What is "stadia rod"

Stadium \Sta"di*um\ (st[=a]"d[i^]*[u^]m), n.; pl. Stadia (st[=a]"d[i^]*[.a]). [L., a stadium (in sense 1), from Gr. sta`dion.]

1. A Greek measure of length, being the chief one used for itinerary distances, also adopted by the Romans for nautical and astronomical measurements. It was equal to 600 Greek or 625 Roman feet, or 125 Roman paces, or to 606 feet 9 inches English. This was also called the Olympic stadium, as being the exact length of the foot-race course at Olympia.
   --Dr. W. Smith.

2. Hence: A race course; especially, the Olympic course for foot races.

3. Hence: A modern structure, with its inclosure, resembling the ancient stadium[2], used for athletic games which are typically played out-of-doors; such stadiums are usually large structures without roofs, though some modern stadiums may have a protective dome overhead. It may be contrasted with the arena, the term commonly used for smaller structures at which indoor games are played.

4. A kind of telemeter for measuring the distance of an object of known dimensions, by observing the angle it subtends; especially (Surveying), a graduated rod used to measure the distance of the place where it stands from an instrument having a telescope, by observing the number of the graduations of the rod that are seen between certain parallel wires (stadia wires) in the field of view of the telescope; -- also called stadia, and stadia rod.