Magnetic Compass Calibration

[NickPick]

Not exactly geocaching, but related, and more approriate here than on my car or scuba diving forums.

 

I've found my old (very old) Silva compass in a draw and since I want to restart hill walking and show the kids how to navigate without batteries, I was wondering if an old compass is still reliable.

 

I intend to measure / calculate some known bearings from maps to check the compass against, but I was wondering if anyone knew whether magnetic compasses lose their accuracy over time, or is it a case of once set, always set (unless it's been stored next to a big magnet or something)

 

Any advice or ideas?

[+MartyBartfast]

Interesting thought.

Purely speculation but I would think the compass either works or it doesn't I don't see how it can become less accurate as it's a simple magnet with a N pole and a S pole, I can't see there's anything in between.

[+Gitchee-Gummee]

Unless that particular compass had been subjected to a strong electromagnetic field, it really shouldn't be a problem. If there is any magnetic qualities retained by the compass, it would still function properly -- although it's possible that it could be reversed, and that wouldn't be a major problem.

 

Now, geomagnetic reversal (which is supposedly going to happen soon), could cause problems while it is occurring -- nobody has a clue as to what the transitional period will be. Afterwards, things should be hunky-dory again.... but reversed.

[+BCNorwich]

Hi,

Yes a compass does have to be recalibrated especially if it's been a few years since last done. Though I suspect not for the reasons you're thinking of. See here for why and how!

 

Regards

Bernard

[Langy]

Wow that post is really interesting to read.

 

I knew of the differences between the geographical and magnetic poles but didn't realise how they were shifting.

It seems to be a case of not so much a new compass but more a case of having new maps.

 

I wonder how the world will cope when the poles reverse and when it will actually happen.

Will this actually be something that causes more issues that the so called millenium bug (ha how we all suffered on that over hyped one).

 

Thanks for the link a real nice article.

 

Langy

[+Amberel]

Will this actually be something that causes more issues that the so called millenium bug (ha how we all suffered on that over hyped one).

Sorry this is not entirely on topic, but that last comment needs answering. The "millenium bug" was not overhyped - it was simply anticipated well in advance and dealt with in sufficient time. Had it NOT been anticipated or dealt with you would have seen a great deal many more, and more serious, problems than there were!

 

Rgds, Andy

[NickPick]

Thanks for the replies. I already knew about declination and needle dip/inclination. I suppose there's also tectonic drift to worry about as well depending on how old the map is.

 

I had a quick play with my compass last night whilst waiting to pick up the kids, and found that the sighting part of it gave a different bearing to the lubber line. Then I remembered that sitting in a steel car is not the best place to test bearings - the compass card changed about 20 degrees from eye level to chest level!

[NickPick]

Well, I had a go yesterday, and in sighting 2 known locations (the corner of a building and the nearby radio mast), from about 500m away, then calculating my co-ordinates and comparing against the GPS (using trigpointing's co-ord conversion to go from WGS84 to OSGB), I got an error of approx 70m, so allowing for sighting accuracy, GPS CEP and co-ordinate conversion errors, I'd say that the compass is OK.

[+BCNorwich]

Hi,

The point of adjusting the Magnetic Declination on your compass is so that you can take a bearing (compass heading) from the map and it automatically has the map north to magnetic north deviation accounted for. Similarly when you know where you are on the map you can take a compass reading / heading to a POI in the distance, place compass on the map and align the compass to point to the POI on the map. There should be no error. This site will give you the differences between Grid North, True North and Magnetic North for the whole of the UK.Regards

 

Bernard

 

 

[Rexetexer]

Hi,

The point of adjusting the Magnetic Declination on your compass is so that you can take a bearing (compass heading) from the map and it automatically has the map north to magnetic north deviation accounted for.

 

Seriously? Has anybody in this country (or any other) ever encountered or set a cache with bearing waypoints far enough apart for the difference to have any effect whatsoever???

[+Original A1]

Firstly I'll admit I'm a non-scientific luddite, but it was Doors Open Day in Edinburgh this weekend and yesterday I went to the British Geological Survey building to have a look around. So it's quite strange to read much of what I read yesterday there on this forum and the links tonight. I only arrived at 3.45pm and it closed at 5pm, so I had nowhere near enough time to get round it all, but it was fascinating nonetheless.

 

And having scanned a QR code, I got this page on my iPhone:

 

http://www.bgs.ac.uk/igeology/

 

Haven't explored it further yet or downloaded it, but it looks really interesting. I wanted to ask someone whether it's possible to overlay the Garmin maps with these geological ones too, but couldn't find anyone free at the time.

 

[+Original A1]

Oh, and I took this web address too:

 

http://www.geomag.bgs.ac.uk/