(This is also posted on the LTP forum)

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... To find your magnetic declination...

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.

That's a new method to me... thanks!

The shadow stick method I remember is: Plant the stick. Mark the shadow's end. Wait, say, 30 minutes. Mark the shadow's end. The line through the shadow's ends is supposedly true E/W. In the northern hemisphere, the first mark you made will be more Westerly than the second mark you made, so you will be facing North if you put your left toe at the first mark and your right toe at the second mark. It's spiffy that it doesn't matter what time of day you do this (or does it? Is it most accurate near noon?)

How does this method compare for accuracy with the shortest-shadow method?

The only way long shadow works is if you have a flat surface. If there is deviation then the shadow will vary in length. You are right though, a longer shadow gives a longer base line to derive a line of sight. Forget about the clock and noon. Think of the arc of the sun at it's highest point creating a shadow that is on a true meridian.

Let's see if I understand: Long shadow can be done any time of day, but requires a flat surface. Short shadow takes more time and has to include local noon, but does not require a flat surface. Did I get that right?

My question about noon was in relation to the "long shadow" technique, not the "short shadow" technique. I was wondering if "long shadow" gives you a true E/W line regardless of the time of day (or is it more accurate at, say, noon).

From what I'm finding on the net, I was wrong about how long-shadow works. It looks like the marks will form an arc, not a line, so you can't just draw a line between two of 'em and have E/W. You need to draw a line between two marks that are each the same time from solar noon. If you're in a hurry and don't need exact E/W, you can just take two marks near noon and use those, but it'll be off. I hope I've got it right this time.

That's fine. I'm only planning on being lost and without a compass around noon anyhow <img src="/images/graemlins/wink.gif" alt="" />

The short shadow also requires flat ground but any slope to the ground will accentuate error, therefore using two long shadows to find an east west line leaves more room for error.

I'm not sure I understand; how you can get a true east west line without taking reading both before and after high noon.

Your understanding is correct (mine, in my first post, was wrong). Here's how I think the long-shadow works: You can get an approximate E/W line using any two marks. The closer to solar noon, the closer to reality the E/W line will be. To get an exact E/W line, you'll need one mark some time before solar noon, and a second mark the same amount of time after solar noon.

I think. This is a lot more astronomy than I ever learned in school <img src="/images/graemlins/smile.gif" alt="" />