Compass basics... the lubber line.

[The Chairman]

Waypoints? ... on a featureless sandy bottom? ... thats another diver, right?
( in our case, you use depth/contour/slope )

I like doing their triangulation dive in Hudson's grotto. We stay at 30 ffw. You can't see SQUAT. Not the bottom, not the surface not even a side.

Good info, for everyone ... you all have compasses ... don't you?

You know, I now dive with a $4.00 compass that I picked up in a Walmart. It sure beat spending $99.95 plus tax that the dive store wanted. It hangs from my BC.

 

[Paco II]

Hope this is not off topic, but regarding compass rear/side windows, all are not created equal. My first compass had a really small window which was a little difficult to read. Got a different compass with a larger window, and it really made navigating much easier.

 

[tracydr]

did you know that you get north and southern hemisphere compasses.

I recently changed hemispheres and all my compasses now have excessive dip.

Actually, you have to correct for true north/magnetic north anywhere in the world. If you look at a map designed for navigation it will have the adjustment on the map. To be perfectly accurate (say for a navigation course) it's important to account for this adjustment.
I think the reason we don't learn this skill in scuba navigation is A:too complicated for students trying to learn so many new skills and B:scuba navigation is relatively inaccurate due to current drift so the need for correction is probably less.

 

[sytech]

Agreed.

Great topic. Just bought a console with compass. Learned some compass basics but want to know more.

Sy

As I dive with other divers from around the world, it appears that the universal flaw in their diving has to do with navigating with a compass: they don't have a CLUE how to use it. So let's discuss some things about the compass that will make it EASY for you to learn it. Most of us know about the cardinal points on a compass: N, S, E & W. Let's expand on that a bit.

First of all, dive compasses are a hybrid. They are part nautical as well as set up like an orienteering style compass. If you look down on it, you can can clearly see that north points to north and that there is a movable bezel which we can align to north. However, if you look on the back side, you will see there is a window which reads only the direction of travel, much like a ship's compass.

Which brings us to the lubber line. This is a fairly misunderstood term. In essence it is our direction of travel. When boats first got compasses it became apparent that some sailors were good at using it and others could not grasp precisely where they were going. Those who were good at it sited their reading along the keel, while the others were a bit to the left or right. Back then, if you were a "nit" on the water, you were called a "lubber": sort of an early iteration of the term "stroke". See? Condescension and arrogance are an old sport! To make things EASIER for these lubbers, they etched a line on the back of the compass which is now called a lubber line. Ever hear of a "Land Lubber"? No, it's not a mispronunciation of the word "lover". It refers to someone who is afraid of the sea and wants to keep that lubber line in a direction pointing to land.

OK, now we know where the term came from, how do we use it? Well, think of yourself as a ship (a submarine in this case). When a ship's compass is set, great care is taken to see that the lubber line is parallel with the keel of the ship. YOUR lubber line has to be in line with your entire body as well. In the beginning, it's important to hold the compass dead center to you with that lubber line laying along that imaginary line that bisects your body. The more carefully you do this, the better your subsequent readings will be. Point your compass at something and look into that window at the back side of the lubber line. Record the number. Now, hold the compass a little lower while pointing at the same object. See where North is pointing? It should be red with a point. Ok, now twist the bezel of your compass and match the north symbol on the bottom of the housing (it moves with the bezel) to the needle. If you look around the outside of the bezel, you will see numbers and probably the cardinal points of a compass. There should be a line just under the bezel at it's very front. Record that number and compare it to the number you obtained through the window. Are they close? If yes, go ahead and practice siting both ways on a number of different objects. If not, you need to figure why your compass holding varies so much.

Big tip: Like most nautical compasses, your compass can probably accept a 30 degree tilt without hanging up the needle. It is in your best interest to keep your compass as level as possible.

Now, once you have mastered aligning things on land, let's see if we can follow our lubber line! OK, you have aligned the bezel to North. If you want to travel in that direction WITHOUT looking at the destination (like you are heading to the boat and you are now underwater), just keep "North" centered over it's symbol and travel in the direction of the lubber line. Now, if you aren't going to look up during this exercise, please be SURE that you are not playing in traffic or in any other area where not looking may put you in danger. Are you ALMOST at your destination? Great! Now, center the south part of the pointer over that North symbol. You have just found your way BACK to where you started. This is called a reciprocal course and it's great to know when you head OUT from you boat, and want to get back.

Big tip: Unlike land based navigation, we have to worry about "drift" caused by currents. This drift is AMPLIFIED when you are swimming into the current and DIMINISHED when you are swimming with the current.

Next discussion: Underwater navigation using the compass.

 

[D_B]

Originally Posted by cancun mark
did you know that you get north and southern hemisphere compasses.

I recently changed hemispheres and all my compasses now have excessive dip.

Actually, you have to correct for true north/magnetic north anywhere in the world. If you look at a map designed for navigation it will have the adjustment on the map. To be perfectly accurate (say for a navigation course) it's important to account for this adjustment.
I think the reason we don't learn this skill in scuba navigation is A:too complicated for students trying to learn so many new skills and B:scuba navigation is relatively inaccurate due to current drift so the need for correction is probably less.

Actually, that is not what cancun mark is referring to.
The magnetic lines do not follow the contour of the ground and in some places can be steeply inclined to it , you have to balance the needle so it's "level" in your area
.. here ...

Compass Balancing (from Suunto )

The horizontal and vertical components of the earth's magnetic field vary considerably in different locations. For this reason Suunto compasses are balanced for 5 different zones. If the compass is used in an adjacent balancing zone, many compass pointers will tilt only slightly. However, the farther a compass is used from its correct zone, the more its pointer tilts. In extreme cases, the pointer will stick. For this reason, it is extremely important to know in which country a compass will be used.

 

[Mako Mark]

Another thing a person unfamiliar with compasses has to deal with, is the "lead" and "lag" when changing directions.

the K

Ahh, but what is the airspeed of an unladen swallow???

 

[The Kraken]

Ahh, but what is the airspeed of an unladen swallow???

In a dive or a climb ????

the K

 

[The Kraken]

You know, it wasn't until I started instructing that I realized that most of the people in the U.S., if not the world, have never even held a compass in their hands, much less have any idea how to use it!

Cub Scouts, Boy Scouts, the U.S. Army, flying helicopters, hunting, canoeing . . . . I just never thought of anyone NOT knowing how a compass works or how to use it.

Not very astute upon my part, I must admit. . .

the K

 

[Mako Mark]

Actually, you have to correct for true north/magnetic north anywhere in the world. If you look at a map designed for navigation it will have the adjustment on the map. To be perfectly accurate (say for a navigation course) it's important to account for this adjustment.
.

Sorry Doc, variation or declination is of very little importance underwater. All you need is a bearing to a reference point.

:scuba navigation is relatively inaccurate due to current drift so the need for correction is probably less.

Exactly, in scuba diving, we basically need to navigate within a few meters/yards over 100 meters /yards or so, so yes, it is not the most accurate or taxing navigation in the world.

If you can find your way from home to work to home, then you should be able to apply the same techniques to find your way back to the boat.

See, you guys can already do this, just do it underwater!!

(those who cant find there way home from work should ignore the last statement)

 

[The Kraken]

Magnetic declination comes into play when you're navigating with a compass and a map.

And, I think it was mentioned in an earlier post, the magnetic declination for a given location changes from year to year, that's why it's important in navigational skills to be able to compute the declination to acommodate for the shift between the time the map/chart was drawn and the date it's being used.

As Mark points out, in scuba, you're simply navigating from one fixed point to another.

the K