What use does a stadia rod serve in the context of surveying?

A stadia rod is primarily used in surveying to facilitate the calculation of distances and elevations. This tool works in conjunction with a level or transit during surveying tasks. When a surveyor takes a reading from a stadia rod placed at a known distance, the sighting through the leveling instrument allows for the determination of the height and depth of benchmarks relative to the instrument's line of sight.

This is critical in establishing relative vertical positions, which aids in various tasks such as topographic surveys, where understanding changes in elevation is essential. Additionally, the graduated markings on the stadia rod enable accurate readings, which are further used in calculations involving the angles measured and the distance to the rod, ultimately translating to precise measurements for distances and elevations on the surveyed land.

The other options, though relevant in broader surveying contexts, do not accurately describe the specific function of a stadia rod.