Before Lasers: How Battleships Measured Distance with Optics and Geometry ?

How did battleships determine the distance to an enemy ship kilometer's away, before laser rangefinders?

The answer lies in precision optics, trained operators and triangulation.

Two important instruments were the coincidence rangefinder and the stereoscopic rangefinder.

Both observed a target through two viewing windows separated by a precisely known distance—the baseline.

1. Coincidence Rangefinder

Mirrors and prisms brought the two views into a common eyepiece. The operator turned a measuring knob until the displaced target images coincided, or their split portions aligned.

The calibrated mechanism translated this adjustment into a range reading.

2. Stereoscopic Rangefinder

The operator viewed the target through two eyepieces, receiving a different perspective in each eye.

Using binocular depth perception, the operator adjusted a measuring mark until it appeared at the same distance as the target.

The instrument then indicated the range.

The mathematics behind the measurement

For a distant target approximately perpendicular to the baseline:

R ≈ B / θ

R = target range

B = baseline length

θ = angle between viewing directions, in radians

Illustrative example:

B = 10 m; θ = 0.0005 rad

R ≈ 10 / 0.0005 = 20,000 m = 20 km

A longer baseline improved sensitivity to small angular differences, explaining the wide horizontal rangefinders mounted on naval directors and gun turrets.

However, measuring distance was only part of the challenge. Fire-control systems also accounted for ship movement and shell flight time. Observed shell splashes helped correct subsequent salvos.

Accuracy depended on visibility, optical alignment, calibration and operator skill. Small angular errors became increasingly significant at longer ranges.

Disclaimer: Based on public historical sources for educational discussion. The numerical example is illustrative, not a specific battleship’s performance claim. Corrections from naval veterans and specialists are welcome.