I am new to using a compass and thought I had it figured but now I'm not so sure. I watched the "How to use a compass" video on REI. I bought a new compass. I set the declination to coincide with the map I'm using.
Then I took a bearing from point A to B with the new compass. After that I took a bearing for the same points on the same map. This time I used an older compass I have that has no declination markings on it.
The bearing was 232º for both of them. Am I missing something? If the bearings are the same, why do I need to set declination?
So, not sure exactly what was being done there, but, just keep in mind that the top of the map points to true north, i.e. the North Pole, the top of the ‘globe’, and , once upon a time the earth’s magnetic pole, however, the magnetic pole is/has moved, and is still moving, meaning the compass no longer points to true north , aligning a map bearing with the compass bearing. The magnetic pole is pulling the compass needle away from the map bearing. The declination is the amount of pull off center, in degrees, and this ‘correction’ realigns the compass bearing with map bearing, or something like that
This is one of the reasons I like living in the Northern Hemisphere - the North Star. Find the North Star and take a bearing on it with compass A and then again with compass B. If one has not been adjusted for declination, they should differ by the declination amount. Are you sure that is the case? Also, be careful with two compasses in the same space. They often do not play together nicely and, for that matter, other metallic objects (iron) can throw them off. Be sure something like that is not affecting your bearings.
That is one of the reasons I am a North Star fan. It is always within one degree of True North and at midnight, it is exactly on. Far more precise, with fewer disturbing factors than any compass (it just has to be in view...)
Magnetic Declination is the difference between true north and magnetic north. It varies with location and moves year to year. Here is a nifty article from NOAA
The declination adjustment sets your compass to point to true north based on the direction the magnetic field points at your location. You must use the correct declination adjustment for your location for this to work. What declination did you set? It is quite easy to set the declination in the wrong direction. I misunderstood the instructions and did that with my compass only discovering it when trying to relate bearings to known points on the map fortunately before I set off into the wilderness.
As @hikermor points out, the North Star is a good reference for true north to check your compass, if you can find it. However it is not that easy to take an accurate bearing on it with a typical baseplate compass since it is too high in the sky. There is a trick you can do with two different length sticks stuck in the ground, using their ends as a sight. Move the shorter stick around to the south to line its end with the longer stick end and the star. The line between the two sticks will be true north. If you have a mirror sighting compass you may be able to do this with the mirror although I have not tried this.
If you have set the declination on your new compass properly for your location and the old compass agrees there may be a number of reasons:
1. the adjustment is small and within the accuracy of a compasses. Even a good baseplate compass can generally only be read within 1 or 2 degrees and in some places the declination is 0 degrees.
2. the old compass has a declination adjustment set
3. one or other of the compasses is not accurate
4. you did not take the bearing correctly. The declination adjustment creates a new point or outline in the compass center to which you must line up the compass needle. If you just line it up with N on the bezel then you will be using the compass without the declination adjustment and it will just read as a compass without one.
My guess is 4.
Thanks for the responses. After reading them over and taking bearings again I think my problem was I didn't take the process far enough. I had stopped after taking each compass off the map. This time I took bearings with each compass and then set them on the floor with "red in the shed" and looked to see where they were pointing. I made sure I kept them far enough apart so they wouldn't influence each other. It was obvious that they were pointed in different directions. The declination where I am is 12º. If I added 12º to the compass without a declination adjustment they pretty much pointed in the same direction.
@KFridman - here is the good news - in most cases, you will hardly use your compass at all. On land, orientation using the lay of the land is most common, especially if there are prominent landmarks and the sun (or North Star) is visible. The problems arise when fog set in, visibility is limited. Then it is time for the compass - when you need it, you really need it. I can only recall two times in decades of fairly active outdoor activity when I needed to use my compass for orientation. At sea, it is a different situation all together.
remember, declination changes through time, although the annual rate of change is usually infinitesimal.....