Compass Bearings

What a bearing is

A bearing is a horizontal angle measured from a north reference line, in a clockwise direction, to a point of interest some distance away from the point of measurement.

North is 0°, east is 90°, south is 180°, west is 270°. A bearing alone defines a line — to fix a location you also need a distance, a second bearing from a different point, or an intersection with a known feature on the map (a road, a stream, a contour line).

A bearing has to be referenced to a north — true, magnetic, or grid. See North References (True, Magnetic, Grid) and Declination for the geometry. The discipline that catches more downstream errors than any single other habit: always write the north reference next to the angle. "45°" is meaningless. "45° magnetic" is unambiguous.

What bearings are for

Three uses:

"Where am I?" (Resection.) Take bearings to known visible landmarks; plot them on the map; you're at the intersection. With only one visible landmark, you can still fix your position by intersecting a single back bearing with a known linear feature you're on (road, trail, ridgeline, stream) — you're where the back bearing crosses the feature on the map. Useful when you can see the landscape but aren't sure where on the trail you are.
"Where is that?" (Intersection.) From two or more known points, take bearings to a distant unknown object — smoke from a fire, a cave entrance across a lake, a person waving from a ridge. Plot the bearings; the target is at the intersection.
Route planning. Plot a sequence of bearing-and-distance legs through the terrain you'll cover. Walk the bearings in the field.

Sighting a bearing in the field

Taking a bearing means measuring the direction from where you're standing to a target — a peak, a tower, a feature you can identify on the map.

The technique:

Hold the compass level. Gravity affects the needle's free movement. A tilted compass gives a tilted reading.
Aim the compass at the target. The mechanics differ between a plain baseplate and a mirror compass:
- Plain baseplate. Hold the compass flat at waist level. Rotate your whole body until the long edge of the baseplate (or the direction-of-travel arrow) points at the target. Your eye alternates between the target ahead and the compass below — practice keeping the line straight across the alternation.
- Mirror compass. Open the lid to about 45°. Hold the compass at eye level and sight through the notch in the lid at the target. The mirror reflects the bezel and needle into your sight line so you can read them without breaking your sight on the target. This setup gives noticeably tighter accuracy than the plain baseplate technique.
Rotate the bezel until the orienting arrow boxes the magnetic needle — the needle parallel to the sides of the orienting arrow, with its north end at the north end. On a mirror compass, you do this while still sighting the target through the lid; on a plain baseplate, take your eye off the target to set the bezel, then re-sight to confirm the aim hasn't drifted.
Read the bezel at the index line. Write the bearing down with its north reference (45° magnetic, not just 45°).

Distant targets sight more accurately than nearby ones. A small angular error on a faraway peak translates to a small ground error; the same angular error on a nearby tree can move the bearing line by tens of feet at distance. Pick targets that are identifiable on the map and visible from where you stand: named peaks, distinctive saddles, towers, road bends.

A baseplate compass without a mirror is reliably good to about ±2°. A mirror compass to about ±1° with care. Beyond that, sighting noise from needle oscillation and eye-compass alignment dominates — don't over-state field-sighted bearings beyond what the tool actually delivers. See Accuracy vs. Precision for the underlying principle.

A correctly-sighted compass will still mislead you if there's ferrous metal close to it. A pen with a metal clip in your hand, a metal pocket knife on your belt, a phone in a chest pocket, or a watch with a steel back can deflect the needle by a few degrees with no visible warning. Before taking a critical bearing, check what's within a foot of the compass. See Compass Uses under Magnetic interference for the full catalog of common-culprit objects.

Following a heading

Following a heading means walking in a chosen direction over the ground — most commonly along a route segment planned on the map. The compass gives you the direction; visible landmarks let you stay on the line.

Set the heading and establish landmarks

Set the desired heading on the compass bezel — magnetic if the compass is un-adjusted, grid or true if it's declination-adjusted. With the compass held level at eye level:

Rotate yourself (not the bezel) until the orienting arrow boxes the magnetic needle. Your body is now facing the heading direction.
Pick a forward landmark on the heading line — a tree, a rock outcrop, something far enough ahead that walking to it will take some time.
Establish a back landmark. Without changing the bezel and without doing any arithmetic, align the SOUTH end of the magnetic needle with the orienting arrow's north — i.e., put the south end where you'd normally put the north end. The same bezel setting now reads the reverse heading; the compass is pointed back along your line. Pick a back landmark on this reverse sight — a feature you started from or just passed.
Walk to the forward landmark without staring at the compass. The two landmarks keep you on the line.

When you reach the forward landmark, re-sight a new forward landmark on the same heading and walk to it. The pattern is landmark-to-landmark, not bezel-staring.

Use both sights to stay on the line

The forward landmark keeps you walking the right direction. The back landmark lets you confirm you've stayed on the line as you've walked. Periodically check both:

Forward sight: north end of the needle in the orienting arrow, lined up with the forward landmark.
Back sight: south end of the needle in the orienting arrow, lined up with the back landmark.

When both line up, you're on the original heading line. If one is drifting, you've drifted off — correct your walk to bring both back into alignment.

The two-landmark back-sight is the most reliable way to follow a heading across terrain. It costs no extra bezel manipulation and no arithmetic, and it gives positive confirmation that you're still on the line rather than open-loop "I think I'm still heading the right way."

