Compass binoculars do two jobs at once: they magnify a distant object and show its magnetic bearing in the same view. Used properly, they give a navigator a position fix from charted landmarks, an early warning of collision risk, and a distance estimate from the reticle, all without power or electronics.
How Do Compass Binoculars Work?
A compass binocular builds a small, fluid-damped magnetic compass into one barrel. Its scale appears in the lower part of the image, so the observer reads the bearing of whatever sits on the center line of the view. Most models also carry a reticle: a vertical and horizontal scale used to estimate distance and size.
| Feature | What it does | Why it matters |
|---|---|---|
| Fluid-damped compass | Steadies the card against vessel motion | Readable bearings in a seaway |
| Illuminated scale | Lights the compass at night | Bearings on lights after dark |
| Reticle | Measures angles in mils | Distance off from a known height |
| 7x magnification | Keeps the image steady by hand | Easier to hold a target on the center line |
The bearing shown is magnetic. Like any compass, it must be corrected for variation, and for any deviation caused by the vessel, before it is plotted. Choosing the binocular itself is covered in the ANS marine binoculars guide.
How to Take an Accurate Bearing With Compass Binoculars
Accuracy depends more on technique than on the binocular. A hand-held compass in good conditions typically reads to within about a degree, according to Starpath's analysis of bearing fix accuracy, and poor technique quickly adds more.
- Choose a spot clear of steel, electronics, and loudspeakers. On a steel vessel, structure near the observer can deflect the compass, so use the same position each time.
- Brace against something solid and hold the binocular level.
- Center the object on the vertical line of the reticle.
- Wait for the compass card to settle, then read the bearing at the center mark.
- Take two or three readings and average them if the vessel is moving.
- Note the time with each bearing, to the minute.
A quick check of the compass error is worth making on each passage. Take a bearing along a charted transit, where two marks line up, and compare it with the bearing on the chart. The difference shows the combined error to apply.
How to Plot a Position Fix From Compass Bearings
A single bearing gives a line of position. Two or three bearings taken close together give a fix.
- Select two or three charted objects that are clearly identifiable: lighthouses, headlands, or prominent buildings. Nearer objects give smaller errors.
- Space them well apart. Three bearings about 120 degrees apart give the strongest fix, while bearings close together give a poor cut.
- Take bearings on objects ahead or astern first, because those change slowly, and the object abeam last.
- Convert each magnetic bearing to true by applying variation from the chart's compass rose, as explained in the ANS guide to magnetic variation vs deviation.
- On the chart, draw each line through the charted object at its true bearing, extending back toward the vessel's track.
- Mark the fix where the lines cross and label it with the time.
Three lines rarely meet at a point, and instead form a small triangle, the "cocked hat." A small triangle indicates a good fix. A large one means a bearing was misread, an object was misidentified, or compass error has not been applied. When near danger, assume the vessel is at the corner of the triangle closest to the hazard. The ANS guide to reading nautical charts covers identifying landmarks and the compass rose, and a fix of this kind restarts the plot in dead reckoning.
Using Compass Bearings to Check Collision Risk
Compass binoculars are also a collision-avoidance tool. COLREGs Rule 7(d)(i) states that risk of collision shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change. The full text of Rule 7 adds that risk may exist even with a bearing change when approaching a very large vessel, a tow, or a vessel at close range.
A practical routine:
- Take a bearing on the approaching vessel and note the time.
- Repeat every few minutes from the same position aboard.
- If the bearing stays steady while the range closes, treat the situation as a risk of collision and act early.
- Cross-check with radar and AIS where fitted, since a hand-held bearing is one input, not the whole picture.
How to Estimate Distance With a Binocular Reticle
The reticle measures angular size in mils, short for milliradians. When the height of an object is known, the distance follows from a simple formula:
Distance = object height × 1,000 ÷ reticle reading in mils
Height and distance come out in the same unit. For example, a lighthouse 30 meters tall that spans 10 mils on the reticle is 30 × 1,000 ÷ 10 = 3,000 meters away, about 1.6 nautical miles.
Three cautions apply:
- Reticle markings differ by model. On many marine binoculars, a minor mark equals 5 mils and a major mark 10 mils, but each manual should be checked.
- Miscounting mils produces a large distance error, so count carefully and repeat.
- Charted heights of lights are measured above a high-water datum, not from the waterline at the time of observation.
Choosing Marine Binoculars With a Compass
The compass models below are all 7x50s, the magnification and lens size best suited to hand-held use on a moving deck.
| Model | Compass | Reticle | Waterproofing |
|---|---|---|---|
| Steiner Navigator 7x50 with Compass | Illuminated, fluid-damped | Not listed | Submersible to 5 meters |
| Steiner Commander 7x50 with HD Compass | Large analog compass | Not listed | Waterproof to 10 meters |
| Fujinon Polaris 7x50 FMTRC-SX | Built-in compass | Yes | Built to US military specification |
| Weems & Plath Marine 7x50 | Illuminated | Rangefinder reticle | Floating design |
| Nikon OceanPro 7x50 with Compass | Built-in compass | Not listed | Nitrogen-filled, O-ring sealed |
For bridge watchkeeping and night bearings on lights, an illuminated compass is the feature to prioritize. For distance estimation, choose a model with a reticle. The ANS roundup of the best marine binoculars compares models by use case, including when image stabilization is worth the cost.
Shop Marine Binoculars
Compass binoculars turn every lighthouse and headland into a navigation reference. With a steady technique, a corrected bearing, and a well-placed second line, they give a fix that needs no satellite and no power.
Shop marine binoculars in the ANS binoculars collection, or contact ANS at +1 (954) 522-3321 or sales@amnautical.com for help choosing a compass model.
Frequently Asked Questions
What is a compass-reticle binocular used for?
A compass-reticle binocular takes magnetic bearings of landmarks and vessels for position fixing and collision-risk checks, and its reticle estimates distance to objects of known height.
Are compass binocular bearings magnetic or true?
Magnetic. Each bearing must be corrected for variation, and for any deviation from the vessel's steel or electronics, before it is plotted on a chart.
How accurate are compass binoculars?
In good conditions, a hand-held compass bearing is typically accurate to about one degree. Vessel motion, nearby metal, and misidentified objects add error, which is why two or three bearings are used for a fix.
How do I get a position fix from compass binoculars?
Take bearings on two or three charted objects spaced well apart, convert them to true, and draw each line through its object on the chart. The fix is where the lines cross.
Can compass binoculars tell if another vessel is on a collision course?
Yes. Under COLREGs Rule 7, risk of collision is deemed to exist if the compass bearing of an approaching vessel does not appreciably change while the range closes.
Why are marine compass binoculars usually 7x50?
Seven-times magnification keeps the image steady enough to hold a target on the compass centerline from a moving deck, and the 50 mm lenses gather light for dawn, dusk, and night bearings.
