For anyone who spends a lot of time in the woods you need to know how to navigate using a map and compass. It's not as easy as you might think. Taking a bearing from the map and utilizing the information you have gotten from the map to move across the ground is a science that needs to be practiced. Its not just a matter of dialing in the bearing and stepping off.

The Norths

There are three different types of North out there and each one has its own use in navigating. True North is the geographic top of the world and the North that coincides with the top of the world is also the one that you read from your GPS if you own one. Magnetic North is approximately 150 West of Frobisher Bay and it moves yearly. It's also the North that your compass points to. Grid North is the top of your map, and it is usually different from True North by a couple of degrees depending on how far North of the equator you are. Think of it this way. The earth is a sphere much like an orange. In order to make a map you need to make a two dimensional representation of something that is three-dimensional. In order to do this you need to "spread" the top of the map in order for it to lie flat. As it spreads the difference between Grid North and True North becomes greater the farther north you go. So in essence we have three different Norths all pointing to different places and giving us differing information. What we need is a way to figure out how to use all the information in order to navigate with precision.

Magnetic North and lines of Declination

Declination

All Maps have some sort of Declination or Magnetic Variation. On a Topo map it can be found as a symbol that looks like this:

This shows the difference between the Norths. The Star equates to True North, The square Equates to Grid North and the Arrow equates to Magnetic North. You can see from our example that the difference between Grid and True North is 0° 32' and the difference between Grid North and Magnetic North is 13°12'. This angle, between Grid and Mag, is known as the Grid Magnetic angle and it is the one we are concerned with when navigating from a map to the ground. In order to get from point "A" to point "B" we need to take a bearing from the map. All a bearing is an angular measurement taken from one point to another with the zero reference being north. In the case of a map that north is Grid North. So when we take a bearing off the map we have, in essence, a Grid Bearing. From our example we can see that there is a difference of a little over 13° between Grid North and Mag North, so if we took our Grid bearing and applied it to our compass we would be out by 13° to start with and the longer the leg we walked the greater our error would be at our arrival point.
So we need to apply the Magnetic Declination to our compass in order to take into account the differences between Grid and Mag. If you remember the following little saying you cant go wrong:

Variation East, Magnetic Least.
Variation West, Magnetic Best.

All that means is that if you have Easterly variation, your magnetic bearing will be the lesser of the two bearings. How can we tell if the variation is Easterly or Westerly? By looking at our example. The Arrow for Mag North lies to the West of Grid North so we have Westerly Variation. If it was on the other side we would have Easterly variation.

So if we take a Grid bearing from the map and we want to make it into a Mag bearing so we can walk across the ground with precision and the Variation is Easterly on our map then we need to subtract the variation from the grid bearing to come up with the correct Mag bearing. If the variation is Westerly on your map then you need to add the variation to the Grid Bearing to come up with the correct Mag bearing.
Now there is one last small calculation that needs to be made in order to be precise in your navigating. The information provided on your topo map is only accurate when the map was printed. In order to be as precise as possible then we need to update the information to today's values. Also included with the declination arrows are when the measurement took place and how much it is changing by. For example

Now we have all the required information we need. In 1995 there was 13°12' declination. It is now 2004, that's a difference of 9 years. There is a change of 3.2' per year. Multiply 3.2' by 9 years gives us 28.8 minutes of change. I round up to 29 in this case. The change is decreasing yearly so we need to subtract our 29 minutes of change from the 13°12' that was measured in 1995. So 13°12 minutes minus 28 minutes leaves us with 12°44' declination. As a rule of thumb if it's below 30 minutes, which is ½ a degree then I round down. More than 30 minutes then I round up. So in this case 12°44' would still be 13° or no real change. You still need to know how to do this especially if you have an older map.

So if you remember the rules for applying the Magnetic Declination to your compass when you are navigating with a map you cant go wrong. Just remember that the bearing you take off your map isn't necessarily the correct one until you convert it.