What does the term 'backsight' refer to during leveling?

The term 'backsight' refers to a measurement taken from the level instrument back toward a reference point, usually the starting point of the leveling survey. It is used to establish a line of sight to a known elevation or benchmark, ensuring accuracy in height differences throughout the leveling process. This measurement is crucial because it allows the surveyor to determine the instrument's height above the ground when the backsight is read against a benchmark, thus facilitating the accurate calculation of elevations at points forward in the leveling run.

In the context of leveling, the backsight contributes to establishing the necessary elevation control, allowing surveyors to compute the difference in elevation between various points. It is essential for creating a reliable level line upon which further measurements can be based.

The other options describe different concepts related to surveying and leveling but do not accurately define 'backsight.' A reference point for leveling pertains to marks or benchmarks used for setting the level but does not capture the action involved in backsighting. The distance measured towards the starting point and the angle measured from the horizontal describe different processes involved in leveling but do not specifically relate to the function of backsight in establishing elevation.