How to Use a Map and Compass
November 01, 2018
At the start of The Legend of Zelda, you get a wooden sword. You don't have to do anything to get it, but you can't get anywhere in the game without it, so it's just given to you, it's a freebie.
In real life, there aren't many freebies, but there still are some. One is the magnetic compass. The idea that there was, in place even before humans arrived, a system to determine your heading from anywhere on the planet, is magic. It's a freebie. You are an explorer, you'll need this, take it and go.
These days, a GPS handheld or smartphone makes way-finding much easier, but there are still many reasons for learning how to use a map and compass. If anything, it's a lightweight backup that will get wet, take a tumble, and still be happy to point you home weeks after your batteries have died. There's also the nagging thought that depending on electronics is at odds with self-reliance and the freedom of the hills.
Whatever your reason, this article is an introduction to navigation using a map and compass. You will need:
- A topographic map (informally, a topo) of the area you're traveling in. For hiking, it should show an elevation contour at least every 200 ft. Maps with elevation shading make it easier to visualize the topography. The National Geographic topos are nicely shaded, targeted at hikers, and are on tear-resistant, waterproof paper.
- A compass with marks for every degree and a clear baseplate so you can lay it on your map and see through. Small keychain compasses are not accurate enough for navigation. Compasses with mirrors make them easier to use (more on this below) and serve as a signal mirror in an emergency or shaving mirror if you're into that sort of thing (multi-use points!). I usually carry a Suunto MC-2, but only after I misplaced my Brunton Type 15. Both are excellent for hiking.
- A clock. A wristwatch works wonderfully. A mechanical watch will never run out of battery, but a quartz watch can have the added benefit of an alarm if you want to do something silly like get up early in the morning.
Navigation, in General
In short, navigation amounts to correlating your position in the real world with your position on a map, so that you can use the map to see farther and make decisions about what to do in the real world. The quality of your decisions will be a function of how good your map is and how accurately you've put yourself on the map.
Understand that you always have some idea of where you are on the map. Even if you're lost in Yosemite National Park, you know that you're in Yosemite National Park, so you can decide, for example, that walking north will eventually bring you to a road. It probably won't be the most efficient way to get to where you want, but it is a way. You weren't completely lost.
That might sound tongue in cheek, but it really is the heart of navigation. In any navigation problem, there will be knowns and unknowns. Your job as navigator will be to accept the unknowns and make the best decisions with what you have.
Learning to use tools increases your knowns. To get started with the compass, there are two basic concepts to understand: heading and bearing.
Heading is the direction something is facing or traveling, relative to some reference frame. In this article, we'll be talking about magnetic heading: your direction relative to the magnetic north pole. It's measured in degrees of angle, clockwise from pointing directly at the pole.
Bearing is a relationship between you with some other feature of the landscape (a mountain peak, for example). It's what direction you're facing when looking directly at the other feature. If your mountain peak is due east of you, you have to face 90° to look directly at the peak, and we say the peak's bearing is 90°.
Using the Hiking Compass to Find Heading/Bearing
Your compass should have a needle that swings around when you change directions, surrounded by a ring with numbers on it (dial), all mounted on a clear plate (baseplate). You can turn the dial, which moves a fat arrow (orienting arrow) underneath the needle. Turning the dial will also turn a set of parallel lines (longitude lines) with the orienting arrow. And finally, on the baseplate will be a prominent pointer that points at the dial and doesn't move (index line).
To find your heading, point the compass in the direction you're facing or traveling, as if the compass were some sort of blowgun and you were sighting along it to line up with your target. Once on target, turn the dial until the orienting arrow lines up with the needle, then read the number on the dial that the index line is pointing at. This number is your heading.
Taking a bearing is the same as taking a heading, except you are pointing the compass at some landmark instead of your direction of travel.
Take the time to make your measurements as accurate as possible. You'll need to hold the compass at eye-level and use the compass sights to sight-in your targets, but this makes the orienting arrow hard to see and moving the compass away from eye-level can introduce errors. For this reason, some compasses come with mirrors that allow you to sight-in your target and, at the same time, see the arrow via reflection in the mirror.
