How to Use a Lensatic Compass: How Military Compasses Work
A lensatic compass is the standard issue magnetic compass of US land forces: a sighting compass with a floating dial, a magnifying lens and luminous markings, built to take and follow a bearing by day or by night. It needs no batteries, no satellite signal and no data link.
This article explains how a magnetic compass works, what each part of a lensatic compass does, how to measure and follow an azimuth, and how mils, declination and tritium illumination fit into military land navigation.
How a Magnetic Compass Works
The short answer to how compasses work is that a magnetised needle or dial turns to line up with the Earth’s magnetic field. The Earth’s field behaves broadly like that of a bar magnet, and a freely pivoted magnet settles along it, pointing towards magnetic north. Every bearing a compass gives is measured from that line.
Two details matter in the field. First, the field lines are not horizontal: they tilt into the ground more steeply the closer you are to a magnetic pole. A compass only uses the horizontal component, so its needle or dial is balanced for the region it will be used in. A compass balanced for the wrong hemisphere tilts and can drag on its housing.
Second, a free magnet swings before it settles. Liquid-filled compasses slow the needle with fluid. The military lensatic compass uses induction damping instead: as the magnet moves, it induces eddy currents in a copper element, and those currents oppose the movement. The braking force exists only while the dial is moving, so it settles the dial quickly without pulling it off the reading, and there is no liquid to freeze, thicken or leak.
What Is a Lensatic Compass?
The lensatic compass is the instrument described in US performance specification MIL-PRF-10436N: an induction-damped, lensatic, unmounted magnetic compass with 5 degree and 20 mil graduations, for individual use by day and at night. It takes its name from the lens in the rear sight, through which the dial is read while the front sighting wire is held on a distant object. That, in short, is how a lensatic compass works: the magnetised dial holds magnetic north while you aim the instrument, and the lens lets you read the angle between the two.
| Part | What it does |
|---|---|
| Cover and sighting wire | Protects the crystal when closed. When open, the vertical sighting wire is aligned on the object whose azimuth is needed. |
| Floating dial | The magnetised dial itself. It is balanced and free floating in use, and carries two scales: degrees (red) and mils (black). |
| Fixed black index line | The stationary reference line on the glass. The azimuth is read where the dial passes under it. |
| Lens and rear sight slot | The lens magnifies the dial; the slot is the rear sight. Folded too far forward, the lens holder locks the dial. |
| Bezel ring | A ratchet ring carrying a luminous line. It has 120 clicks in a full turn, each equal to 3 degrees, so a bearing can be set by touch. |
| Thumb loop | Secures the compass closed and steadies it in the centre-hold technique. |
| Luminous markings | The two dots at the ends of the sighting wire, the line on the north-seeking arrow and the line on the bezel. On tritium versions they glow without charging. |
Degrees and Mils
A military compass carries two scales because two communities use it. Degrees divide a circle into 360. Mils divide it into 6400, the NATO convention, which gives finer resolution and a convenient rule of thumb for fire control and observation.
The practical point is consistency. A patrol, an observer and a fire support team must quote bearings in the same unit. Mixing degrees and mils is a simple error with a large consequence. The two scales are printed in different colours, red and black, so always confirm which one you are reading.
How to Use a Lensatic Compass, Step by Step
There are two ways to hold the compass, and each has its job. The centre-hold technique is fast and suits moving on a bearing. The compass-to-cheek technique is more accurate and is used when an exact azimuth is needed, for example for intersection and resection.
- Move away from metal. Check the distances in the table further down before taking any reading. A rifle, a vehicle or a power line will pull the dial.
- Centre-hold. Open the cover fully so it forms a straight edge with the base and move the lens holder back to 90 degrees. Put your thumb through the thumb loop, rest the base on it, lay your index finger along the edge, and hold the compass level at stomach height with your elbows at your sides. Turn your whole body, not the compass, towards the object or direction of travel.
- Compass-to-cheek. Open the cover to 90 degrees and fold the lens holder slightly forward until the dial can be read through the lens, but not so far that it locks the dial. Look through the rear sight slot, align the sighting wire with the object, then glance down at the dial and read the azimuth under the index line.
- Preset a bearing. Hold the compass level and rotate it until the required azimuth sits under the black index line. Turn the bezel until its luminous line lies over the north-seeking arrow. Then, in the centre-hold, turn until the arrow is back under the luminous line and move in the direction of the sighting wire.
- Set a bearing in darkness. The same setting can be made by touch: each bezel click is 3 degrees, so dividing the azimuth by three gives the number of clicks. On the move, keep the north arrow under the luminous line and follow the two luminous dots of the sighting wire.
- Use the back azimuth. To return, or to check a position, add 180 degrees to an azimuth of 180 degrees or less, and subtract 180 degrees from a larger one.
Declination: Grid, Magnetic and True North
A compass points to magnetic north. A military map is drawn on grid north, the direction of its vertical grid lines. True north, the direction of the geographic pole, is a third line. The angle between grid north and magnetic north is the grid-magnetic angle, or G-M angle, and it is printed in the declination diagram in the map margin.
