Adjusting compass bearings

Is there an easy way to remeber how to allow for magnetic deviation? I know that deviation changes based on geographic location and is changing over time but not how the +/- and east/west changes work. What I am looking for is a way to include this information on a card in a PSK.

East is Least (-) and West is Best (+)

>Is there an easy way to remeber how to allow for magnetic deviation?

This is going back into my boating training... might be a little foggy, bear with me...

Often, when people talk about "deviation", they're actually talking about "variation". I think that's probably what you're asking about. On most maps and charts, there will be a compass rose of some sort that will show magnetic and true north. The difference between them is the local [color:"blue"]variation[/color] at that point on the chart.

For example, here in Massachusetts, on the coast, I have a nautical chart that shows a variation of 15 degrees 45 minutes West. When my boat compass is reading North (magnetic), my boat is actually facing west of True North. I'm on a heading of 344 degrees 15 minutes true, even though the compass says 0. I subtracted the variation from the magnetic heading to figure out my true heading.

[color:"red"]A general rule of thumb for variation is this:
Think of variation as an error in your compass relative to finding True North. You want to correct that. If the variation is East, you add it to arrive at true north, Think CAE, Correct Add East. If it's west, like my example above, you subtract. [/color]

Deviation, is normally used to refer to the influence that electronics, wiring, and other magnetic objects have on what your compass will read. On a ship, where most of the things that might affect the compass are in a fixed location, there are methods for determining deviation. For the most part, in the wilderness, deviation is assumed to be zero if you take your compass readings away from metallic objects and electrical influences.

For instructions in a survival kit, you might not want to even worry about deviation, other than saying you should take readings away from metalic objects. For variation if they have a chart, it probably has magnetic north on it, so they can use that when plotting their courses. Half the time, in survival situations, I'd think that the compass will be used to just keep them going in the same general direction, rather than a specific one. They may not even have a map.

Sorry for the long winded answer, I'm sure there are some details I left out that someone else can fill in. I don't know the rule of thumb elsewhere, but, in the US, there is something called the "Agonic Line", where variation is zero. It runs from the North geographic pole, through the North Magnetic pole, and down through the US through a point west of Tampa, FL. Anywhere east of that line has a West variation, and if you're west of that line, you'll have an East variation. Fun eh?

There are a couple places that do a nice job of getting into the details of this. I have done a lot of hiking and orienteering , but, I learned compass and mapping skills originally through boating. Specifically, the US Power Squadron and Coast Guard Auxiliary have some great courses that help with navigation skills on and off the water. A great reference, in a book, is "Chapman's Piloting". It's the bible for boaters. For hiking, I think that several of the outdoor schools do good work with map and compass skills, but, you basically need to get out there and do it when you don't need to, so you're comfortable with it when you do.

I hope you'll share your survival kit info sheet with us when you're done. <img src="/images/graemlins/smile.gif" alt="" />

>East is Least (-) and West is Best (+)

This is probably easier to remember than my "Correct Add East". Someone looking at our two posts might think they were opposites, but, actually, we just were looking at it from different directions.

If you're going from True to Magnetic, the "east is least -" works. If you're going the other way, from a compass bearing to a true north bearing, it's the opposite, which is what I was illustrating.

I think they taught us the concept of "correcting" the compass bearing, (Correct Add East) to help us keep straight whether we were going from magnetic, (which must be corrected for variation) to true,(which is, well, true). Keep smilin.

Yah, I'm going from a flying background, where we steer by a magnetic course. You draw your course on the map, measure true course, then correct it to magnetic and steer that.

In the Marine Corps, we use Right Add, Left Subtract. Meaning, if the deviation is to the right, you add it to your grid, or map direction, and subtract it, if it is to the left. Also, some maps have it listed right near the declination diagram.

http://www.geo-orbit.org/sizepgs/magmapsp.html

I guess I just cheat because if I need to be that precise I used an adjustible compass and set it ahead of time for the area that I will be using it in.

Be sure to update it every few years as the magnetic north pole is on the move! See Earth's Inconstant Magnetic Field on the Science @ NASA website. <img src="/images/graemlins/crazy.gif" alt="" />

Thats exactly what I was looking for. I just started the additional sheets to include in the kit. It's to supplement/correct the information on a set of the Brunton cards. The sum information on direction finding in these is instructions on how to float the disk compass. Actual direction finding is summed up as "REMEMBER: The "N" points north". Nothing about the other methods such as stick & shadow or analog watch.

Your right about the usual use of the compass to maintain a general direction of travel. I ran across an illustration of the effect of only a couple degrees of variation combined with detours around obstacles can have in the woods. I don't remember the distances involved (they were relatively short) but they would have been the difference between a night in the woods on short supplies. The example was a camp located on a creek and traveling to a location and back. You could have found the creek without much problem but once you found it, do you go upstream or down?

