4	TRUE NORTH AND THE UNFAITHFUL NEEDLE

How to Correct for Declination

Sometimes reading explanations of how to reconcile true north and magnetic north is like watching a clown cross his arms, point in opposite directions, and say, “He went thataway.” But fear not: There are simple, easy-to-comprehend ways to solve declination problems. The best way, as I mentioned in Chapter 2, is to buy a set-and-forget compass. I’ll show you how to use one later in this chapter, after I’ve laid the groundwork for you to understand the explanation.

Even if you didn’t buy a set-and-forget compass, you can still learn to correct for declination. You don’t need to memorize rules or silly rhymes; you just need to remember some straightforward logic.

Declination, as I’ve already mentioned, is the difference in direction between magnetic north and true north. Magnetic north is defined by the direction a compass needle points. True north, also called geographic north, is defined by the direction to the geographic North Pole—one end of the Earth’s axis of rotation. Declination, therefore, is an angle. It’s measured with true north as the starting point. If magnetic north lies to the east of true north (to the right, or clockwise, as we look at a map), we say the declination is east. If magnetic north lies to the west of true north (to the left, or counterclockwise), we say the declination is west. East declination is measured clockwise from true north; west declination is measured counterclockwise. (This is in contrast to bearings and courses, which are always measured clockwise.) In other words, if magnetic north lies 10 degrees east of true north, the declination is 10 degrees east. If magnetic north lies 10 degrees west of true north, the declination is 10 degrees west, not 350 degrees.

Declination in the United States varies from about 20 degrees west in Maine to 21 degrees east in Washington and as much as 30 degrees east in parts of Alaska. The agonic line, where the declination is zero, runs from the Great Lakes to Florida. Ignoring the correction for declination can lead you seriously astray and raise the rude possibility of eating shoe leather for dinner as you bask in the warmth of your cigarette lighter. For each degree that your course is in error and each mile that you travel, you’ll be off by 92 feet. If the declination is 20 degrees, you’ll be off by 1,834 feet—one-third of a mile—after hiking just 1 mile. It’s pretty hard to relocate your tent if a third of a mile of timber or fog separates you from it.

Strictly speaking, it’s incorrect to say, “The compass needle points to the magnetic north pole.” What the needle actually does is align itself with the Earth’s magnetic field. That magnetic field resembles a skein of yarn when the cat’s done playing with it. Compass needles may or may not actually point at the magnetic north pole itself, which in 2007 was near Canada’s Ellesmere Island and moving toward Russia at a rate of around 35 miles per year. For the sake of simplicity, let’s just say the compass needle points to magnetic north.

For a wilderness traveler the exact position of the magnetic north pole doesn’t matter. If you’re using the most recent USGS map available, the declination it gives will be accurate within a degree or two. If you need the most up-to-date value for the declination, visit www.ngdc.noaa.gov/geomagmodels/IGRF.jsp, a website run by the National Geophysical Data Center, a division of the National Oceanic and Atmospheric Administration. There you can search for the current declination by entering the latitude and longitude of the region where you’ll be traveling, or search by zip code. Handheld compasses, even mirror-sight ones, are only accurate plus or minus about 2 degrees, so such precision is unnecessary unless you are using a tripod-mounted pocket transit, which is essentially a highly accurate compass and inclinometer. I actually purchased a Brunton Pocket Transit when I was working on a series of photographs in Arches National Park where I needed extreme precision (plus or minus half a degree) in calculating the position of the sun and moon, but I’ve never needed my pocket transit for navigation.

Although declination changes slowly as you travel east or west, you can assume that the needle always points in the same direction within the bounds of the area you can cover in a typical human-powered trip. On the East Coast, for example, where declination changes relatively quickly, you’d have to travel east or west at least 50 miles for the declination to change 1 degree.

All USGS maps have a declination diagram in the bottom margin similar to the one shown in figure 4-1, which happens to show a declination of 14 degrees west. True north is always indicated by the line with a star, which symbolizes Polaris, the North Star. The magnetic north line is indicated by “MN.” On some maps, grid north, a concept we don’t need to worry about, is indicated “GN.” The amount of declination, in degrees, is written beside the diagram.

Correcting for Declination with a Standard Compass

Let’s assume for the moment that you didn’t spend the extra money to buy a set-and-forget compass. (If you did, consider your money well spent and skip ahead to the section below labeled “Using a Set-and-Forget Compass.”) Before you can understand how to correct for declination with a standard baseplate compass, you need to ingrain two facts in your mind. First, every angle that you measure on a map is measured clockwise, with true north as the starting point. The needle is irrelevant. You’re simply using the compass as a protractor. Second, every angle that you measure in the field by placing the needle in the gate of the compass is also measured clockwise, but the starting point is magnetic north.

Let’s refer to angles with true north as the starting point as true north bearings or true north courses. (They’re essentially the same. A bearing is just a direction to a landmark; a course is a direction you’ll follow.) All angles measured on a map, starting from true north, will be true north bearings or true north courses. All angles measured with a standard compass, using magnetic north as the starting point, will be magnetic north bearings or magnetic north courses.