The angle is not fixed. The magnetic poles migrate, so the G-M angle on an older map must be updated with the annual change given in its margin. The same drift is why the World Magnetic Model, produced by the US National Geospatial-Intelligence Agency and the UK Defence Geographic Centre for defence and navigation use, is reissued every five years. It is the standard model for navigation and heading systems that use the geomagnetic field, and the current edition, WMM2025, was released in December 2024.
Why it matters: each degree of uncorrected error puts you about 17 metres off line for every kilometre travelled, so a 7 degree G-M angle ignored over a 5 km night move leaves you more than 600 metres from the objective.
Luminous Markings for Night Navigation
Night navigation is where the compass earns its place. A torch used to read a dial is a light signature, however dim. The luminous markings let the user hold a bearing in the dark without any light source at all.
There are two ways to make those markings glow. Phosphorescent paint stores light and gives it back, so it must be charged and fades through the night. Tritium uses small sealed glass light sources in which the beta decay of tritium gas excites a phosphor coating, giving a steady glow for years with no charging. Our article on tritium illumination explains the physics, and the comparison of tritium vs glow in the dark sets out the trade-offs.
The Tritium Compass 3H is a tritium lensatic compass under NSN 6605-01-196-6971, with induction damping, a non-liquid needle housing and a choice of Northern or Southern Hemisphere balancing. It is also offered with phosphorescent markings.
Lensatic, Prismatic and Hand Bearing Compasses
Military users meet three main sighting compass designs. They all work on the same magnetic principle; they differ in how the bearing is read while the target is in view.
| Type | How the bearing is read | Typical military use |
|---|---|---|
| Lensatic | Sighting wire on the object, dial read through a lens in the rear sight. | Individual land navigation; standard issue in US forces. |
| Prismatic | A prism shows the dial and the object at the same time, so the bearing is read while sighting. | Land navigation where a precise bearing is needed; also used in surveying. |
| Hand bearing | The card is viewed through a prism against a sighting line while the compass is held at eye level. | Bearings from boats and small craft, coastal and riverine work. |
| Baseplate (orienteering) | Liquid-filled capsule on a transparent baseplate, read from above; no sighting optics. | Fast map work and plotting; less suited to precise long-range sightings. |
For waterborne users the tritium hand bearing compass combines a prism sight with one-degree graduations and tritium illumination for bearings at night.
Common Errors in the Field
Most compass errors are not faults in the instrument. They come from the user, the map or the metal around them.
| Source of interference | Minimum distance |
|---|---|
| High tension power lines | 55 m |
| Field gun, truck or tank | 18 m |
| Telephone wire or barbed wire | 10 m |
| Machine gun | 2 m |
| Service rifle | 0.5 m |
- Reading inside a vehicle: a hull or cab deflects the dial far more than any small item of kit. Dismount and walk clear before taking a bearing.
- Ignoring the G-M angle: plotting a compass bearing straight onto a grid map, or the reverse, builds in the full declination error.
- Tilting the compass: a dial that touches its housing does not settle on north. Hold it level and let it float.
- Mixing scales: reading the black mil scale as degrees, or the reverse, gives a bearing that looks plausible and is wrong.
- Following terrain instead of the bearing: pick a steering mark on the bearing, walk to it, then take the next one, rather than drifting with a path or a ridge.
Why a Compass Still Matters Alongside GPS
Satellite navigation is accurate and fast, and it is also a single point of failure. GPS signals can be blocked by terrain and buildings, and they can be jammed or spoofed. Receivers need power. A magnetic compass needs neither power nor a signal, emits nothing and cannot be jammed, which is why it remains the fallback for every soldier and every vehicle crew that may have to navigate when the electronics are down.
The skills transfer the other way as well. A navigator who understands azimuths, mils and the G-M angle reads a digital display critically, and notices when it is wrong.
Frequently Asked Questions
Only with care. The dial is balanced against the downward pull of the magnetic field in the region it was made for. Far from that region the dial tilts, and in the worst case it drags on its housing and gives false readings. For deployments south of the equator, order the Southern Hemisphere version, or specify the balancing when compasses are bought for a mixed theatre.
For many years, but not for ever. Tritium decays steadily, so the glow dims gradually over the life of the compass rather than failing suddenly. The Tritium Compass 3H is specified for more than 12 years of continuous illumination and the tritium hand bearing compass for a service life of 10 years. Plan replacement around that figure, and return or dispose of expired units through a route that accepts tritium light sources rather than as general waste.
Not exactly. A true milliradian divides a circle into about 6283 parts; the NATO mil rounds that to 6400. Other armies have used 6000 (the former Soviet Union and Finland) and 6300 (Sweden). The one metre at one thousand metres rule works as an approximation for all of them, but the values are not interchangeable, so check which mil a compass, a sight or a fire control document uses before working across nations.