All of us have magnetic directional compasses in our kits, the button size compasses are very small and useful only for general locating, plus or minus 20 degrees at best, wrist watch compasses may be slightly better. Most larger handheld compasses that have the feature of a rotatable needle ring (the graduated circle) will allow the user to set the magnetic declination for that particular region, and establish a line of sight.
A simple definition of magnetic variation is, the difference in degrees, and minutes between magnetic north and true north. Knowledge of your regional magnetic declination is essential if you are to use your compass for any kind of accurate work.
Declination Explained Magnetic variation changes over the earths surface in irregular and inconsistent patterns. One can determine the latest declination readings for your region here
Find your magnetic declination here. Historically, explorers, surveyors and cartographers didn't understand the why and how of declination, however they did have ways of correcting the errors of the magnetic needle. Today, these primitive methods could be very useful to persons who find themselves in an area where the magnetic declination is unknown.
In order to find a true meridian (north south line) one must turn to astronomical references. While the north star is always a good general reference, one method to accurately determine true north and your local magnetic declination is to use a shadow stick.
The shadow stick is easy to use but takes time in one location to allow the sun to pass through the midday hours. On a relatively level piece of ground, drive a straight staff, or stick into the ground and make it plumb (use a weight at the end of a string so you have a plumb line reference). If you are in the Northern hemisphere place yourself on the north side of the staff. From mid morning, begin to plot, either with small stones or sticks, the end of the shadow left by the top of the staff. If you continue plotting these points through the arc of the sun at it's highest point and past, you will end up with longer shadows in the morning and afternoon and short shadows as the sun is at it's highest point. Find the shortest shadow point. Next plot an extended line through this point and through the center of the staff. (again. using a plumb line allows you to extend a visual line several yards).
This line you have established is a true north/south meridian or very close to it. Establish a line of sight with your compass along this true north line and turn the needle ring until the zero azimuth is pointing to true north. You have now established your general magnetic declination because your needle is not pointing at your true north line it is now reading the declination east or west of true north.
There are many other methods of establishing true north when you are in a pinch and don't know exactly where you are.... This is an easy one to remember.
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