Compass basics... the lubber line.

[knotical]

For clarification, declination and dip are slightly different phenomena resulting from the same cause.
Declination is horizontal “error”, dip is vertical.
Mark and the K were discussing dip. BKP and NetDoc are discussing declination.

 

[knotical]

Excellent article, Pete. I look forward to the next installment.

It should be red with a point.

A minor point: - They're not all red.

 

[vladimir]

I have yet to see a diver's compass with an adjustable declination.

The Suunto D9's compass has an adjustable declination.

 

[SNorman]

Good info here... some people are confused but the right info is in the thread somewhere

NetDoc, I realize you are right, if you are swimming into the current you will be pushed off course to the side farther than swimming with the current!

As for the compass, the compass needle orients itself with the magnetic lines of force running from the North pole to the South pole. Ever put some ferric particles on a piece of paper, and put a magnet underneath, and visually see lines form from one end of the magnet to the other?

In the Northern hemisphere, these lines of force are angled up toward the sky pointing at the equator (toward the SOUTH). In the southern hemisphere it is the same (up toward the hemisphere), resulting in the lines of force angling up toward the NORTH.

This is not the same as declination. There are compasses made to be used in the Southern hemisphere to account for the opposite dip from the Northern hemisphere to help keep the needle spinning freely.

 

[SNorman]

From http://www.allstar.fiu.edu/aero/MagComp.htm, because I think this compass dip is interesting:

C. MAGNETIC DIP

The lines of force in the earth's magnetic field pass through the center of the earth, exit at both magnetic poles, and bend around to re-enter at the opposite pole (see the Magnetic Dip figure, right). Near the Equator, these lines become almost parallel to the surface of the earth. However, as they near the poles, they tilt toward. the earth until in the immediate area of the magnetic poles they dip rather sharply into the earth. Because the poles of a compass tend to align themselves with the magnet lines of force, the magnet within the compass tends to tilt or dip toward the earth in the same manner as the lines of force.

 

[Mako Mark]

Dip? I am unfamiliar with this term. Is it nautical in nature? I know that I have used a compass below the equator and found no difference in it's use.

magnetic lines of flux do not flow parrallel to the earths surface:

so except at the magnetic equator, one of the needles points slightly down. Near the poles, it would be almost straight down which is why compasses dont work well there.

This angle is called dip and there are various ways to overcome it. One way is to place the fulcrum away from the center of balance of the needle or card. In the northern hemisphere, the south needle would be longer than the north needle in order to try and keep the needle horizontal..

This can cause errors such as acceleration errors and turn errors, but divers generally dont move fast enough for this to matter.

 

[Charlie99]

The magnet pulls in a straight line to the pole. The pole in the southern hemisphere is, more or less, directly through the center of the earth, so the compass will "dip" down toward the ground.

the K

Most people have seen the demonstration where iron filings are spread around on a sheet of paper above a bar magnet. The filings will show the lines of magnetic flux. The lines of flux around the earth take on a similar pattern, but with the magnetic south and north poles replacing the two ends fo the bar magnet.

The magnetic lines are parallel with the earth surface only at the magnetic equator, which is more or less around the geographical equator. Near either pole, the magnetic fields are nearly vertical.

This vertical angle between the magnetic lines and the local horizontal is called "magnetic dip" or "magnetic inclination" (That's INclination, as in INcline).

A compass needle or compass card that aligned itself with the magnetic field in both horizontal and vertical directions would be tilted. Some compass makers will unbalance the compass card a bit so that it will be close to horizontal with a certain amount of dip. One which has been dip compensated in such a fashion for the southern hemisphere will have noticeable tilt when used in the northern hemisphere, or vice versa.

Charlie Allen

 

[Mako Mark]

wow, for a thread about compass basics, this got complicated WAY fast.

Sorry Doc

 

[The Chairman]

Well K, I take it back. Here, I learned something about the compass as well. Not sure that I will ever use that info while diving, but it is good to know nonetheless!

Since most nautical compasses can accommodate a 30 degree tilt and that includes diver's compasses, the need to worry about a dip (other than the immutable "K") is pretty moot for our discussions.

 

[Mako Mark]

there is also deviation which is graphically demonstrated in This thread