In the context of surveying, what does a leveling rod measure?

A leveling rod is primarily used to measure the elevation differences between various points in surveying. When used in conjunction with a leveling instrument, the leveling rod provides a visual indication of height differences relative to a reference point, typically a horizontal line of sight established by the leveling instrument. This process is essential for establishing datum points and determining elevations for construction, land development, and various engineering projects.

The other options do not accurately describe the function of a leveling rod. For instance, measuring horizontal distances requires tape measures or total stations, not a leveling rod. The angles of elevation from a baseline pertain more to the use of theodolites rather than leveling rods. Lastly, while surveying does involve accounting for the curvature of the Earth, a leveling rod itself does not directly measure this curvature; it focuses on relative elevations in a specified area.