Compass Uses

What a compass is used for

In wilderness navigation, a compass is used to do five things:

• General orientation to the four cardinal directions — which way is north, south, east, and west.
• Orienting a map to north — rotating the map so the directions on the paper line up with the directions on the ground.
• Traveling along a heading — walking in a chosen compass direction, either to maintain a general direction of travel or to follow a precise bearing.
• Finding the location of an unknown thing — sighting bearings to it from two or more known positions, then plotting those bearings on the map (intersection).
• Finding your own location — sighting bearings to known landmarks from where you stand, then plotting the back-bearings on the map (resection).

Each of these uses is real. Each one also has limits, and several of them are routinely overestimated.

A compass is one tool among several. For some uses, almost any compass — including a $5 zipper-pull — is enough. For others, you need a real baseplate compass and practiced skill, and even then another tool often does the job better. In particular, for the "where am I?" question, a GPS is faster, easier, more reliable, and more accurate than a sighted-and-plotted compass bearing.

That position runs against decades of land-navigation training tradition, much of which persists because that's how it was taught, not because it's still the best method available today.

What the compass is not is a piece of safety gear that protects you by being in your pocket. Carrying a compass without the skill to use it doesn't make you less likely to be lost. What actually keeps you found is a map, the ability to read it, awareness of the terrain around you, and — for fixing your position — a GPS.

Where the compass does come into its own is in conditions where the visible landmarks navigation usually relies on disappear: fog, whiteout, dense forest, night travel. In those conditions traveling along a heading goes from optional to essential, and the compass — paired with the skill to use it — goes from extra weight to load-bearing gear.

The compass earns its place in specific, named uses; what follows is what those uses are and where they end.

Orientation to the cardinal directions

The compass tells you which way is north — and from there, which way is south, east, and west.

Most of the time, you can get this right without a compass. The sun, the time of day, prevailing wind, knowledge of which mountains lie which way, and the situational awareness you've built up walking in — all give you enough to know which way is north. The compass adds little once you're actually paying attention to your surroundings.

This is the same principle the next section uses for the simplest approach to orienting the map — just applied to your own awareness rather than to the paper. In one direction you rotate the map to match the world; in the other you rotate your sense of direction to match what the compass tells you. The skill is the same and the situations that call for it are the same.

A simple, low-cost compass is good enough for this use — a zipper-pull or button compass is plenty.

Orienting your map to north

Orienting the map means rotating it so the directions on the paper line up with the directions in front of you — the north edge of the map points to actual north, the road that runs east on the map runs east in your field of view.

There are three approaches to orienting the map, each more accurate and more effortful than the last.

Cardinal awareness and visible terrain

Use your awareness of the cardinal directions, combined with visible terrain features. Rotate the map until features on the paper point in the same direction as those features in front of you.

The least accurate of the three, but often more than adequate. Fast and easy. No compass needed at all.

The limits are visibility and feature recognition: in dense cover, fog, or featureless terrain, this method doesn't work — which is the situation the next two approaches exist for.

Align the needle with the magnetic-north arrow

Place a compass on the map near the magnetic-north arrow of the map's declination diagram. Rotate the map and compass together until the compass needle points in the same direction as that arrow.

More accurate than the previous approach, and more difficult. The arrow must be the magnetic-north arrow, not the true-north or grid-north arrow — see North References (True, Magnetic, Grid) and Declination for the geometry. Line up with the wrong arrow and the map ends up off by the local declination, which can be tens of degrees.

Set the compass to account for declination

Set the compass to account for the magnetic declination, either by dialing it in on the bezel directly or by using the compass's declination adjustment. Align the orienting lines on the baseplate with the true-north (or grid-north) lines on the map, and rotate the map and compass together until the magnetic needle is boxed in the orienting arrow.

The most accurate of the three methods, but somewhat tedious and error-prone. Accuracy at this level is seldom required for orienting the map.

The mechanics here — declination adjustment, baseplate orienting lines, boxing the magnetic needle — are shared with bearing work, which is why the technique often gets taught alongside bearings. But accurately orienting the map is not a prerequisite for taking or plotting bearings with a baseplate or protractor compass; the two are independent operations. See North References (True, Magnetic, Grid) and Declination for the underlying geometry.

Traveling along a heading

The compass is used for travel in two distinct ways. They share mechanics — you're using the compass to keep yourself pointed in a chosen direction — but they're applied to different problems and tolerate very different amounts of error.

Traveling in a general direction

Use the compass to keep yourself walking in roughly the right direction. Doesn't require much accuracy. The compass's job here is to prevent the slow drift into wandering or walking in circles that happens when you're following no fixed reference at all.

The compass is particularly useful for this at night and in poor visibility, when the terrain features and sun cues that would normally keep you oriented disappear.

Any compass works for this — a zipper-pull is enough.

Traveling along a specific heading

Use the compass to walk a precise compass direction, like 48° M, often as part of a route plotted from the map.

Before reaching for this use, be clear about what it costs. Straight-line travel on land is rarely the best route between two points — often it's an exceedingly difficult one. For most route-finding decisions, straight-line bearing-following should not be high on the list of options. See Route Planning and Following for the alternatives that almost always do better.

Where specific-heading travel does earn its place: routes designed to avoid specific hazards on either side of the line, typically combined with night travel or poor visibility.

