I made good use of my compass while working as a forest technician in northern Ontario. The model I used came complete with a magnetic-tipped needle, a compass housing marked with an orienteering arrow and orienteering lines, a graduated dial, a baseplate which doubled as a ruler (measuring in inches and centimetres) and an index line.
However, it also had a few extra gizmos, like a preset declination (a must to have in my opinion) and a mirror with a “bull’s-eye” on top so you can read a bearing while holding the compass at eye level—and check your hair in the morning.
Let’s look at how it works, starting with declination. (More to come in part two!)
The basics behind using a compass is to first note that the red end of the floating arrow always points to “magnetic” north, not “true” north. Since the needle is magnetized, it will always point toward the Earth’s magnetic field (made up of molten iron).
To help confuse the issue, however, this is a different place than “true” north or what’s better known as the “geographical” North Pole. The difference in degrees between these two northern points is called “declination.” To confuse the issue even further, the amount of compass error (declination) is not only dependent on where you are on the Earth, it also changes on a yearly basis. This variance is usually only a few degrees (30 degrees is the largest), but the further a distance you follow it, the more you’re going to head off in the wrong direction.
To compensate for declination, you must first check what the declination is in the area you are traveling. The easiest way to do this is to contact a government agency for that area (Ministry of Natural Resources, Department of Mines, Department of the Interior, etc.) and ask what the declination is. Or Google it. It’s also noted on the margins of the topographic map (usually the upper right-hand corner). Look for a sketch of three arrows: True North (TN), Grid North (GN) and Magnetic North (MN).
With me? OK—let's keep going.
True North marks 360 degrees (0 degrees). Grid North represents parallel vertical lines as seen on a topo map. Magnetic North is marked to either the left (west) or right (east) of the True North arrow, with the variation of degrees shown between TN and GN. You actually want the variation between TN and MN, so either use what’s given, since the amount between TN and GN is never that much to worry about, or add or subtract the difference.
It’s best to own a compass with a declination screw. You can reset the declination by turning the screw (west or east) to the proper degree and never have to worry about it the entire trip. If you don’t have a declination screw, then you’re going to have to do some math to help you find your way.
Remember the phrase: “east is least and west is best.”
If the declination is to the east, then you would subtract the amount of degrees to your bearing. If the declination is to the west, then you would add it to your bearing.
If the declination is 10 degrees W, and your bearing is 42 degrees, you would then follow 52 degrees. If the declination was 10 degrees E, and your bearing is 42 degrees, then you would follow 32 degrees.
This is where having a declination screw comes in handy. This all sounds confusing, which is why having the declination already adjusted on your compass is a good idea; you’ll never have to add or subtract each time you take a separate bearing.
A field bearing is a degree (heading) calculated without the aid of a topographic map. This technique actually isn’t used that often out there. Mostly, we keep an eye on the map and use obvious land forms (i.e. islands, large hills, creek mouths) to keep on track. But let’s say you find yourself “confused” while crossing a big lake and you want to stay in the right direction. Or how about when you just want to take a day-hike up a ridge you’ve spotted from your campsite? Here’s how you do it:
- Hold your compass level, at chest height.
- Point the direction-of-travel arrow (the top of the compass) at the point you want to go (i.e. other side of the lake or a particular ridgetop).
- Rotate the compass housing (the round dial with all the degrees marked on it) until the orienteering arrow (the one that doesn’t float) is perfectly lined up with the compass needle (the red pointed arrow that floats).
- Read off the bearing (set in degrees) that is lined up with the direction-of-travel arrow.
- Keep following that bearing by constantly aiming off on an object ahead; put the compass away, walk to the object, take the compass back out, and aim off another object.
- To get back to where you once were, just figure out the reciprocal (the opposite bearing). This can be found by either subtracting 180 degrees from your previous heading or just look at where the other end of the red needle is pointing to.
Stay tuned for part two!