CROSSING A CURRENT: You know you need to aim upcurrent in order to compensate for the flow and arrive at your target. But how much upcurrent? As a rule of thumb, aim two tick marks upcurrent for every half-mph of current.
Here's why: the best speed most divers can maintain over a distance--not a sprint but faster than cruising--is about 11/2 mph. If you swim at 11/2 mph at a right angle to a current of one-half mph, it will sweep you off course by about 20 degrees, or two tick marks. Obviously, if the current is more than one knot--four tick marks--you'd better think carefully before you try it. If the current is not at right angles to your course, or you are prepared to swim harder, you can figure on less than two ticks per half knot.
TRIANGULATING A DIVE SITE: Dive site triangulation is the classic method for fixing a dive site location on the surface of the water. It depends on taking compass bearings to fixed and obvious landmarks on shore. Where the compass courses from those landmarks cross is the spot over the wreck.
Courses from two landmarks are enough if you are careful, but a third is better because it is a check on the first two. Try to pick landmarks so that at least two courses cross as near to a right angle as possible. The chance of error is much greater when there are fewer than 4 ticks or more than 14 between them. (To find a right angle from your course, substitute the adjacent cardinal point. A right angle from N +4 is E +4 or W+4.)
SWIMMING A BOX COURSE: This requirement appears sometimes in underwater navigation courses, underwater ballet routines and probably nowhere else. (OK, in a search and recovery operation.) Still, it's a useful training exercise for practicing course changes. It's a succession of right-angle turns, so you substitute adjacent cardinal points.