Thanks for all the replies. This was exactly what I was looking for.

There is so much good information in this thread that it is probably confusing. One thing that may not be clear is whether you need to convert true to magnetic or magnetic to true, and the truth is that it could be either depending on the situation. But the simplest and most common case where an adjustment would be needed is the following:

1) You have a local topo map or aviation chart, the map's grid is True North (which is most common) and there is an indication of local magnetic deviation in the margin.

2) You use the map to determine your desired direction of travel. This will be relative to True North since that is how the map is gridded.

3) You have a compass with a rotating bezel and degree markers. Set the bezel to read your intended direction of travel in degrees true. Now you are faced with making the adjustment. In this case, add declination West.

Example, you want to travel true north (0 degrees). Local declination is 4 degrees West. Add 0 and 4 to get 4 degrees magnetic for your compass heading. Your compass bezel now reads 4 instead of 0 so you will be travelling 4 degrees magnetic (but still 0 degrees true). Since you are adding west and subtracting east declination, the saying "East is least, West is best" is applicable here. Least means to subtract, best means to add.

One way to verify this with common sense is to look out to the horizon and visualize the magnetic north pole being 4 degrees west of the true North pole. The declination image on the map will also illustrate this. Your compass needle is pointing to the magnetic pole and your direction of travel arrow is pointing to the true North pole and they match your visualization of where they should be.

I think this is the only conversion method you need for basic survival instructions.

The most common reason you would need to convert in the other direction is if you are triangulating your position on a map, by taking sightings on distant landmarks. But that is a more sophisticated procedure requiring a sighting compass, pencil, straight egde and some more knowledge, and is probably beyond the scope of a basic survival kit and basic survival instructions. Presumably, anyone knowledgable enough and equipped to triangulate their position would know how to make the conversion in the proper direction for that application. In this case, the "Correct add East" (CAE) method is used, but I would ignore this and stick with the East/Least method for basic navigation.

If your base compass does not have an adjustable bezel you can prepare your map in advance by drawing parallel lines indicating magnetic north. Adjusting for the declination at home is a lot easier than trying to compute it in the field. Especially for those with little or sporadic map and compass experience.

Spend the extra few dollars and get a real compass with adjustable declination and sighting mirror. 90% of compasses are next to useless for map navigation.

I highly recommend the Silva Ranger models.

Flip

That last is a perfect description of my normal situation. Most of my map use is interpreting the angle between two surveyor's bearings (N XX Degrees XX minutes XX seconds E) . Does anyone know where they came up with this notation? It seems that degrees measured clockwise from north would be much easier.

My guess is that it's a hybrid of "cardinal" points- N-NNE-NE-ENE-E-ESE-SE-SSE-etc

There a two basic compass errors that one needs to deal with, variation and deviation. Variation is the error due to the offset of the magnetic pole from the geographic pole and will be on the chart. Deviation is a compass error due to magnetic material in the vicinity of the compass and for a hand held compass should be zero.

There are a couple of aide memoires that help with the conversion from True (the chart) to Compass (what you read on the compass):

"True -> Variation -> Magnetic -> Deviation -> Compass, Add West"
i.e when converting from True to Compass add west variation and deviation, subtract east.

I usually remember it using the aide memoire:
"True Virgins Make Dull Companions, Add Whisky"

The opposite direction can be deduced by simply reversing the above.

Andy

I keep a little map of magnetic variation in my PSK. There's around a 40-degree variation across the continental U.S. Here are some links with info and maps:
ttp://www.geolab.nrcan.gc.ca/geomag/compass_e.shtml , http://www.thecompassstore.com/decvar.html , and http://www.windtesting.com/4.html .

Steve

It's a relative bearing off of north or south. ie N 45 degrees 0 minutes 0 seconds W would be northwest. N 45 degrees E is northeast. N 135 degrees W would be southwest. N 135 degrees E is southeast. The points would reverse if the notation were S 45 E = southeast. The only place I have seen this notation used is from surveyors or people such as contractors and property deeds that work from their information.

I just wanted to add one more thought about magnetic deviation before this thread becomes inactive:

You can determine your current local deviation by locating the North Star (Polaris) and determining its magnetic bearing. I won't go into how to locate Polaris because this is well covered elsewhere. Making a compass sighting on something high above the horizon is difficult, but can be eased with a plumb line.

Polaris is always within 1 degree of true north and can be used from much of the northern hemisphere. This would give you a value for deviation if you have no access to charts, etc. or would allow you to confirm the value you got from a (possibly outdated) chart. It could also resolve any lingering doubt or confusion about whether to add or subtract the deviation, since you are directly observing true north.

Excellent suggestion! <img src="/images/graemlins/smile.gif" alt="" /> If you know the deviation for the area you're heading to, you could also compare it before you head out or when you are at a known location.