Compass 101

[Mario S Caner]

Compass Balancing

The horizontal and vertical components of the earths magnetic field vary considerably in different locations. This is why compasses are balanced specifically for the region they are sold in.

This suggests that a compass purchased locally in Seattle, will be off in Australia. If the compass is used in an adjacent zone, the needle will only be off balance slightly. The variance will get bigger as you go further, and in extreme cases the compass can stick. For this reason, travellers need to be careful.

Check This Link to see the different zones.

 

[TexasMike]

Very interesting Mario. Until you mentioned it, I wouldn't have thought about compasses being affected by locale. But it does make sense.

Does this apply to the electronic ones as well?

 

[omar]

That's why the declination from true north is printed on USGS topo's. The declination also varies over time so if you have a 50 year old map then it will be off by 1-3 degrees.

Another point is that the declination will not vary to any great extent over the small distance that that you will be traveling underwater. The offset will always stay the same and if you keep track of the cardinal points and turn accordingly the "problem" is really moot.

What are you trying to do Mario, confuse all of the beginners?

omar

 

[Rick Murchison]

Declination, or, more commonly variation, is the difference in magnetic north and true north - and that isn't what Mario's talking about. A perfectly balanced compass needle will point toward the magnetic north pole - simple enough, right? Well, not quite... if you grab a globe, and point a needle straight at the north pole, you'll see that you're pointing *through* the earth, or *down* to some degree, with the downward angle increasing the further south you get. To counteract this effect, compass cards have a tiny counterweight on the south side of the card, so it'll be level rather than pointing downward. The further south you get, the more weight is required - so a compass optimized for the mid latitudes of the northern hemisphere (Continental United States), for example, will have a little bit of a downward dip on the north side of the card if you take it to Australia. This usually isn't enough of a difference to cause a problem as most diving compasses have adequate tilt tolerance, but it will cause sticking sooner should you tilt it to the south side.

Rick from the backwaters of Alabama

EDIT - 8/15/06 good grief! I wrote that? I wuz confused... the tilt factor has more to do with the "lines of magnetic force" than actual direction of the magnetic pole, and the compensating weight does *not* increase as you go south, but rather as you go away from the equator
Rick

 

[joewr]

Gents (and any Ladies who duck in),

You guys make it sound like you use dive compasses the way I used compasses while backpacking, flying and jeeping. I guess I never have used one that way while diving. We are only down for an hour or so and cannot really go that far: I use it to take a bearing on the boat or the shore location where I entered in the water, etc. That is I do not use it for map reading.

Maybe I have not done enough "adventure" diving? To navigate a boat: I do not think I would use a dive compass. Geez, I hate to be so damn critical. Forgive me if I am off base. And I know Jack S--- about cave diving--maybe that is where you need that kind of accuracy??? But I thought reels and lines were used--like I said I know Jack S--- about cave diving.

Oh yes, I understand about the phenomenon that Rick discussed--but that is just a physical problem associated with compass reading. I have been as far south as Indonesia without having a serious problem.

Educate me--even from the Backwaters of LA!

Joewr

 

[Warhammer]

See, Joe, your problem is that you can always see your target location and beyond, therefore you don't have to navigate to it. Us backwaters folks often can't see our hand in front of our face, so a compass helps.

 

[joewr]

Hi, Warhammer!

Actually I have been far enough away and with objects in the way to not have been able to see my objective. But I took my bearings, remembered my course and returned. Again, I was not trying to use my compass to read a course off of a USGS map or the like.

Well, I guess this is just another example of Candy A-- Joewr!

Joewr

 

[Warhammer]

LOL!! Sorry, Joe, couldn't resist.

 

[joewr]

Warhammer,

Do you mean you were picking on me! Now, cut that out!

Joewr (the Defenseless, but not lost!)

 

[Rick Murchison]

As I said, the difference in card level isn't usually a problem - more of a curiosity than a consideration we need concern ourselves with. And variation is something that matters in airplanes, not Scuba diving.
Personally I think Mario was just casting a little bait out to see who'd bite...
Rick (hook, line & sinker)