In the early days the only way to find the distance to the target was using binoculars without any built-in reference. The position of the observer was placed on the map. The map was divided in parts, for instance a rectangular system with coordinates. In this way reference points for the observer and the calculator were created. The observer spotted the target and the direction was taken from the map and computed into aiming directions. The problem with this procedure is that the observer had to stay in one place and could only rotate in horizontal and vertical axes. These are circular movements, which are not easy to translate to a map which is devided into squares. The classical matrix of degrees, minutes and seconds are not very fast references in the stress of the battle. So, a more simple system was designed.

Translated in: thousands, 1/1.000 or mills (US). Army and Airforce used this system, while the Navy maintained the traditional designation in degrees and the spoken directions of the compass rose. Let us explain. The aiming circle in the Strich system was divided in 64.000 segments. A circle with a radius of 1.000 meters has a circumference of 2 x pi x radius = 6.280 meter. This is not a pratical value. But when we take the next rounded number, 6.400, it can easily be divided by 2 or the multiples of 2. The margin of error is small. North, 0 degrees, lies at 0 Strich, but also at 6.400. Some translations to the Navy system:

- east, 90 degrees, is 1.600 Strich
- north-east, 45 degrees, is 800
- north-north-east, 22 degrees, 30 minutes, is 400 Strich
- north-north-east-northern, 11 degrees, 15 minutes, is 200
- north-north-east-northern-by-north, 5 degrees, 37 minutes, 30 seconds, is a simple 100 Strich

It is easier and much faster in communication.

The full circle is 6.400 Strich, moving one mill, you cover 1 meter at a distance of 1.000 meters. At 3 kilometers this is 3 meter. This makes it possible to calculate the distance to a target, however you have to know the dimensions of that target.

A Lancaster bomber has a span of 34 meters. Let us assume that the plane is flying straight towards the observer. He puts the middle of the cross hair on the nose of the bomber. He reads the scales: over the wingtips 17 Strich and to the ground 62 Strich. Next formula gives the horizontal distance: 34 x 1.000 / 17, so 2.000 meters. 62 x 2 gives a height of 134 meter. If the guns fired, the observer could see the explosions and give corrections. In this way a simple firecontrol was born. This is all theory. The practical result depended on the skills (experience) of the observer, the quality of the optics and the accuracy of guns and ammunition. Fixed weapon positions were pre-sited. Gun parameters were noted during test runs.

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