Deviation between compasses.

[onestep]

I just purchased a new compass for use as a backup. It's made by Silva. Same brand as my 'ole faithful...

What puzzles me is there is a 20 degree difference between the two when pointing in the same direction! Neither has an adjustable declination. When I first suspected they did not read the same I devised a test. I aligned N on each compass dial with the "line of sight" arrow on the base plate. I then placed the front side of the base place against a wall and compared readings. Each compass reads different. The difference is 20 degrees, i.e. if one reads 140degrees, the other reads 160.

What’s up with that?

When taking these readings I made sure to keep one compass away from the other to eliminate any magnetic interference between the two.

Onestep

 

[teejay]

Nails in the wall. Just a guess. Try moving one compass slowly on the wall to where the other one was and see what happens.

teejay

 

[sardog1]

Methinks thou dost need to get outside more often, with your compasses that is.

Your test methodology suggests the possibility that something inside the structure, either nails in the studs or wires behind the wallboard, is having a little fun at your expense.

The compasses will not affect each other all that much. Take both outside, well away from all power lines, vehicles, and rodeo belt buckles. Repeat your test while you hold them side-by-side at waist height in front of you, then report back here please with the result.

 

[Tom Rankin]

What’s up with that?

Sounds like one of them is whacked!

If you want due South, go out and sight the Sun at 12:54 PM tomorrow. (For an explantion of why that time is correct, see http://en.wikipedia.org/wiki/Equation_of_time )

 

[onestep]

Nails in the wall. Just a guess. Try moving one compass slowly on the wall to where the other one was and see what happens.

teejay

Initially I thought the same thing so I made sure I used the same spot on the wall for both compasses. I tested one compass at a time, not both together. I then went into the kitchen and repeated the test along the side of the kitchen table, again using the same spot to test each. Same result. Finally, I took my compasses out to the front porch and used the wood railing as my fixed straight edge... still 20 degrees off.

Onestep

 

[TCD]

Is it possible one could be an oddball compass calibrated in grads, rather than degrees? Are there 360 units in the full circle? I've never heard of a compass with grads on it, but there are 100 grads in 90 degrees (400 grads in a circle).

 

[teejay]

The next question is "Which one is right?" If it's the new one, that might explain "So thaaaat's what my problem was."

Try TFR's idea.

teejay

 

[lumberzac]

I I find it very interesting that there is a difference of 20*. I’ve had two compasses fail over the past two years. Both needles ended up reversing, but were off by about 20*.

 

[Nessmuk]

I just purchased a new compass for use as a backup. It's made by Silva. Same brand as my 'ole faithful...

What puzzles me is there is a 20 degree difference between the two when pointing in the same direction! Neither has an adjustable declination. When I first suspected they did not read the same I devised a test. I aligned N on each compass dial with the "line of sight" arrow on the base plate. I then placed the front side of the base place against a wall and compared readings. Each compass reads different. The difference is 20 degrees, i.e. if one reads 140degrees, the other reads 160.

What’s up with that?

When taking these readings I made sure to keep one compass away from the other to eliminate any magnetic interference between the two.

Onestep

In purely open (outdoor) space, there should be no reason for one compass to read 20 degrees different than the other. There must be some external (magnetic or ferrous) influence you are not taking into account. I once had a student with a similar problem. I finally discovered that every time she used the compass she was wadding the lanyard up in her hand under the compass, to which she had attached a metal split ring. Bingo.

If you want to determine which compass is correct, go out tonight, find Polaris (the north star) and align your compass to it. Here in the NE it should read the positive value of your published declination, plus or minus one degree (Polaris is one degree away from the true pole). In the Adirondack region that would be 14 or 15 degrees.

 

[sardog1]

To give you an idea of how small the object influencing the compass can be --
I was laying out a compass course for students and marking it with short wire flags of the type used by utility workers. A single flag held between my elbow and my chest, while sighting with a mirror at eye level, was enough to affect the needle.

Nessmuk and I are available for exorcisms . . .

 

[Tom Rankin]

If you want to determine which compass is correct, go out tonight, find Polaris (the north star) and align your compass to it. Here in the NE it should read the positive value of your published declination, plus or minus one degree (Polaris is one degree away from the true pole). In the Adirondack region that would be 14 or 15 degrees.

I figured the Sun would be easier to find than Polaris! But your suggestion of Polaris as (almost) never moving is a good one.

 

[RoySwkr]

Depending on where you live, there is probably a long straight stretch of highway whose bearing you can determine from the map. I find it hard to sight on an object almost directly overhead such as the sun at noon or Polaris.

 

[onestep]

I once had a student with a similar problem. I finally discovered that every time she used the compass she was wadding the lanyard up in her hand under the compass, to which she had attached a metal split ring. Bingo.

Problem solved! This evening I went outside with both compasses, a pencil, and a short length of 2x4 (no nails in it). I laid the 2x4 flat on the ground and then put my new compass on it. I drew a pencil line on the 2x4 along the side of the compass base plate. I then rotated the 2x4 till the compass needle pointed N. I then carefully removed the compass from the 2x4 and laid the old compass on the pencil line. The old compass pointed to 20 degrees! WTF I thought... I was sure it was nails in the wall or maybe electrical wires, or whatever, that was "bending" the needle. I came back inside, fired up VFTT and read the new posts on this thread... voila!

My old compass is attached to the a lanyard with a metal clip which is just below a 7/8" long metal crimp that holds the lanyard cord together!!! I went back outside, removed the lanyard from the old compass, and repeated the test. This time, without the lanyard, both compasses pointed in the same direction!!!

Now that I have a compass that truly points north I'll probably get really turned around!

Onestep

 

[Nessmuk]

Depending on where you live, there is probably a long straight stretch of highway whose bearing you can determine from the map. I find it hard to sight on an object almost directly overhead such as the sun at noon or Polaris.

Polaris's elevation is your co-latitude... ~90-44=46 degrees. Yeah, it can be a little more difficult to sight at that elevation than something on the horizon. But it is the way to determine your declination (assuming your compass is not screwy). You can use a trick of holding a weighted vertical line through Polaris to find a distant horizon object at the same bearing. If you can measure accurately enough, the one degree offset of Polaris from the true pole is in the direction of the end of the handle of the Big Dipper.