Now let’s assume that you’re hiking in the Colorado Rockies, where the declination is always east. Take a look at figure 4-2. True north is marked zero degrees. Magnetic north is marked 10 degrees. It lies to the east of true north. In other words, the declination is 10 degrees east. Now let’s say you measure a course on the map to a landmark and find it to be 90 degrees. That’s a true north course; you started measuring at true north, just as the diagram shows. Now if you take a bearing on that same landmark with your compass, you’ll find the bearing to be 80 degrees. That’s a magnetic north bearing, since you started measuring at magnetic north.

The true north angle, measured on the map, is greater than the magnetic north angle, measured in the field with the compass by placing the needle in the gate. Furthermore, the difference is 10 degrees—exactly the amount of the declination. And that leads to our first conclusion: When the declination is east, true north angles (bearings and courses) are always going to be greater than magnetic north angles (bearings and courses). If you measure an angle on the map and want to transfer it to your compass, you must subtract the declination from the true north angle, because magnetic north angles are always smaller than true north angles when the declination is east. If you measure an angle with your compass and want to transfer it to the map, you must add the declination, because true north angles are always greater than magnetic north angles when the declination is east.

You don’t need to memorize these rules. Just remember the logic behind them. If you need to jog your memory, look at the declination diagram on the bottom of the map. Notice how it closely resembles the part of figure 4-2 that is drawn with heavy lines. If magnetic north is east (clockwise, or to the right) of true north, then every angle measured clockwise from true north must be greater than the same angle measured clockwise from magnetic north. To reinforce this concept one more time, look at figure 4-3. It shows the relationship between magnetic north and true north for a situation where the true north bearing is 270 degrees.

The same logic applies if the declination is west, as it is on the East Coast. Look at figure 4-4. The declination is again 10 degrees, but this time it’s west. Let’s say you measure an angle on the map (starting at true north) as 90 degrees. If you measure the same angle in the field with your compass, starting at magnetic north, you’ll get 100 degrees. The difference, 10 degrees, is equal to the declination. That leads to our second conclusion: When the declination is west, true north angles (bearings and courses) are always going to be smaller than magnetic north angles (bearings and courses).

If you measure an angle on the map and want to transfer it to your compass, you must add the declination to the true north angle, because magnetic north angles are always greater than true north angles when the declination is west. If you measure an angle with your compass and want to transfer it to the map, you must subtract the declination from the magnetic angle for the same reason: True north angles are always smaller than magnetic north angles when the declination is west. Again, the declination diagram in the bottom margin of the map should help refresh your memory should you ever forget.

Figure 4-5 gives another example of the relation between magnetic and true north when the declination is west, this time for a true north angle of 270 degrees.

Sometimes in the East you’ll measure a true north angle on the map and then find when you add the west declination that you’ve gone past 360 degrees. For example, you might measure a true north angle on the map as 355 degrees, then need to add a declination of 10 degrees west to get the magnetic angle: 355 + 10 = 365. An angle of 365 degrees is the same as an angle of 5 degrees. You can also just rotate the compass dial counterclockwise 10 degrees to add 10 degrees to the true north angle of 355 degrees and get the correct magnetic north angle of 5 degrees. Figure 4-6 shows this graphically.

Sometimes in the West you’ll measure a true north angle on the map, find it’s less than the declination, then need to subtract the declination from the true north angle to get the magnetic north angle. For example, the true north angle, measured on the map, might be 5 degrees, and you’ll have to subtract a declination of 10 degrees east. Five degrees is the same as 365 degrees, so you can just subtract 10 from 365 and get 355 degrees as the correct magnetic north angle. You can also just rotate the dial clockwise 10 degrees from its original setting of 5 degrees to do the subtraction and reach the correct magnetic north setting of 355 degrees. Figure 4-7 shows this graphically.

Using a Set-and-Forget Compass

If you’re using a set-and-forget compass, draw a big sigh of relief at this point. You can forget about adding and subtracting angles once you’ve set the declination and double-checked that you set it in the right direction. With most such compasses setting the declination is idiot-proof. Usually you turn a small screw or perform some other simple operation to adjust the compass gate so it points to the angle representing the declination. If the declination is 20 degrees east, for example, the compass gate, after adjustment, will point to 20 degrees. If the declination is 20 degrees west, the compass gate, after adjustment, will point to 340 degrees (360 – 20 = 340). When you’re measuring an angle on the map with a set-and-forget compass, you ignore the needle (as always) and the compass gate (which no longer points to north on the capsule). Instead you always use the capsule’s north-south lines when you’re orienting the capsule north and south, making sure north on the capsule points to north on the map. To transfer that angle to your compass, simply place the needle in the gate. Angles measured with your compass can be transferred directly to the map, again using the north-south lines in the capsule, not the gate. As you can see, set-and-forget compasses let you avoid mental gymnastics when you’re cold and wet and would much rather be thinking about dinner courses than compass courses. If you plan to go hiking more than once this year, buy a set-and-forget compass. You’ll never regret it.