Practical Value of Rangefinders

As optical instruments, rangefinders have finite accuracy and precision. Manufacturers and naval services attempted to identify and quantify sources of error and limitation in an effort to make their instruments more helpful in the fire control process.

This article will focus first on Coincidence Rangefinders, as they are easiest to characterise in optical and mechanical terms.

Sources of Error

The Royal Navy estimated that the human eye can detect misalignment of half images in a coincidence rangefinder when the angle at the eye reaches 12 arc seconds, or 0.0000582 in circular measure. For a rangefinder, this meant that the uncertainty of range estimation was:

error = (0.0000582 * range * range) / (magnification * baselength)

In other words, the error in ranging would vary with the square of the range, and would be reduced linearly by the baselength of the instrument and by its magnifying power.

Errors in Practice

John Brooks cites late-war Grand Fleet reports that

An examination of rangefinder plots obtained at full calibre firings during the third quarter of 1917 shows that small rangefinder spreads were the exception, and that the gun range in many cases differed from the mean of the rangefinders by as much as 1000 yds. In more than one instance, an individual rangefinder altered its divergence from the gun range by 2000 yds. during the firing. Rangefinder rates on the whole were good, but in some cases all rangefinders of a ship indicated a rate of 200 to 300 yds. per minute in error. ...for want of a better explanation, these errors have been attributed to refraction, i.e. the bending of the rays from the target to the rangefinder, due to the opacity of the atmosphere, smoke, or light effects. </quote> Investigation by Barr and Stroud discounted reports of refraction and placed the blame predominantly on heating of the instruments under direct sunlight.

Footnotes

Bibliography

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