Once you have a bearing, you can go to your map and start piecing together your position there.
What you want to plot is a line of position (LOP). Given a landmark and a bearing to that landmark, there is only one line in space that you can be on to see that landmark at that bearing. In other words, the bearing narrows down the possibilities of where you are to a single line.
To visualize this LOP, make sure the compass dial is still set to the bearing you just took, align the compass's longitude lines with the map's longitude (north-south) lines, and at the same time move the compass so that the edge of the baseplate intersects the landmark you were shooting. Use this edge as a ruler and draw a line. You are somewhere on this line. Write along this line the bearing value you were plotting. If your plotting goes awry, this will help you find out where things went wrong.
If you have some other piece of information in addition to the bearing, like you know what trail you're on, you're done. Just look at the LOP you just drew, find where it intersects your trail, and that's your position. Other common references are if you know you're on the edge of a lake or along a ridge line. If none of these clues are available, don't worry, just find another landmark to get another LOP off of. Once you have two LOPs, the point where they intersect will be your position.
Once you know your position, draw a dot at the point and then a circle around the dot. In the art of navigation, this is a fix, that is, a high-confidence guess of your position. Also, write the time you made the fix at next to the circle. Always write times as four-digit numbers (24-hr clock) and headings/bearings as three-digit numbers so you don't get confused which is which later on. If for whatever reason you continue traveling but can't get a fix later on (e.g. there's fog and you can't see any landmarks anymore) the time traveled since your last fix will help you make a good guess.
Following a Heading
From here, you either are happy that you know where you are on the map and get on with your travels, or you also need to figure out which direction to travel from here. If the latter, you want to get a heading.
Finding a heading is kind of like plotting a bearing backwards. On the map, line up the edge of your compass with your position you plotted above and with the point you want to get to. Then, turn the dial until the longitude lines line up with those of the map. The number on the index line will be the heading you want to travel to get to your target position. Put the map away, turn yourself until the needle is in the orienting arrow, and you will be facing your target heading. Start walking.
To avoid having to walk constantly looking down at your compass, pick a distant landmark that lines up with your route once you establish your heading. Then, as you walk, you can just walk towards that landmark. Alternatively, take note of where the sun is hitting you when you're on course, and try to keep the sun in the same position relative to you as you walk.
In practice, it'll be a rare case where the terrain lets you walk in a straight line. There will be brush and streams and bumps in the land that encourage you to find ways around them. Aside from maybe marking where your tent is, rough positions are good enough to keep you moving along. Finding precise positions and headings is secondary to reading and moving with the terrain. Practice matching up what you see around you with the topography charted on the map.
You may have noticed that, on the map, the longitude lines are lined up north-south relative to true north while your compass has been pointing at magnetic north the whole time. The angle between magnetic and true north is called declination, and is different depending on your longitude. In the area around the Mississippi River, for example, the magnetic pole is seen directly in front of the true pole, so magnetic compasses point true north, and navigators there don't have to worry about declination. As you move west of of the Mississippi, compasses will start to point east of true, and vice versa on the other side.
Luckily, the declination value is pretty stable and will be printed on the map you're using. Most compasses will have a declination adjustment that lets you dial in the declination offset while you're getting ready at home and then you don't have to worry about it for the rest of your trip. If your compass doesn't have this, or you forget to set it, you have to remember to add/subtract the declination value each time you move between the map and the compass.
If you're hiking a very long hike moving east to west or west to east, your declination will change along the way, and you should remember to update the offset as you go.
Learning to use the map and compass is a first step towards finding your own way in the backcountry. For those who have always hiked by following trails and blazes, breaking off from the established trail snips the last tether linking you back to society. The likelihood of seeing other people falls to near zero, the ground is untrodden, and the viewpoints you encounter may be yours alone for hundreds of years in both past and future. The result is a feeling of independence and acceptance by the wilderness.