Magnetic Compass Accuracy

Does anyone have any data, information, sources, and referenes regarding the accuracy of a hand held magnetic compass?

I'd like to compare the accuracy of the basic ball compass, the Brunton baseplate type (e.g., 8010G) and the Silva Rangers.

Blake

www.outdoorquest.blogspot.com

Blake,

I don't have references right at hand, but IIRC the textbook for the class I took a few years back Mountaineering: The Freedom of the Hills listed sighting compasses like the Silva Ranger as having a degree of accuracy of +/- two degrees.

Of course, it's not just the compass itself but how it is read. A baseplate compass (without sighting mirror) might be just as accurate, but how do you accurately take the sighting? The mechanics of sighting and then reading the sighting will make a reading from a baseplate compass generally less accurate than from a sighting compass.

And of course, there's no substitute for training, experience, and practice. Having an accurate compass that you don't know how to use correctly pays no dividend.

HJ

Chances are pretty good that you already have all 3. Most of us outdoors geeks do. (Well, maybe not a Ranger - but a mirror compass anyway.)

Try taking the measurements yourself on a topo and a known spot.

I have this as an exercise for students in a course using my grab-bag of compasses and find that getting angles to 5 degrees is about all you can get from first-timers and not using a mirror. I've not really spent the time to see if I could do better with lots of practice.

My point for the exercise was to have them plot their bearings and get a triangle rather than a pinpoint to demonstrate that there is ALWAYS uncertainty in measurement.

To compare all three - stand in one spot.

Sight on an object fairly close, then farther away, then farther still. Not the readings each time for each compass. Note any differences.

Now do what unimogbert said to do. If you have a protractor you will know the bearing. Again, use all three compasses. With luck, one's right.

My dad had a compass course in our backyard. After many tries the best I could do was +/- 2 meters over about a half mile. This was using bearing and pacecount only. He had it down to 2 feet at one point.

In my view, the largest inaccuracy is YOU using the compass.

I think you'd be hard pressed to do a two degree accuracy when sighting on distant landmarks, or finding the compass course on the map and find landmarks in that direction.

For that matter, two degree accuracy is rarely required when orienting a map to align with terrain features. A good many compass roses are only marked to five degree intervals, if that.

As a matter of practice, it is only possible to consistantly read a compass (or any needle like meter) to one-half of the smallest marking. Five degrees means two and a half precision is possible. MostlyHarmless is correct, the biggest question is the user. Without a good sighting system, as HikinJim indicated, even that ability is moot because of the ability to transfer that to a place in the world. My Brunton Pocket transit reads to one degree. Sighting it to that level is difficult even as large as the sighting system is. Best on a tripod. Not something that is necessary for hiking. If you are talking about the intrensic capability of a magnetic compass, it is so high that even the one-tenth degree markings on my transit are not up to the capability of a magnetic compass (but that requires exact time and good corrections for deviation, etc.).

Izzy, the body will make no difference to at least one-tenth. What you are wearing (or carrying) can make a huge difference. Knives, watches and belt buckles are the most common offenders.

Respectfully,

Jerry

when I was a kid in high school, the jr ROTC unit I was in had at one time done compass work on the drivers ed course with the cadets under a refrigerator box.

they had to stop the year before I got there due to the needs of the the driver ed classes. it was just in use too much.

I was told by some of the older cadets that had actually gone thru it that some people got to the point where they could reliably get within ten feet of where they were headed with a course they were given hand written on a piece of paper and a flashlight to read the course with.

Others were lucky to get within 50 feet. The biggest problem seemed to be that very few people have consistent paces. Keeping a straight course is not real hard, but keeping your paces consistent is all but impossible without a lot of practice, and even then it is not real easy.

I know your looking for the most accurate magnetic compass but even with a "perfect" compass the magnetic field changes as others have pointed out.

http://www.ngdc.noaa.gov/geomag-web/#declination

Northern Minnesota has huge deposits of iron ore right under or at the surface. Compasses don't work very well there. In foggy conditions before the days of lighthouses and GPS, ship captains near the north shore of Lake Superior followed their compass bearings right into the rocks.

Getting an early start on climbing Orizaba a "few" years ago, my companion was navigating by compass; I was following the North Star. The compass was off by 45 degrees due to iron in the surrounding volcanic basalt. Not very critical in that situation, because the critical directions were up and down.

totally subjective comment:

30 years ago, working for the USFS as a survey contract inspector, i used a personally-owned Ranger Type 15CLQ daily. part of my job included locating 100+ yr old survey makers.

let me describe my best "find": i had to go one mile from a known survey starting point though a very thick lodgepole-pine thicket. counting my steps and checking the Ranger every few paces, i eventually stopped when i thought i was in the right place. scanning around i didn't see any eye-level tree markings usually associated with a ground level cap or rock-marker. i was about to give up when i looked down. i was standing on it!

if it hadn't happened to me i never would have believed it. i give full credit to the compass. it never let me down and always got me very close to where i needed to be, even over a many mile trek.

fyi - my impression over the years was that i could be consistently accurate to within half a degree, so it surprised me to read the above comment in post #2, that they are only accurate to +/- 2 degrees...

Lot's of very thoughtful input.

I really appreciate it.

Blake

www.outdoorquest.biz

Just to give some credit where credit is due, the modern descendant of that Ranger compass in the U.S. is the Brunton 15DTCL. Unfortunately, looking at Brunton's web site it looks like they are rolling out an entirely new line of O.S.S. compasses ... and will no longer be offering the many styles they've sold for years.

Some web sites still have the Brunton 15DTCL for sale though.

Honestly not trying to hijack this thread (Sorry!!), but I just stumbled upon this article about Brunton moving its manufacturing from China back to Riverton, Wyoming ... and in the article there are pictures of the new style O.S.S. compass. That must be the back story on the new line of compasses. I hope it works for them.

http://county10.com/2012/03/14/a-360-tur...ton-from-china/