Until the First World War, guns were fired over open sights, or nearly so, and were therefore located from their muzzle flash and the smoke issuing from the barrel. Increasing ranges called for other techniques. But let us first see how muzzle-flash spotting could theoretically have been sufficient—and why it was not…
Visual detection, acoustic detection ------------------------------------
The essential instrument for visual detection is the theodolite, a device used to measure angles.

A theodolite in military dress. This model, however, dates from after the Great War, in the 1950s.