Even in these cases, a preprogrammed GPS route is usually a better option: a GPS receiver shows you when you're drifting off the route, while a compass heading only tells you which direction to walk — not whether you're still on it.

Finding the location of an unknown thing

You can see something — a feature in the landscape, a column of smoke, a person on a distant ridge — and you want to know where it is on the map. From two known positions, sight a bearing to the unknown thing. Plot both bearings on the map. The thing is at the intersection. This is intersection.

This use case is uncommon in recreational outdoor travel but real in specialized work. For something already named on the map (a peak, a road junction, a labeled feature), you don't need bearings — you just look it up.

The limits:

• You have to be at known positions yourself, or able to move to known positions between sightings.
• The unknown thing has to stay visible from both points (smoke drifts; people move).
• Bearings have to be measured and plotted against a consistent north reference.
• Misidentifying the landmark you're standing on is the most common failure mode — and silently produces a wildly wrong fix.

Two bearings give you a point. Three or more produce a small triangle whose size tells you about your error. See Compass Bearings for the procedure and the triangle-of-uncertainty discussion.

GPS doesn't directly help here: a GPS tells you where you are, not where a thing you can see is. For this specific use case, a compass and map are the right tools — provided you can be at known positions while the thing stays visible.

Finding your own location

You're not sure exactly where you are. You can see two or more landmarks that you can identify on the map. Sight a bearing to each landmark from where you stand. On the map, draw the back-bearing from each landmark — the line going in the opposite direction of the bearing, which passes through your position. You're where they cross. This is resection.

This is the canonical "I'm lost, where am I?" use case. It's also the use case where another tool now does the job better. A GPS gives you your position directly — faster, more accurately, and without requiring visible landmarks. For the "where am I?" question on a working GPS, resection isn't the right answer; the GPS is.

The limits:

• You need visible, identifiable landmarks. That rules out the conditions you most often need a position fix in: fog, dense cover, featureless terrain, night.
• You need to correctly identify each landmark on the map. Peak misidentification is the most common failure mode, and silently produces a wildly wrong fix.
• The identifiable landmarks should ideally all sit on the same map sheet that covers your position. A landmark on an adjacent sheet is workable but awkward — the bearing line has to be extended across the sheet boundary. Landmarks two or more sheets away are virtually unworkable in field conditions.
• Bearings have to be measured and plotted against a consistent north reference.
• The procedure requires real practice — sighting in the field, converting north references, plotting back-bearings — all working together under field conditions, often with cold hands and wind.

Resection earns its place when a GPS isn't available or working and you have the map and the practiced skill to use it. Otherwise the GPS does this job better.

The procedure (sighting, back-bearings, plotting, the triangle of uncertainty) lives on Compass Bearings.

Magnetic interference

Every compass operation assumes the needle points to Magnetic North. When it doesn't, the operation produces a wrong answer with no warning indicator. Interference comes from two sources.

Things on or near you

Anything ferrous or magnetic close to the compass will pull the needle. The common culprits:

• A phone in a chest pocket or held near the compass.
• A camera on a lanyard.
• Belt buckles, metal-cased pocket knives, watches with a steel back.
• Resting the compass on a vehicle hood, a metal gate, or a steel-framed table.

A useful habit: when a bearing surprises you, before second-guessing the map, check what's within a foot of the compass.

Things in the ground

Local geology can deflect the needle from the regional declination. In most places the effect is small enough to ignore. In a few places it's large enough that the compass is rendered useless. Three of the better-known cases:

• Ramapo Mountains, northeastern New Jersey — iron ore deposits.
• Malpais lava flows near Grants, New Mexico, north of the Gila Wilderness.
• Kingston Harbor, Ontario — magnetite and ilmenite deposits produce 16.3° W to 15.5° E of anomalous declination over two kilometers.

See North References (True, Magnetic, Grid) and Declination for the full list and the geometry of how anomalous declination is plotted and corrected.

In unfamiliar terrain, a useful sanity check before relying on the compass: take a bearing between two map features whose true relative direction you can read from the map. If the compass agrees, you're fine. If it doesn't, you've learned something important about where you are.

"I have a compass, so I'm safe"

The notion that simply having a compass makes you less likely to get lost is a placebo — common, comforting, and wrong. The compass is a precision tool that rewards skill; carrying one without the skill to use it confers no real safety.

What actually keeps you found is:

• A map you can read.
• Awareness of the terrain around you and where you've been on it.
• For the "where am I?" question specifically, a GPS.

The compass earns its place in those named uses — most of which can be done with a very simple compass, and some of which (notably resection) are done better by another tool. Outside those specific uses, the compass in your pocket is comfort, not safety.

How much compass do you actually need?

Each named use has a different answer to "what's the simplest compass that can do this?"

Most outdoor travel only needs the first few rows. If the only uses you actually rely on are cardinal orientation, the simpler approaches to orienting the map, and traveling in a general direction, a $5 zipper-pull is enough compass. The precision baseplate-with-mirror in your pack is mostly weight you're carrying for uses you don't actually do.

The honest question: which of these uses do you actually rely on in practice? Not which uses you've been taught the technique for — which ones do you actually use when navigating?

Further reading on maptools.com

• 21st Century Navigation Handouts (PDF) — printable reference handouts covering the modern compass-and-GPS workflow

Common student misconceptions