[Mike Irwin]

"THEY USED TRIANGULATION."

Triangulation is good if you have the time and equipment needed to take the measurements.

You need at a minimum a measuring device (if it is of a known length, even a piece of rope will work) and a device to measure the angles.

Then you need to be able to run the computation which, while relatively simple mathematics, still was likely beyond most of the infantry soldiers of the time.

Triangulation would have been a tool used primarily by artillery and engineers, and would probably be most useful when estimating the distance to a fixed point, and less useful on a fluidly moving battlefield.

As others have noted, most soldiers would have simply guessed at the range based on personal experience.