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#35533 - 12/21/04 03:39 PM Re: Adjusting compass bearings

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.

#35534 - 12/21/04 08:50 PM Re: Adjusting compass bearings
billvann Offline
Old Hand

Registered: 05/10/01
Posts: 780
Loc: NE Illinois, USA (42:19:08N 08...
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.
Willie Vannerson
McHenry, IL

#35535 - 12/21/04 10:12 PM Re: Adjusting compass bearings

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.


#35536 - 12/22/04 02:15 AM Re: Adjusting compass bearings
UTAlumnus Offline
Old Hand

Registered: 03/08/03
Posts: 957
Loc: East Tennessee near Bristol
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.

Edited by UTAlumnus (12/22/04 02:18 AM)

#35537 - 12/22/04 03:33 AM Re: Adjusting compass bearings
GoatRider Offline
Old Hand

Registered: 08/28/04
Posts: 817
Loc: Maple Grove, MN
My guess is that it's a hybrid of "cardinal" points- N-NNE-NE-ENE-E-ESE-SE-SSE-etc
- Benton

#35538 - 12/22/04 03:09 PM Re: Adjusting compass bearings
pteron Offline

Registered: 10/01/01
Posts: 59
Loc: UK
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.


#35539 - 12/23/04 12:00 AM Re: Adjusting compass bearings
Steve Offline

Registered: 05/29/04
Posts: 84
Loc: North Carolina
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 .

"After I had solaced my mind with the comfortable part of my condition, I
began to look round me, to see what kind of place I was in, and what was
next to be done"

#35540 - 12/25/04 12:05 AM Re: Adjusting compass bearings
UTAlumnus Offline
Old Hand

Registered: 03/08/03
Posts: 957
Loc: East Tennessee near Bristol
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.

#35541 - 12/27/04 09:32 PM Re: Adjusting compass bearings

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.

#35542 - 12/28/04 03:18 AM Re: Adjusting compass bearings
UTAlumnus Offline
Old Hand

Registered: 03/08/03
Posts: 957
Loc: East Tennessee near Bristol
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.

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