So you’ve just purchased a shiny new compass, now it’s time for an epic backcountry adventure, right?
Not quite so fast there. Before you go out into the field and attempt to navigate with your compass, there are a few things you should know how to do. Though your compass is one of the ‘Ten Essentials’ you should always have handy when traveling into the backcountry, simply having it won’t be much help to you unless you know how to properly use it.
That’s why we’ve created a different list of essentials: 3 Essential Compass Skills. These are a few basic compass techniques you should be familiar with before attempting to navigate with your compass in the field. These skills won’t turn you into a master navigator overnight, but they will give you enough demonstrable information to get started, and provide a basis from which to continue learning.
Adjusting for Declination
Now you probably already understand that you need to turn the compass horizontally so that the red end of the needle points toward the “N” on the compass housing. What you may not know, though, is that this North is not necessarily the same as the North represented on your topographic map, and before you try to use the two in tandem, you’ll need to reconcile that difference, known as ‘declination.’
Compasses determine North by picking up the Earth’s natural magnetic field. However, for a variety of reasons including man-made sources of magnetic interference and the shifting of molten metals within the Earth’s core, this ‘magnetic north’ often points in a direction up to 30 degrees off of ‘true north’ — that is, the direction toward the geographic north pole.
Near the bottom of every USGS map is a diagram that tells you the magnetic declination for the area depicted in the map (Because declination can change significantly over time, it’s crucial to make sure your map is current). In the diagram, true north is represented by a star, while magnetic north is abbreviated as “MN.” The diagram will also have an entry for ‘grid north,’ (“GN”), but we won’t be using that measurement. Magnetic declination is the number of degrees between true north and magnetic north. This declination value will be positive if magnetic north is east of true north, and negative if magnetic north is west of true north.
How you adjust for declination will depend on whether yours is positive or negative. If the declination in your area is positive, then you’ll need to subtract that degree value from your map bearing in order to follow it in the field, and add the value to any field bearings to plot them on the map. What if, on the other hand, your declination is negative? Add the value to any map bearings before following them in the field, and subtract the value from any field bearings before plotting them on your map.
Before you set off into the wilderness, you’ll need to determine where you are on your map, and one of the most accurate ways to do this is known as ‘triangulation.’
To triangulate your location, you’ll first have to find two landmarks in sight that you can identify on your map with a significant amount of confidence. When you’ve chosen your landmarks and found them on the map, you’ll need to take a bearing on one of them (We’ll call this one landmark A). Once you’ve taken a bearing for landmark A, adjust for declination to convert it to a map bearing, and set the compass to it. Then, place your compass on the map with one of the longer edges of the baseplate intersecting landmark A. With the baseplate’s edge still on landmark A, rotate the entire compass until its orienteering lines match are parallel to the map’s meridian lines, ensuring that the orienteering arrow points north and not south. Use a pencil to lightly (but visibly) trace a line along the edge of the baseplate that is intersecting the landmark.
Perform the same process with landmark B, and your pencil lines should intersect (If not, simple extend them until they do). Provided you’ve done everything correctly, the point where the lines intersect will be your position.
Calculating a Back Bearing
When in the wilderness you may find it necessary to retrace your steps. Whether you’ve discovered that your path is impassible and decided to turn back, or just need to return to camp after gathering water, you’ll almost certainly need to go back where you came from at some point. Just turning around and retracing what you think was your path leaves too much room for error, though, and can potentially get you lost.
In order to retrace your steps accurately, you’ll need to calculate what’s known as a ‘back bearing’ or ‘back azimuth.’ Luckily, doing so is exceptionally simple. If your initial bearing is 180 degrees or less, just add another 180 degrees to determine your back bearing. Conversely, if the bearing you were traveling is greater than 180 degrees, subtract 180 degrees from it to determine your back bearing.
Taking the time and effort to calculate a back bearing may seem excessive, but this simple calculation can mean the difference between making it back to camp safely and getting totally lost.
While these skills will certainly get you started, they’re far from all there is to know about compass navigation. If you’re interested in learning more about navigating with a map and compass, we recommend seeking out an instructional course in your area. Many colleges, universities, and even some outdoor outfitters offer classes on navigation that will allow you to get hands-on practice and personalized instruction.