stadia

Latin supplies this plural, and it covers every meaning of stadium, not only sporting venues. Stadium can also name a telemeter, a surveyor's rod read between stadia wires, or a life stage of an organism. So in a biology text, stadia are developmental stages, not grounds. Context, not form, tells them apart.

A surveyor's word that came into English from Italian. Despite its look, this stadia is not a plural: it names a single rod. Distance is read by counting how many of the rod's graduations appear between the stadia hairs in the telescope. The word often works attributively, as in stadia hairs and stadia wires. The Greek and Latin plurals spelled the same way are separate words.

Here the graduations move from the rod into the optic. With the surveyor's rod, the scale stands in the field and the hairs sit in the telescope. In a riflescope the marks are etched on the lens, and the target becomes the rod. That is why the method needs a target of known height, such as a soldier or a game animal. The technical name for the technique is stadiametric rangefinding.

The Greek plural, taken straight from στάδια. A stadion was a unit of distance of about 185.4 meters, fixed by standardized footraces. The word could also mean the racecourse itself, so stadia here can mean lengths or tracks. Because stadion and stadium overlap in both meanings, this form also coincides with the Latin plural.

A historical instrument, so the word belongs in accounts of past soldiering rather than descriptions of current equipment. It works on the same principle as the riflescope markings: a soldier or horse of roughly known height serves as the yardstick. The difference is that the scale was engraved on a hand-held brass triangle instead of etched into glass.