About compasses

[BlueTrain]

I love maps yet I rarely seem to use compasses. But they have proved their worth on a couple of occasions. The problem on one of those occasions was that I didn't have one! But on to my question.

Everyone knows you need to step away from large metal objects when using a compass, although I have no idea how far and so on. However, my question is about storing your compass when you aren't using it. Does any lasting harm come from having your compass lying in a box next to, say, a knife? In the past I have paid no particular attention to this question and so far I have apparently paid no penalty. Is this likely to be true? Can a compass go bad and loose whatever it is that makes it work? My compass is an ordinary compass and not one that locks the pointer in place.

The other instance, by the way, when the compass was useful was this: There is a national park (the Shenandoah) to the west of Washington, D.C. I have often wondered if it was possible to see the Washington Monument from a certain point in the park (North Marshall, for those of you who are familiar). It is a distance of perhaps 75 miles, perhaps a lot more. So far it has always been much too hazy to see that far but once when studying my map, I realized I was never looking in the right direction to begin with, not that it made any difference. Washington is almost directly east, or slightly northeast of Mt. Marshall. The mountain range runs in a Northeast/Southwest direction and I was looking at right angles to the front, if you follow me. Knowing which direction to look may have made a difference.

 

[spamel]

A good and valid question. As a member of the Armed Forces, I carry the obligatory Survival Tin with me, which has amongst other things a nice shiny brass button compass. This is wrapped in toilet paper to save it from being broken, but it is nestled up against wire saws, surgical blades, bits of hacksaw blade all inside a metal box! Does this affect it at all? I'm not too sure, but it doesn't seem to be out at all. Maybe if there was a slight magnetic force in the other items, there may be a problem.

 

[BlueTrain]

Since we are talking maps and compasses, does anyone know if there is a place on line to check the current magnetic deviation for any particular place in the world, at least for North American and Europe? Some of my maps are twenty or thirty years old and it is possible that information is slightly out of date.

 

[spamel]

I don't know for sure, but there should be something on the map telling you the annual variation, but of course, how long does it go east before turning west again and vice versa?

 

[JohnC]

That would be magnetic declination

http://www.learn-orienteering.org/old/

 

[daved]

If you have a reliable map, an alternative approach to work out the current declination for the area is to sight along a linear feature (eg. a wall) and compare it's bearing to the one on the map. You can adapt this approach by taking a bearing on any prominent feature, if you are at a known location (e.g. between two trig. points) - you just need to compare the observed bearing to the one on the map.

Some compasses, like the Silva 15TDCL have a little screw so you can "lock-in" the declination adjustment and save on all the arithmetic. (By the way, if you are looking for a compass, that one is highly recommended.)

 

[Ogri the trog]

Blue Train,
For my perspective as a lay person, I do not think that your compass is likely to be affected, if it is a simple needle type. In close proximity to a magentic field, the needle would align itself (or minutely affect the locality) and that would be that until you came to use it again. If the compass had a disk type pointer however, the disk could get locked into place by virtue of being tilted and then it might succumb to some gradual effects of a nearby magnetic influence, possibly making it inaccurate. I would be wary of strong magnetic influences which could demagnetise or remagnetise the needle and I suppose close proximity to electrical fields should be avoided for the same reason.

As to stepping away from large metal objects, I would keep going until there is no visible change of influence on the needle and then keep checking your compass as you continue your journey to ensure the consistency of the bearing.

I do not have any experience of a compas that locks the pointer in place - believing that anything that can interfere with a correct bearing can and will at the most critical moment leading to possibly dire consequences.

ATB

Ogri the trog

 

[Goose]

Since we are talking maps and compasses, does anyone know if there is a place on line to check the current magnetic deviation for any particular place in the world, at least for North American and Europe? Some of my maps are twenty or thirty years old and it is possible that information is slightly out of date.

Something like this?
http://www.pangolin.co.nz/almanac/magvar.php

 

[daved]

I do not have any experience of a compas that locks the pointer in place - believing that anything that can interfere with a correct bearing can and will at the most critical moment leading to possibly dire consequences.

Just to avoid any potential confusion. The type of compass I was talking about does not lock the pointer. It sets the declination adjustment by rotating the north arrow on the base plate by the appropriate amount. Really saves a lot of grief with doing all those little calculations and no mistakes about whether to add or subtract when you are tired!

Would have to agree with Ogri that a compass that locks the pointer offers a big potential for mistakes.

I would also agree with the comments on storing compasses - the problem is more theoretical than practical. Storing a compass next to a magnet would be unwise. Iron/Steel can pick up a magnetic charge if they are left in a strong magnetic or electric field (try sitting a video tape on top of a loudspeaker for a few weeks if you don't believe me ). I have also heard that you can magnetise things by repeatedly hammering them or stroking them with a silk cloth (although I have never tried it.) So, it is possible that your metal items might aquire a weak polarisation without you realising. Unlikely to be enough to harm your compass but I would generally err on the side of caution and store your compass away from iron and steel if possible. This is straying more in to the area of school physics experiments but better not to take the risk.

Spamel - don't think there is any need to worry about your button compass though. Not really designed for precision navigation in the first place.

 

[AJB]

Since we are talking maps and compasses, does anyone know if there is a place on line to check the current magnetic deviation for any particular place in the world, at least for North American and Europe? Some of my maps are twenty or thirty years old and it is possible that information is slightly out of date.