Forward bearings vs. back bearings

More generally, a back bearing (also called a reciprocal bearing) is the exact reverse of any bearing — 180° in the opposite direction, the same line of sight pointed the other way. If a forward bearing reads 045°, its back bearing is 225°; if 200°, its back bearing is 020°. Whether you call a given bearing "forward" or "back" depends on which endpoint you're standing at when you sight it:

Forward bearing. Sighted from a known location to an unknown location. You're at a known point on the map; you sight to a target you want to locate (a column of smoke, a person on a distant ridge). Plotted on the map starting at your known point, drawn in the bearing's direction. The natural case — "where is that?"
Back bearing. Sighted from an unknown location to a known location. You're at an unknown point on the ground; you sight to a landmark you can identify on the map. To use the bearing on the map, the line needs to start at the landmark and extend backward through it toward your unknown position.

The plotting move for a back bearing — without any arithmetic — is to plot it on the map as if it were a forward bearing from the known landmark, then extend the same line backward through the landmark. Your position is somewhere along that backward extension.

Procedure:

Treat the measured bearing as a forward bearing from the known landmark.
Plot it at the landmark with your normal tool, drawn in the bearing's direction.
Extend the same line backward through the landmark.

That backward extension points to your unknown position. No ±180° calculation needed.

Two or more back bearings from different landmarks produce a resection — the intersection of the backward extensions is your location.

Plotting tools

Three baseplate-style tools, plus a brief note on the lensatic compass:

Protractor

The simplest tool: no moving parts and conceptually transparent. A clear plastic protractor with a center hole, marked in degrees clockwise from a north arrow.

Procedure (with a sighted bearing of, say, 60° magnetic, plotting toward an unknown position from a known landmark):

Convert the bearing to grid (or whatever north reference your map uses). If the local declination is 5° E, magnetic 60° = grid 65°. (See North References (True, Magnetic, Grid) and Declination for the formula.)
Center the protractor on the known point on the map.
Rotate the protractor so its north arrow aligns with the map's Grid North reference lines.
Mark the map at the desired bearing along the protractor's edge.
Use a straight edge to extend the bearing line through the marked point. (Sewing thread or dental floss strung through the center hole works well for long lines.)

A useful sanity check before starting: identify the rough cardinal direction (N, NE, E, SE, S, SW, W, NW) the target should be in. If your plotted line points 80° away from that rough direction, you've made a mistake.

Baseplate compass (unadjusted)

A baseplate compass with no declination correction set on the bezel. The compass functions as both a sighted bearing-taker and a plotting protractor. Procedure to plot a sighted bearing:

Convert to grid. The compass is reading magnetic; the map is referenced to grid. For a 60° magnetic bearing with 5° E declination: add 5°, plot 65° grid.
Set the converted bearing on the compass bezel.
Place the compass's edge at the known point on the map.
Align the compass capsule's orienting lines with the map's north reference lines — not with the magnetic needle. The magnetic needle is irrelevant during plotting and may point in any direction.
Draw the bearing line along the compass edge.

Baseplate compass (declination-adjusted)

A baseplate compass with the declination angle pre-set on the bezel. The compass reads grid (or true) directly, no field conversion required.

Since we adjusted our compass to read directly relative to Grid North, no adjustment needs to be made to our bearing.

Same procedure as the unadjusted compass, minus the conversion step. Faster in the field, but the compass must be re-set when local declination is different. Whenever you move to an area where the declination differs, you'll need to readjust.

The same warning applies: the magnetic needle is not used during plotting and may point in any direction.

Lensatic compass

Lensatic compasses use a sighting and plotting procedure substantially different from baseplate-style technique. They have a significant user base, particularly in military contexts where the lensatic is the standard-issue tool. The procedure isn't covered here; lensatic users will want lensatic-specific instructions.

More bearings, smaller error

A single bearing gives you a line. Two bearings give you an intersection — but compass and plotting errors mean two "intersecting" bearings rarely meet at a single mathematical point. Three bearings tell you something about the size of your error.

The geometry of the landmarks matters as much as the precision of the bearings. Two bearings that cross at nearly right angles produce a sharp intersection; two bearings meeting at a shallow angle (close to 0° or 180° apart) produce a vague one — the same bearing error translates to a much larger position offset.

Pick landmarks spread widely in azimuth: roughly 60°–120° apart for a two-landmark resection, ~120° spacing for three. Two landmarks in nearly the same direction give a poor fix; two landmarks separated by close to 180° (one ahead, one directly behind) are worse — the back bearings overlap as a single line with no useful intersection at all. The same principle applies to intersection (pick base points so the target subtends ~90° at the bearing crossing) and to the single-bearing-plus-linear-feature case (the bearing should cross the feature as close to perpendicular as possible).

The smaller the triangle, the smaller the errors.

Plotting a third bearing produces a small triangle (the triangle of uncertainty) bounding your true position. If the triangle is larger than expected, the four likely culprits are:

Sighting error when taking the bearing (most common).
Map plotting error when drawing the bearing line.
Misidentification of the landmark — you took a bearing to one peak but plotted from another.
Map vs. terrain mismatch — old map with shifted shorelines, washed-out roads, etc.

A third bearing usually identifies which: if it forms a tight triangle with the other two, you've got measurement noise. If it's wildly off, you misidentified something.

When the triangle is small enough to work with and you need to commit to a single point, pick the most likely position from terrain context — for instance, if the bearings intersect just off a mapped shoreline, the shoreline is more likely the true position than a point sitting in open water.

Reading conventions

Three digits, spoken. Over the radio, read bearings as separate digits with leading zeros for values under 100° — "zero-four-five degrees magnetic," not "forty-five magnetic." Not universally followed, but worth the discipline: radio noise turns "forty-five" into "forty-something" more readily than it garbles "zero-four-five."
Always include the north reference. 45° magnetic, 45° grid, 45° true. Don't make people guess.
Bearings are always clockwise from north. Counterclockwise conventions exist in math but not in field navigation.
"Heading," "bearing," and "azimuth" are used interchangeably in most contexts; aviation and marine have stricter distinctions that don't matter for land nav.