Hi Blue Train,

Although this isn't of much use to you, it might be to our UK members.

http://www.threelittlemaids.co.uk/magdec/index1.html

Andy

 

[lardbloke]

I have an old Silva compass (military) that I have used for near on twenty years. A compass should not be affected by a metallic source when in storage. As for error margins, if I am walking from point to point, you should be able to compensate the effect by following the waypoint and a nearest object. If there are no landmarks in the terrain and are moving over long distances, then I would use an error margin. I would always buy an up to date map if I am off on a walk about just to be on the safe side i.e new footpaths, routes, roads etc.
For me a compass is usually a last resort when I am lost or unsure. If I cannot follow what is on the map, on the ground and in my head then I am having problems. This is usually in adverse conditionds, such as fog, so will resort to a compass. On mountains you have to be carefull due to the nature of the rock, which can throw the compass out completely (Skye, Cullins). As a last resort I would use GPS, but I do not rely on this.

 

[BlueTrain]

Thanks for all the comments and especially the link to the almanac site.

First, concerning compasses, I believe military issue compasses and other more sophisticated than any I am likely to have have dials which lock in place when the compass is folded. I am referring to those which have sights.

Now, on to maps.

I dug out a couple of maps of the Appalachian Trail as published by the Potomac Applachian Trail Conference, which is responsible for trail maintenance from Pine Grove Furnance in Pennsylvania south to Waynesboro in Virginia where the A-trail crosses I-64. There is a series of 13 overlapping maps of the Applachian Trail (which go beyond their limits of trail maintenance), plus a few other areas for which they publish maps.

I have in front of me maps 9 and 10, which cover the North District and the Central District. Map 10 is from June 1977. It states the compass deviation was 5 1/2 degrees West. Map 9 is a revision of June 1996 and states the magnetic north 1994 declination was 9 degrees, 15 minutes west. So we can note that not only has new language been used but that there are almost four additional degrees of magnetic variance, which sounds like quite a lot. Evidently deviation was no longer an acceptable word in 1994.

I did a quick look at the website of the United States Geologic Survey but did not see any (easy) reference to magnetic variation but I'm sure there is something there somewhere. However, the website references in one of the replies appears to be exactly what I was asking about.

I might also add the these PATC maps are not oriented in the usual north-south fashion but rather so that the portion of the Applachian Trail that is covered is pretty much in the center of the map. I have other USGS maps of places further west and I will have to check the details on those maps for comparison. It is interesting to compare different editions of the same map but the differences are slight, even though I am aware of some changes in trail locations here and there. Frankly, things don't change that much around here. The maps have improved, however, and the new ones are made of a very weather-proof plastic coated material.

For comparison, the northern section of the Shenandoah Park is 31.5 miles long and Skyline Drive, located entirely within the park, is 105 miles long. The park entrance is about 60 miles as the crow flies from the Washington Monument, which I referred to earlier, and about the same distance from Mt. Marshall, which is an especially good vantage point in the northern section.

 

[daved]

The magnetic pole does wander so the declination will change from year to year. UK maps usually have marked on them the current declination and an annual adjustment to calculate the current value. I can't remember approximately how much the variation is each year and unfortunately don't have any maps to hand to check the reasonableness of the figures you have. Anybody out there with a map handy they can check? Don't forget, there are three Norths to consider: magnetic, grid and true north - need to make sure we are comparing like with like.

Some possible explanations:
The figures are correct
The maps are unreliable
The grid is orientated differently on each sheet
The map grid, projection or the geode was adjusted between editions
Quoted declination was magnetic to grid on one and magnetic to true on the other (unlikely).

Whatever figure you come up with, it will still pay to ground-truth it when you are in the area - just compare an actual to bearing to the one on the map.

Maps do vary in quality. We are rather spoiled in the UK with the high quality ordnance survey maps. I have used French/Italian maps too and they are no comparison. Of the ones you mention, I would put more faith in the USGS ones.

 

[BlueTrain]

I have plenty of faith in the maps published by the PATC but I have no idea where they begin. I don't imagine they start from scratch but rather from the USGS maps. It is interesting, however, how in other parts of the country that certain features (towns or villages) can disappear completely, even though they remain marked on maps for quite a while afterwards. The places I am thinking of were old coal mining villages (coal camps) in the southern part of West Virginia. The mines gave out, the miners moved on and the houses torn down. You would never believe there had ever been a thriving little village there.

Speaking once again of the northern district of the Shenandoah Park, I had the opportunity last year to fly over that area on my way to Louisville, Kentucky, where I happened to go on Derby weekend. The plane crossed the park around dusk. It was completely dark, naturally (I just mean no lights on the ground) but I was surprised at how many features I recognized, even though the passing was brief. It also leveled the ground, too. It seems much rougher when you are actually on the ground but that's probably some optical illusion.

 

[BlueTrain]

I dug out my stock of U.S. Geological Survey maps to compare with those published by the Potomac Applachian Trail Club. They are not of the same area but otherwise, the PATC maps are exactly the same sort of topographical maps, but obviously are produced for a narrower purpose. The PATC series of maps are not all oriented the same relative to the grid. There is also no reference on those maps to declination from grid.

The USGS maps do give a declination from grid, which is something new to me. I assume it is the distortion, to pick a word, caused by printing a flat map of a curved surface. The grid variation is .07' east. I assume that never changes.

Of the half dozen maps I have with me at the moment, most have a reference year of 1976 when the magnetic north declination was 4 1/2 degrees west. One map is 1968 when the declination was 3 1/2 degrees west. The map also gives the same variation in mils, which is curious.